|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 107-149|
Princeton University, Princeton, New Jersey
The Late Triassic Dachstein Formation (Lofer facies) of the Northern Limestone Alps in the region of Salzburg is a lagoonal deposit containing about 200 cyclothems. Cyclothems in the mid-part of the formation show a basal disconformity. Underlying desiccation and solution cavities are filled by an insoluble-rich, commonly red limestone, interpreted as a reworked soil. Above lies a thin unit of dolomitic limestone with algal mats, containing a variety of desiccation structures; this is interpreted as intertidal. The main and last unit of the cyclothem is a massive limestone with varied biota, considered as subtidal These bedded rocks are cut by neptunian dikes--limestone fissure fillings which show distinctive faunas and shrinkage structures. A new terminology is proposed for shrinkage structures. The cyclothems are attributed to a eustatic fluctuation of low amplitude and a period of between 20,000 and 100,000 years. A megacyclic grouping of these cyclothems in sets of 5 to 8 is deduced from graphic space-time analysis and is attributed to rhythmic variation in the rate of tectonic subsidence.
The Late Triassic (Norian-Rhaetian) rocks of the Northern Limestone Alps consist mainly of carbonates. In Salzburg and adjacent regions (Fig. 1) they form a great reef complex (Fig. 2; Spengler 1951; Zapfe 1959, 1962a). A barrier reef, the Dachstein Riffkalk or Hochgebirge-Korallenkalk, is preserved in remnants along the southern margin of the Northern Limestone Alps, and occurs north of this margin in some tectonically transported masses. The foreslope talus of this barrier-reef belt interfingers southward with the thin, partly red, ammonite-bearing Hallstatt facies, which I interpret as a starved-basin sediment formed in waters of considerable depth (for a contrary opinion see, Zapfe, 1959). Northward the reefs pass into a sequence of bedded limestones and dolomitic limestones, the Dachstein Formation in the narrow sense (Fig. 1).
Figure 1--Index map showing five geological provinces: (1) Molasse Basin, of Oligocene-Miocene sediments, only partly involved in alpine folding; (2) Late Cretaceous-Eocene Flysch, folded and faulted; (3) Northern Limestone Alps, belt of Mesozoic sediments (mainly limestone), moderately to intensely folded and faulted; (4) Graywacke Zone, Paleozoic sediments and volcanics, intensely folded and mildly metamorphosed and (5) Alpine Core of Precambrian, Paleozoic (?) and Mesozoic rocks intensely deformed and metamorphosed. Northern Limestone Alps, together with Graywacke zone, form large thrust sheet (Oberostalpin) which has moved far from south, across Alpine core, onto Flysch. Rear boundary is not shown as thrust here, for in writer's view it represents later vertical fault with vertical and extensive lateral movement. Major units within Alpine core, and subsidiary thrust masses within Northern Limestone Alps, are not differentiated. Late Triassic rocks in major ranges in southern part of Northern Limestone Alps have been differentiated, lagoonal Lofer facies (Dachstein Limestone sensu stricto) being shown by vertical lines, barrier reef facies by cross shading. Three short sections described in paper are shown by circles. Base map from Yetters (1933) and others. A larger version of this figure is available.
Figure 2--Diagrammatic restoration of Late Triassic facies in Northern Limestone Alps (Salzburg region). Cycles described here occur in Lofer back-reef facies. A larger version of this figure is available.
The bedded Dachstein Formation comprises a sequence 1,000 to 1,500 meters thick, and some tens of kilometers broad. The lower part, the Norian, is commonly dolomite (Dachstein Dolomite, or tongue of the Hauptdolomit, discussed below), whereas the middle and upper parts (Norian to Rhaetian) are dominantly limestone, with minor dolomitic intercalations. The sequence is clearly of "back-reef" or lagoonal origin (Spengler 1951; Zapfe 1957, 1959), deposited in shallow waters at times unfavorable to normal marine life. Its cyclic stratofabric is the subject of this paper.
Northward the Norian parts of the Dachstein sequence are more dolomitic and pass into the ultra-back-reef facies of the Hauptdolomit. The Hauptdolomit facies extended toward the Vindelician shoreline and evaporites of the German Keuper Basin. In places the late Norian is represented by a fine-grained, platy, commonly bituminous limestone, the Plattenkalk. The Rhaetian parts of the Dachstein Limestone pass northward into the marly, bituminous Kössen beds, evidently a sediment of slightly deeper water, rimmed in late Rhaetian time by smaller barrier and patch reefs.
Cyclothems were first recognized in the Dachstein Formation by Sander (1936). In much of the formation, thin (less than one meter thick) units of dolomitic limestone, partly showing a striking millimeter-lamination, alternate with massive limestone units generally several meters thick. Sander termed this rhythmic facies, the Lofer facies, because of its excellent exposure in the Loferer Steinberge near Lofer (Fig. 1).
Sander wrote of the complex fabrics of the laminated rocks, and discussed cycles and their origin. Subsequent work of Schwarzacher and my work have essentially confirmed his conclusions and have added new data.
Schwarzacher (1948) dealt with cycles in general, and with the laminated facies of the Dachstein Formation in particular, agreeing with Sander that this represented annual varving. He also noted a megacyclic grouping of cyclothems, a subject more fully discussed in his 1954 paper. Sander attributed the cycle to changes in sea level, but saw no evidences of emergence. The thin, dolomitic, laminated beds, however, offered conflicting evidence of water depth. The organic crusts and algal threads in them suggested shallow water, yet the presence of such fine laminations suggested to Sander a depth below wave base (which he took to be 200 meters) . Here, they are interpreted as intertidal deposits.
The red limestones, which form much of "member A" in the cyclothem division here employed, were studied by Leuchs and Udluft (1926) and by Leuchs (1928).
The present paper is based on the writer's work on the late Triassic Alpine reef complexes. The Dachstein Limestone was investigated in traverses across the Loferer Steinberge, Reiteralm and Lattengebirge at Berchtesgaden, Steinerne Meer, Göll-Brett Massif (in which the back-reef facies is well exposed along the road to the Kehlstein tea house), Tennengebirge, Dachstein, and in some of the ranges farther to the east. The stratigraphic development of the cyclothems was specifically investigated in three short sample sections which provide fresh rock surfaces, free of the usual crust of boring algae and enclosing lichens. Field work was supplemented with hand specimens, peels, and thin sections, as well as some insoluble residues.
The sections measured are shown in Figure 1. The section in the Loferer Steinberge was measured on the south wall of the cirque behind the Schmidt-Zabirow hut (Fig. 5A). It includes five cyclothems and is shown graphically in Figure 3.
Figure 3--Section measured in Loferer Steinberge, separated into constituent cyclothems (for explanation see, Fig. 4).
Figure 4--Section measured in Steinernes Meer, separated into constituent cyclothems. A larger version of this figure is available.
The section in the Steinernes Meer, which consists of twenty cyclothems (Fig. 4), was measured on the north wall of the cirque between the Riemann House and the Breithorn (Fig. 5B). The section on the Dachstein containing ten cyclothems (Fig. 6), was measured near the foot of the Hallstatt glacier, below the trail leading from the Simony hut over the glacier to the Dachstein peak. None of these sections was stratigraphically pinpointed within the formation; the Loferer Steinberge section is probably not above the middle, Steinernes Meer section at or about the middle, and Dachstein section some hundreds of meters below the top of the formation.
Figure 5--Aerial photographs showing landscape expression of Lofer cyclothems. A, face in southern part of Steinberge (Leoganger Steinberge). Height of face shown about 1,000 meters. Cyclic bedding is more pronounced in upper, more calcitic part of section than in lower, wholly dolomitic portion. B, Breithorn in Steinernes Meer. Massive beds are subtidal megalodont-bearing limestone "member C." Intervening recesses are formed by somewhat dolomitic intertidal beds. Mean thickness of cyclothem between 5 and 6 meters.
Figure 6--Section measured on Dachstein, separated into constituent cyclothems (for explanation see, Fig. 4). A larger version of this figure is available.
The main conclusion is that the Lofer cyclothem (Fig. 7) typically consists of: (1) a disconformity at the base; (2) a basal argillaceous member (red or green) which is commonly restricted to solution or desiccation cavities in the underlying rock; (3) an intertidal member containing algal mats and other sediments showing a variety of shrinkage features attributed to desiccation; and (4) a subtidal massive limestone member with a varied biota. The bedded sequence is cut by neptunian dikes--fissures filled by calcareous sediments, some of them characterized by a distinctive fauna.
Figure 7--Diagrammatic representation of Lofer cyclothem. A, basal, argillaceous member, representing reworked residue of weathered material (red or green), commonly confined to cavities in underlying limestone. B, intertidal member of "loferites" with algal mats and abundant desiccation features. C, subtidal "megalodont limestone" member, with cavities produced by desiccation and solution during succeeding drop in sea level.
The unusual structures found in these rocks are derived mainly from the near-surface lithification of each cyclothem by desiccation, dolomitization and weathering processes, in which compaction played only a minor role. Similar cyclothems and similar structures may be more common elsewhere than is generally known, but their recognition in the Alps has been favored by the excellence of exposure, and by color contrasts between the red or green basal member, white-weathering dolomitic intertidal carbonates, and gray-weathering subtidal member. Origin of neptunian dikes is sought in contemporaneous fracturing of the region and in deep mudcracking.
These sediments accumulated in a setting in which tectonic subsidence was closely balanced by carbonate sedimentation, so that the surface of the sediment (i. e., of the lithosphere) remained closely adjusted to sea level. The cyclic nature of the sediments records cyclic deviations from a perfect adjustment, and thus provides opportunity to inquire into the interplay between tectonics and eustatic movements. The graphic time-space analysis may find application in the study of other cyclothem sequences. Even with the work of Sander, Schwarzacher, and the author, knowledge of the Lofer cyclothems, their petrography and their sedimentary structures, remains in a preliminary stage. Many of the interpretations made here are speculative, in need of further documentation (or refutation), in field and laboratory. Many aspects of these rocks have not been dealt with at all. Hopefully this paper will help to kindle further studies.Acknowledgments--The U. S. National Science Foundation supported this study through grant G 11588, and through a Senior Postdoctoral Fellowship (during a sabbatical leave of absence from Princeton University) at the University of Innsbruck. Professor W. Heissel at Innsbruck furnished space and facilities of the Geological Institute. It is a special pleasure to acknewledge the stimulation received from Professor Emeritus Bruno Sander, who has followed this study with much interest. Dr. B. Hauser, Professor Ladurner, and Dr. F. Purtscheller provided photographic facilities, and Dr. Schäfer provided thin-sections of Norian dolomites from the Dolomites. Discussions with Drs. J. L. Wilson and A. Wells of the Shell Oil Co., and with Messrs. D. Shearman and G. Evans of Imperial College, London, about recent carbonate sedimentation in the Persian Gulf, have strengthened my faith in the ecologic interpretations here presented. To Dr. Alan Lees of the University of Reading I am especially indebted for discussions concerning pore space and internal sedimentation. Dr. Robert Ginsburg of the Shell Oil Co. provided literature on algal mat sediments. For help in fieldwork I am grateful to Drs. H. Ohlen and E. Cotter, and to Mr. J. Gross; for extended field discussions especially to Drs. Ch. Pendexter and H. Zankl, and to Mr. Claude Monty.
One of the most distinctive characters of the rocks dealt with here is the occurrence of cavities which appear to have originated by shrinkage of muddy sediment (mt). [Note: See glossary at end of this paper for new or unfamiliar terms.] These cavities were filled or partly filled during early diagenesis by internal sedimentation, which generally involved a combination of mechanically introduced carbonate mud, m2 or bottom mud (Sander's k2 or d2), and chemically precipitated calcite or dolomite spar, forming linings and fillings of the remaining voids. Such shrinkage structures are classified into three main types.
Prism cracks--The most familiar type of shrinkage structure is the "mudcrack" which develops in regular or irregular polygonal patterns on the surface of drying mud puddles. To distinguish this type of crack from others which also develop in mud it is here termed prism crack, inasmuch as it tends to break the sediment into prisms standing normal to bedding (Fig. 8). The Lofer cyclothems show prism cracks developed at three different scales. Prisms on the order of a half meter in diameter, their cracks filled partly with argillaceous limestone and partly with radiaxial calcite, are sometimes found below the unconformity at the base of the cycle (Fig. 9). Prisms with a diameter and depth on the order of a decimeter are found in the algal-mat facies and in pellet-calcilutites of the intertidal member (Fig. 10, 11, 12A, 13). In these, the indivdual beds are generally concave upward, and disturbed along the cracks; cracks are on the order of 1 to 2 centimeters wide. They are filled mainly with dolomite (or calcite) mud, but may show spar-filled segments (Fig. 13B); the mud fills (m2), in turn, commonly show spar-filled cracks (Fig 11C). Prisms on a scale of 2 to 3 centimeters and a depth of only 2 to 3 millimeters also occur in the algalmat facies (Fig. 12B, 14A, 14B). These show no concavity upward and are generally spar filled; the cracks tend to be triangular in cross section, tapering upward. Perhaps such cracks formed not directly at the surface but under cover of a tough algal layer; alternatively, they may have formed at the surface during a brief spell of shallow desiccation, and may have been overgrown by the next algal layer before acquiring a mud filling (Fig. 8B).
Figure 8--Diagrammatic representation of prism cracks and sheet cracks. A, large prism cracks, open to surface, which have broken sediment into prismatic columns. Sheet cracks of varying size have developed parallel to bedding. B, small (internal?) prism cracks have divided certain beds into polygonal plates.
Figure 9--Unconformity at base of cyclothem. Surface of megalodont limestone, somewhat ground by glacial scour and etched by subsequent solution. Elevated floor of cirque behind Schmidt-Zabirow hut, Loferer Steinberge. A, erosion-modified prism cracks in megalodont limestone filled with greenish argillaceous limestone ("member A") of succeeding cyclothem (light colored). B, detail, showing mudfills and megalodont pelecypod shells. It is uncertain whether truncation of shells is due to intercycle unconformity or to subsequent glacial scour. Shells had been partly dissolved away before deposition of succeeding argillaceous lime mud. C, prism cracks partly filled with radiaxial calcite rather than with lime mud.
Figure 10--Prism crack, sheet cracks, and shrinkage pores. Drawn from peel illustrated in Figure 13. Original pore space shown in black, no matter whether now filled by m2 or by spar. Göll-Brett complex; road to Kehlstein tea house.
Figure 11--Intertidal sediments: Mudcracked algal mats. A, algal mats divided into prismatic columns by deep mudcracks; side view, width of columns 10-20 cm.; outcrop, Steinernes Meer. B, prism-cracked algal mats in bedding-plane view; outcrop, along trail below Schmidt-Zabirow hut, Loferer Steinberge. C, peel, negative print X 1, of bedding plane parallel section; algal laminae are upwardly concave in each column, therefore obliquely cut by section. Algal laminations highly dolomitic, mud-filled prism cracks less dolomitic; spar appears black on print. Vertical sections shown in Figures 12 and 13. Locality same as B.
Figure 12--Intertidal sediments: Mudcracked algal mats. Peels, negative prints X 1. A, vertical section of specimen illustrated in Figure 11C; Part of column bounded by prism cracks. B, bedding parallel section of small prism cracks, peel, negative print X 1; cracks are about 2 mm. deep, mainly filled with spar (black), and triangular in cross section, pinching out upward; dolomitization (light gray) has occurred preferentially along walls of mudcracks. Larger mudcrack of type illustrated in A cuts specimen from left to right. Both specimens from trail below Schmidt-Zabirow hut, Loferer Steinberge.
Figure 13-Intertidal sediments: Prism cracks and sheet cracks. A, peel, negative print X 1, of specimen illustrated in Figures 11C and 12A; note spar-filled microsheet cracks between laminations, and larger, irregular shrinkage pores in more homogeneous mud. B, peel, negative print X 3, of calcitic pellet loferite and calcilutite, cut by prism crack which passes into sheet cracks (outlined with ink); entire bottom edge of picture lies in large sheet crack. Abundant shrinkage pores as well as prism crack and sheet cracks are filled partly with m2, partly with spar. Abrupt termination of mud-fill in upper sheet crack (light) against spar fill of prism crack (dark) suggests that mudcrack opened to present width after sheet crack had become filled. Göll-Brett complex, road to Kehlstein tea house. Compare drawing (Fig. 10).
Figure 14--lntertidal sediments: Crinkled algal mats. Peels, negative prints X 3. A, bedding parallel section across domal structure (center) showing large and small vesicles; below this an irregular shrinkage crack (black). Schwerwand, Tennengebirge. B, bedding parallel section showing domal structure in lower left and microprism cracks on right side. Dolomitic layers (white) contain many small rounded pores which may be cross sections of algal thread molds; locality same as A. C, Vertical section through typical crinkled algal-mat rock. Golling quarry (Hagengebirge).
There is a distinct possibility that these sediments are also prism cracks on a much larger order of magnitude, too large to be readily apprehended on the ground. Such large-scale mudcracking, with prisms tens of meters in diameters was pointed out to the writer by Mr. Wallace Pratt, from the air, in the saltflats west of the Guadalupe Mountains in Texas. The sediments involved here, however, are mainly algal-mat, laminated gypsum deposits, which accumulate in temporary playa lakes. Such prism cracks might be expected to open to widths of decimenters or a meter. Their presence in the Lofer cyclothem sequence is suggested by numerous submarine fissure fillings-neptunian dikes, discussed below. However, some of these neptunian dikes seem to owe their origin to tectonic fracturing.
Sheet cracks--Another type of mudcrack does not assume a polygonal pattern normal to the depositional surface, but shows a planar form. It may form simple sheets of spar or internal mud fill in the sediment, or may occur in parallel sets which produce a zebralike banding (zebra limestone) in cross section. (Fig. 15, 16F, 17, 18).
Figure 15--Neptunian dike of zebra limestone. Massive limetsone ("member C," stippled) dipping toward left is cut at right angles by dike with complex filling. Later fault movement has sheared away right side of dike in area of picture, so that only about one-half of dike remains. Sequence of events: (1) sedimentation of calcarenite; (2) shattering of rock by small-scale faulting and opening of fault planes; fillings of fibrous calcite, shown by closely spaced lines; individual fractures deviate from main fracture zone into calcarenite; repeated movements led to many successive episodes of spar deposition; (3) main fault zone opens wide, and is filled by Triassic calcilutite of mauve-gray color (shown in black), with scattered ostracode shells; (4) shrinkage of dike mud, resulting in closely spaced, bedding parallel sheet cracks (zebra rock); a peel of rock is shown in Figure 16F; and (5) tilting of rocks and faulting oblique to dike, removing right side. Tennengebirge, above Werfen, along trail from Oedl house to Eisriesenwelt cave.
Figure 16--Algal fabrics, shrinkage pores and zebra limestone. A, thin section, positive print X 40, showing algal filaments of fine to intermediate diameter, standing mostly normal to bedding, in fine dololutite layer between vesicular dolomite laminae (algal crusts). Detail of slide illustrated in Figure 19C; Mt. Schlern, Dolomites. B, thin section, positive print X 40, of vesicular dolomite layer (algal crust); small vesicles are probably random cross sections of algal filaments or clumps; origin of larger blisters (note thin dolomitic wall) unknown; Mt. Pomagagnon near Cortina d'Ampezzo, Dolomites. C, thin section, positive print X 40, of fine- to medium-diameter algal threads oriented roughly normal to bedding, in a fine dololutite band between vesicular dolomite layers; from same slide as "B." D, thin section, positive print X 40, showing dolomitic pellet loferite with large shrinkage pore. Note m2 at bottom and spar above. Cycle 8, Dachstein section. E, thin section, positive print X 40, of shrinkage pore from section illustrated in Figure 20C; cycle 14 of Steinernes Meer section. F, peel, negative print X 3, of homogeneous gray calcilutite cut by parallel, spar-filled sheet cracks (zebra limestone). From a neptunian dike at Eisriesenwelt ice cave, near Werfen (Tennengebirge; Fig. 15).
In bedded sediments such sheet cracks are generally parallel to bedding whereas in neptunian dikes they may develop either parallel to bedding (Fig. 15) or parallel to the walls.
The relationships of sheet cracks to prism cracks are shown in Figures 10 and 13B, and diagrammatically in Figure 8. Sheet cracks occur through a great range of scale, from the microscopic ones illustrated in Figure 20C through the centimeter scale shown in the pictures of zebra limestone, to the decimeter wide, meter-long sheet cracks developed in massive limestones below the unconformity at the base of the Lofer cyclothem (Figure 21, 23).
Microscopic sheet cracks of the rocks termed loferites (Fig 20C) are either sparfilled, or show a combination of bottom mud with succeeding spar. Larger sheet cracks of the zebra limestones and of the type found below the unconformity commonly show more complex fillings (Fig. 17, 18, 21). Fragments may have dropped off the roof and lodged on the floor; then there is commonly a lining of radiaxial spar, followed by deposition of carbonate bottom mud, which may alternate with further episodes of spar precipitation. In the zebra limestone shown in Figures 17 and 18 each of three sheet cracks illustrated shows the same sequence of events.
Figure 17--Zebra rock; peel, negative print X 3. Primary lime mud (m1) of dark-gray color, in itself somewhat complex, is cut by three parallel sheet cracks, each of which shows same sequence of internal sediments: thin spar-crust (s1), overlain by flesh-colored calcilutite (m2), succeeded by thin-spar crust (s2), followed by light-red calcilutite (m3), followed by third layer of spar (s3) which meets spar crust which grew downward from roof. Irregular and intraclastic upper part of m1 under each sheet crack probably represents material caved from roof of sheet crack before deposition of s1. Talus block, Schwerwand, Tennengebirge; it is uncertain whether this is p art of bedded sequence or out of cavity fill or neptunian dike.
Figure 18--Simplified drawing of Figure 17; spar shown in black.
Figure 19--Intertidal sediments: Algal mats. A, peel, negative print X 3, of specimen showing presumed algal mats lacking internal structure; note disruption of sediment along minute prism cracks and sheet cracks. Göll-Brett complex (road to Kehlstein tea house). B, peel, negative print X 3, showing crinkled and vesicular algal crusts, with cavities filled by m2 (white) and spar (black). Some cavities appear to owe their origin to primary discordances in layering of algal crusts-upper crust having bridged depressions in lower one. Base of cycle 1, Loferer Steinberge section. C, thin section, negative print X 3, of dolomitic vesicular algal mat rock from top of Mt. Schlern in Dolomites; details illustrated in Figure 16A.
Figure 20--Intertidal sediment: various loferites. A, thin section, negative print X 3, of pellet loferite with small shrinkage pores, passing upward into more homogeneous mud with larger shrinkage pores which have lower parts filled with m2 (light gray) and upper parts with spar (black). Below loferites lies sheet crack with filling of m2 (light gray) containing intraclasts (white). Above this mud lies spar (black) which extends into loferite in an embayment which may represent cross section of prism crack. Within sheet-crack m2 lies secondary sheet crack, filled with spar. B, thin section, negative print X 7. Calcilutite with dolomitic intraclasts passes upward into vesicular dolomite, containing large and small shrinkage pores. Lower parts are filled with mud (medium gray), upper parts with spar (black). Cycle 1, Dachstein section. C, thin section, negative print, X 3, of pellet loferite in which shrinkage pores grade into microsheet cracks. m2 dark gray, spar black. Cycle 14 of Steinernes Meer section. D, peel, negative print X 3, of loferite showing gradation of shrinkage pores into microsheet cracks; m2 in pores (dark gray) grades upward into spar. Cycle 4, Steinernes Meer section.
Figure 21--Filled cavities in subtidal megalodont limestone ("member C"). A, a sheet crack, filled with thin spar lining and argillaceous, brown and red limestone. Note rounded roof pendants and free rounded block at right. Offset by small vertical faults. Length of picture about 1.5 meters. B, close-up of smaller sheet crack or solution cavity, showing lamination of lime. mud filling, and "inverted unconformity" where laminae meet irregular roof. C, contact of megalodont bed ("member C") with overlying cycle ("member B," massive at base, grading upward into speckled and laminated loferite). Hammer stands on contact. "Member A" of upper cycle is represented only in cavity fills of red (black) and brown (dotted) limestone, in megalodont limestone. Cavities include discontinuous sheet crack just below unconformity, and larger solution cavities below. Dachstein section. D, peel, negative print, X 3, of typical red cavity fill. Base of cavity not shown, top at upper left. Fill consists of alternating beds of mechanically introduced lime mud and chemically precipitated radiaxial calcite. Mud laminae contain many mud pellets (fecal pellets?) which are concentrated at top, giving each lamina an inversely graded character. Transition to succeeding spar crust is gradual, highest pellets of each lamina floating in spar matrix. After deposition of fifth lamina shown, laminae increased in thickness and assumed different orientation. Roof was lined with radiaxial calcite only; parts of lining dropped out from time to time--one piece is buried at base of fifth main mud lamina, and other pieces are floating in last mud layer. Large black spots are air bubbles in peel. Tennengebirge.
Shrinkage pores--Much of the fine-grained carbonate rock here placed in the intertidal member of the Lofer cycle is riddled by millimeter-scale pores of highly irregular shape (Fig. 13B, 16D, 16E, 19A, 20). They are not intergranular pores, being larger than the particles of the sediment, though they grade into intergranular pores in intraformational conglomerates (Fig. 22). They are generally too irregular in shape and size to warrant interpretation as burrows, though in individual cases the differentiation from burrows is not always possible. They grade into microscopic sheet cracks (Fig. 19A, 20C, 20D) and into small prism cracks (Fig. 14A, 14B). They commonly give the impression of having been formed by a pulling-apart of their walls (Fig. 16E). In pellet muds the shrinkage pores tend to be small and closely spaced; in more homogeneous lutites they are larger and farther apart. The gradation from the former to the latter may be seen in Figures 13B and 20A. The sum total of observations suggests that these pores owe their origin to shrinkage of sediment away from weak spots-intergranular pores, burrows etc.
Figure 22--Intertidal sediments: Homogeneous facies and intraformational conglomerates. A, thin section, negative print X 7. Dolomitic lutite containing small gastropods is cut obliquely by an irregularly bounded band of calcilutite (gray) containing dololutite intraclasts. Calcilutite is probably filling of an oblique prism crack or sheet crack. Note horizontal mud-levels of dololutite in gastropods. After partial filling of shells, aragonitic shell matter was leached away; then calcite mud was introduced, forming partial cast replicas of shells (gray, at base of shells). Finally spar (black) filled remaining voids. Steinernes Meer section, cycle 9. B, peel, negative print, X 3. Cheesy dolomite (white) at base of picture is probably in place. It is overlain by calcilutite (gray, speckled) with abundant dolomite intraclasts. Abundant, large pores filled with combination of calcilutite m2 (smooth gray) in bottoms and calcite spar of two generations: an early radiaxial s1 (dark gray), and later s2 (black). Drawing of specimen is shown in Figure 25B. Pass Lueg, Tennengebirge. C, acetic acid peel, negative print, X 1. Highly dolomitic series of laminae (lighter bands and islands are partly calcite), succeeded by doloInitic calcilutite with angular intraclasts of dolomitic laminae. Contact is discordant, dolomite having been eroded more deeply on right than on left. Göll-Brett complex, road to Kehlstein tea house, at base of "member B."
These pores commonly show a partial filling by bottom mud, followed by precipitation of spar. The spar commonly shows two generations-an early lining of somewhat cloudy radiaxial spar (s1) and a later filling of the remaining void by clear, coarse carbonate (s2) (Fig 19B). In the Lofer facies the internal fillings are almost universally of calcite, even though the surrounding sediment is generally partly (in some cases wholly) dolomitic. Sander has described some pores in which radiaxial dolomite formed on the floor, prior to introduction of bottom mud. In the wholly dolomitic rocks of the Dolomites the shrinkage pores show precisely the same appearance, but the bottom muds and the succeeding sparry deposits are composed of dolomite (Fig. 19C).
On a weathered surface the sparry carbonate spots which represent chemical fillings of these cavities tend to weather out conspicuously; in freshly broken rock they show up as glittering flecks. In American literature such sparry blebs have been referred to as birdseyes, and such rocks as birdseye limestones. I have rejected the term for two reasons: (I) it is uncertain how many of the birdseye limestones owe their character to the same type of porosity (rather than to sparry fillings of fossils, intergranular porosity etc.) ; and (2) the term birdseye can only refer to patches of glittery spar; yet the pores are commonly partly, and occasionally wholly, filled with internal mud (m2), or may be open. Tebbutt, Conley, and Boyd (1965) have made a detailed study of American Permian loferites and have coined the term fenestra for my "shrinkage pore." Perhaps this more objective term of theirs will be found preferable.
Origin of shrinkage phenomena--The question of whether lime muds can contract and acquire shrinkage structures under water appears to be unsettled. Whether they can or not--the combined occurrence of prism cracks, sheet cracks, shrinkage pores, and algal-mat sediments, above the unconformity in this cyclic squence, leaves little doubt that we are dealing here with littoral sediments which shrank as a result of desiccation. The largescale prism cracks and sheet cracks which occur below the unconformity, in the massive subtidal limestone member, are attributed to desiccation of this member during low stands of sea level. Subaerial versus subaqueous origin of the zebra sheet cracking in neptunian dikes remains an open question.
The Lofer cyclothem is developed in its simplest form and in predominantly gray, white, and greenish colors in the Loferer Steinberge section (Fig. 3). In the Steinernes Meer (Fig. 4) and Dachstein (Fig. 6) sections it assumes somewhat greater complexity and partial red coloration. There is, however, enough similarity between all of these sections to permit joint treatment.
The base of the cyclothem is normally marked by a disconformity. The massive megalodont-bearing limestone which normally forms the main member and end member of the Lofer cyclothem generally shows one or more of the following evidences of exposure, desiccation and erosion: (1) Large-scale prism cracks (Fig. 9), apparently modified by erosion; along these the basal deposits of the succeeding cycle ("member A") commonly penetrate some decimeters, but locally some meters into the underlying limestone. (2) An irregular surface which truncates structures in the underlying limestone (Fig. 23). (3) Large-scale sheet cracks (Fig. 21A, 23), filled by the succeeding "member A." (4) Cavities of irregular shape, commonly roughly aligned parallel to bedding, which are here attributed to solution along temporary water tables (Fig. 21C), and which are likewise filled largely by the succeeding "member A;" these may occur some meters below the disconformity. (5) Presence of a basal conglomerate of limestone cobbles above the disconformity. (6) Partial leaching of shells, the molds thus formed being filled by "member A" (Fig. 9B).
Figure 23--Disconformities. Field sketches, Mt. Dachstein. Left: Megalodont limestone (C1) became emergent, and developed a sheet crack and prism cracks. Spar lined prism cracks and roof of sheet crack, while red argillaceous lime mud (A2) accumulated on fioor of sheet crack. Then erosion truncated prism cracks and spar filling. Finally, disconformity was covered by intertidal sediments (B2), consisting of a basal pink doloInitic calcilutite, succeeded by banded, speckled loferite. Right: Megalodont limestone (C1) became emergent, and solution cavities developed in it. These became filled with red argillaceous lime mud (A2). Cobbles of limestone developed on disconformity, and became enclosed in intertidal deposits (B2), a basal pink dolomitic lutite, grading up into banded loferite. This is abruptly terminated by a mudcracked surface, upon which the succeeding megalodont limestone (C2) comes to rest in disconformable relationship, and with an intraclastic base.
These features are particularly well shown where the succeeding "member A" is red. The red fills of solution cavities and sheet cracks are particularly conspicuous in the Steinernes Meer and the Dachstein sections. Some of them have long been known in the German literature as "schwimmende Scherben" (floating potsherds), an apt descriptive name in view of their angular outlines, though a misleading one inasmuch as it suggests clasts rather than an internal sediment.
The general nature of the various members of the Lofer cyclothem is shown in Table 1. Most of the cycles have a thin basal unit of shale or argillaceous limestone which is here termed member A. In many instances it is merely a shaly bedding plane; in many others it is entirely restricted to cavity fillings (see above) in the underlying limestone ("schwimmende Scherben" and long sheet-crack fills). In still other places it forms the matrix of a basal conglomerate. It also forms most of the neptunian dikes.Table 1--Distribution of rock types, biotas, and structures in members of Lofer cyclothem.
|Homogenous carbonate lutites||x||x||x||x|
|Laminated carbonate lutites||x|
|Biota||Filamentous algae in mats and crusts||xx|
|Filamentous algae in oncoids||x|
|Rhodophytes, dasycladaceans, codiaceans||xx|
Most commonly, member A consists of greenish-gray, ochreous-brown, or brick-red, argillaceous limestone. Greenish to yellowish colors predominate in the Loferer Steinberge, reds and browns in the Steinernes Meer and Dachstein. Much of this calcilutite is conspicuously pelleted, and occurs in distinct beds which show an inverse grading (Fig. 21D): from a finely silty lime mud at the base they grade upward into a mud speckled with small pellets, and upward into a sediment of pellets only, cemented by calcite spar; this in turn is succeeded by a thin crust of pure spar (in the lower part of which there occur isolated "floating" pellets). This crust shows a sharp upper boundary, upon which lies another bed of mud, similarly graded. Some of these cavity fills also show a kind of crossbedding. While mud and spar deposition alternated on the floor, the roof acquired a lining of pure radiaxial spar. Figure 21D shows a case in which a part of this roof lining dropped out and came to be lodged in the floor deposits.
The pellets may be mainly of fossil origin (fecal pellets). Some of them are highly ferruginous. Several types of problematical microfossils as well as ostracode shells are present. Bedded red-pellet limestones of some neptunian dikes were burrowed by some organism. Other neptunian dikes are filled with brachiopods, packed in a matrix of red limestone which is identical with the red limestones of member A (Fig. 24). This special matter will be discussed under neptunian dikes.
Figure 24--Brachiopods in neptunian dikes. A, neptunian dike of deep red pellet calcilutite with Halorelia pedata; peel, negative print X 1, Tennengebirge (Scheiblingkogel). B, same, X 3, reversed. Shell material appears to have been largely removed in solution after introduction of internal mud sediment, and before growth of spar (dark). C, brachiopods fontring sedimentary framework in neptunian dike, voids filled by intermittent introduction of spar and of red mud. Thin section, X 3, negative print. Steinernes Meer.
Leuchs and Udluft (1926) and Leuchs (1928) investigated these red limestones. They recognized that some represent cavity fills, but interpreted the sheet-crack fillings as surficial beds. This forced Leuchs (1928) to consider the radiaxial calcite spar linings of cavities as calcite crusts chemically deposited on the sea floor. Working without stratigraphic control, they did not recognize the cyclic nature of the Lofer facies as a whole, and did not relate the red sediments to the disconformity at the base of the cycle. They assumed derivation of the red sediment from adjacent land areas, in occasional catastrophic influxes which killed the carbonate precipitating organisms (Leuchs interpreted the entire Dachstein Formation as a reef deposit). In order to explain angular blocks of gray limestone in the red matrix, Leuchs assumed simultaneous diastrophic movements (a cessation of subsidence, or an episode of uplift).
With Leuchs and Udluft, I interpret the red sediments of member A (as well as the greenish ones of the Loferer Steinberge) as a modified and redeposited soil. In contrast to these authors I envision a local origin of this soil--mainly by solution along the disconformity at the base of the cyclothem. That most of the red material forms internal sediment in cavities, sheet cracks and neptunian dikes is clear from the field evidence. The high carbonate content, presence of fossils, and pelleted nature of much of the red sediment necessitate the assumption that most of the weathering mantle produced on the disconformity was mixed with carbonate sediment, and was eaten and pelleted by organisms, during the return of the sea, and prior to trapping in the cavities below the disconformity.
Lutites lacking shrinkage pores--Homogeneous carbonate lutites, ranging from pure calcite to dolomite, are commonly found above member A or above the disconformity, and commonly occur interbedded with the loferites described below. They are more conspicuous in the Steinernes Meer and Dachstein sections than in the Loferer Steinberge, where the loferites predominate in member B. They vary in color from gray to brown, pink and red. Commonly they show a red and black speckling. Some of these rocks appear unfossiliferous, whereas others contain an abundance of microscopic fossils, belonging to a very few species. Most conspicuous among these are tiny snails (Fig. 22A), associated with one or two species of Foraminifera. Ostracodes and fragments of very small pelecypod shells occur more rarely.
The occurrence of beds or lenses of the "Hallstatt facies" is frequently reported in the "Dachstein limestone." In part, such reports refer to bona fide Hallstatt starved-basin sediments with their characteristic faunas, inter-fingered with tongues of reef talus in the foreslope of the barrier reefs. In part, they refer to the reddish, conchoidally fracturing calcilutites described here, in members A and B of the Lofer cyclothem. Leuchs (1928) and Sander (1936) assumed such a facies identity. The association and paleogeographic position of these sediments suggest from the beginning that this lithic resemblance does not express identical settings of origin. Insoluble residues have borne out the difference. Whereas acetic acid residues of the Hallstatt rocks consistently yield conodonts, and dolomitized echinoderm remains (a discovery made by Dr. H. Zankl, communicated to me, and confirmed by myself); twelve residues of the reddish calcilutites in member B of the Lofer cycle have only yielded dolomitized ostracodes, or no fossils at all.
Laminated calcilutites of dark-brown color, containing ostracodes and small pelecypods, and laminated dololutities form less common variants of the homogeneous lutites.
Laminated as well as homogeneous lutites of this type are frequently found reworked into intraformational conglomerates. Figure 22A shows a snail-bearing homogeneous dololutite reworked into such a conglomerate with calcitic matrix, which in this case appears to form a large mudcrack filling, and is in itself loferitic, showing shrinkage pores (black). Figure 22C shows a laminated dololutite reworked into a flatpebble conglomerate. These examples are proof of very early lithification and dolomitization.
Loferites--The term loferite is here suggested for limestones or dolomites which are riddled by shrinkage pores. The term is thus partly synonymous with birdseye limestone. However, it is likely that not all birdseye limestones contain shrinkage pores; neither need loferites have "birdseyes," i. e. small blebs of sparry calcite. Shrinkage pores and related cavities generally form from 15 to 30 percent of rock volume.
The shrinkage pores tend to be aligned along the bedding, and are commonly confluent, to produce irregular microscopic sheet cracks (Fig. 20C, 20D). They are commonly filled partly with m2 and partly with spar. (Fig. 16D, 16E, 20). Shrinkage pores wholly filled with spar are common, but complete fillings by mud are rare. The muds range from granular and silty ones to exceedingly fine ones showing a waxy translucence in thin section; such very fine muds tend to grade into the overlying spar, suggesting that they were still very watery in their upper part when spar growth commenced. Commonly the m2 is somewhat graded (Fig. 20B).
In the Lofer facies the framework of the loferites (m1) is generally partly to largely dolomitic, whereas the m2 and the spar are calcite. Sander (1936) has described cases in which radiaxial dolomite crusts formed on the floor of such cavities prior to the entrance of m2. In the Hauptdolomit of the Southern Limestone Alps (Dolomites) the m2 and succeeding spar are composed of dolomite. They are in no way blurred or obliterated by recrystallization, the m2 showing in part the same waxy translucence and exceedingly fine grain found in the calcitic ones. With Sander, one can only assume that this represents essentially primary dolomite.
The loferites here considered are of four main types, and grade into each other: algal mat, pellet, homogeneous, and conglomeratic.
Algal-mat Loferites--These correspond essentially to Sander's organogenic crusts. The individual laminations are generally microvesicular dolomitic crusts (Fig. 14, 16B, 19C), flat or wrinkled, which are separated by irregular sparry blisters (in part perhaps shrinkage pores, gas blisters, animal burrows, or simply watery voids formed under the tough algal mat) which tend to merge into irregular sheet cracks. In some of them the crusts are also separated by thin laminae of dololutite (Fig. 16A, 16C). The vesicles in the dolomite crusts are presumably the carbonate-encrusted molds of the algae and perhaps other organisms which formed the mat; the scarcity of elongate threads in the sections suggests that they were mostly rounded cells or cell aggregates, rather than filamentous. The thin lutite layers, on the other hand, show well-defined threads by which the filamentous members of the colony evidently grew toward the surface, after having been covered (Fig. 16A, 16C). The algal mats are abundantly but not universally prism cracked (Fig. 11, 12, 13A, 14A, 14B) . These prism cracks were seen by Schwarzacher, who described them in his unpublished thesis (University of Innsbruck) and noted their resemblance to desiccation mudcracks, but hesitated to attribute them to subaerial desiccation. They seem not to have been recorded in the literature.
Although no thin sections of modern algal mats appear to have been published, the studies of Black (1933), Ginsburg (1957, 1960), and Ginsburg and others (1954) leave little doubt that these fossil sediments are comparable to the algal mats growing today in intertidal carbonate areas.
Pellet Loferites--Pellet loferites consist largely of pellets of microscopically unresolvable carbonate (calcite, dolomite, or both), ranging in size from tens of microns to one mm. Their outlines tend to merge where in contact, and the rock can thus grade into a homogeneous calcilutite. The pellets may be partly of fecal origin. Kornicker and Purdy (1957) described coarser gastropod-pellet sediments from the intertidal zone of the Bahamas. On the other hand, the possibility that the pellets represent in part carbonate accretions around small, rounded algal colonies cannot be ruled out. The rock may be comparatively homogeneous, or may show a millimeter lamination due to changes in pellet size. The shrinkage pores in these pellet rocks tend to be small and abundant, and are embayed around individual pellets (Fig. 16D, 20). Rocks of this type, though of coarser grain, have been described from the Permian of North America by Boyd (1958a, 1958b), and attributed to algal-mat origin.
Homogeneous Loferites--Beds of pellet loferite commonly lose their structure upward, to pass into homogeneous lutites; in these the shrinkage pores tend to be larger and farther apart (Fig. 13B, 20A).
Loferite Conglomerates--They contain mainly dolomitic intraclasts in a generally more calcitic, lutite matrix which shows pores resembling shrinkage pores. Such a rock in which dolomitic intraclasts are embedded in a calcitic matrix is illustrated in Figures 22B and 25. Many of the pores here lie under intraclasts, suggesting that the porosity pattern arose in part from the settling of a watery matrix within a somewhat self-supporting framework of intraclasts. Desiccation shrinkage may be only a minor, modifying feature in the pattern shown here. In the specimen shown in Figure 20B, the conglomeratic nature is clear in the lower part, but whether the irregular cavities in the upper part represent spaces between larger intraclasts or shrinkage features in a lutite is not clear. Intraclasts also occur in normal pellet loferites (Fig. 20C). Some rocks, such as illustrated in Figure 19A, show laminations of semiconsolidated, curdy sediment, disrupted by shrinkage and evidently somewhat moved before burial; they are on the borderline of intraformational conglomerates.
Figure 25--Dolomite riddled with shrinkage pores and cut by sheet crack passes upward into intraformational conglomerate of dolomite intraclasts in calcilutite matrix (stippled lightly). This contains many larger pores, which may represent combination of (1) settling of watery mud matrix between and beneath intraclasts which are abundant enough to form framework, and (2) shrinkage porosity induced by desiccation. Internal sediments in cavities shown by stippling (m2) and by solid black (spar). Whole is cut obliquely by branched vein. Specimen also illustrated in Figure 22B. Tennengebirge (Pass Lueg).
Biota of Loferites--The loferite biota is very restricted. The algal mats contain vesicles and filaments (Fig. 16A, 16B, 16C) which are taken to be encrustations of algal cells and colonies of various shapes and dimensions. These are best preserved in the wholly dolomitic sequences of the southern Alps, but similar features are found in the type loferites. Foraminifera and ostracodes are distinctly rare. Larger spar-filled tubules, with diameters of 0.5 mm to 2 mm, bifurcating and oriented in various directions, probably represent the burrows of worms or other small invertebrates.
Observations and Interpretations on Dolomite--Algal mats are dolomitic throughout; pellet lutites and homogeneous lutites are generally calcitic near the reef, and increasingly dolomitic into the back-reef area. The reworking of dolomitic rocks into dolomitic intraclasts in calcitic matrix leaves little doubt that dolomite was primary or resulted from replacement which occurred soon after deposition, essentially at the surface.
Early diagenetic dolomitization of lime muds is shown by dolomitic halos around some shrinkage pores and around prism cracks (Fig. 12B). This is the belteroporic dolomitization of Sander.
Precipitation of dolomite in cavities during diagenesis was shown by Sander (1936).
Genetic Setting of Member B--The algal mats are indicative of shallow-water conditions, and strongly suggest an intertidal origin (Ginsburg 1957,1960). The shrinkage phenomena in these and associated rocks (prism cracks, sheet cracks, and shrinkage pores) show that these sediments were not dewatered by burial and compaction under a gradually increasing overburden, but at the surface, by desiccation. Thus an intertidal origin seems established for the loferites, and by association, for all of member B. The term intertidal is here used not in the sense of daily tides, but for the bottoms exposed and covered by the tidal extremes during the year.
Within the Lofer facies of the Dachstein Limestone, dolomite is largely restricted to the intertidal sediments of member B, and is not present in appreciable quantities in the subtidal member C., described below. Thus, the early origin of the dolomite, restricted biota, and presence of extensive algal mats, all combine to suggest a setting not unlike that of tidal flats around the Persian Gulf (Wells 1962; Curtis, Evans, Kinsman, and Shearman 1963), known as sabkhas.
The main and terminal member of the Lofer cyclothem is the megalodont limestone, member C, ranging in thickness from 1 to 20 meters (Fig. 26). It ranges from well-winnowed, sparry calcarenites (Fig. 27D, 28B) through calcarenites with a muddy carbonate matrix (Fig. 28A) to featureless calcilutites. The calcarenites consist of fossil fragments, featureless pellets, and compound lumps. Ooidal overgrowths are common, as are oncoids up to several centimeters in diameter (Fig. 27A, 27B, 27C).
Figure 26--Frequency distribution of thickness of intertidal members (cross hatched), and subtidal members (plain) in the three sections studied.
Figure 27--Subtidal deposits; calcarenites and oncoids. A, thin section, negative print X 3, showing sparry calcarenite containing irregular oncoids with little concentric structure but abundant vesicles. Calcarenitic particles include Foraminifera, echinoderm fragments (dark), and molluscan debris, and commonly show thin oolitic overgrowths. Tennengebirge (Wieselstein). B, thin section, negative print X 7, of oncoids--one with sphinctozoan sponge as core, one with codiacean alga. Core surrounded by fine-grained, but somewhat vesicular oncoidal overgrowth; outer rim contains abundant encrusting foraminifera (shown in print as irregular white-walled vesicles). Göll-Brett complex (road to Kehlstein tea house). C, thin section, negative print X 3, of a vesicular oncolite; small dark spots may represent molds of algal filaments, origin of larger vesicles unknown. D, thin section, negative print X 7, of calcarenite with sparry matrix. Among particles are Foraminifera, fragments of dasycladacean algae, pieces of corals (upper right), snails, pelecypod fragments, echinoderm debris. Most grains show oolitic overgrowths. Grimming Range, at Mittemdorf (east of area shown in Fig. 1).
Figure 28--Subtidal sediments; fossiliferous calcarenites. Thin sections, negative prints, X 7. A, calcarenite with muddy matrix, containing molluscan fragments (clear spar, shown black), echinoderm debris (medium to dark gray), vesicular oncolitic bodies, and an admixture of minor constituents ranging from codiacean algae to Foraminifera, bryozoan, and brachiopod chips. Göll-Brett complex; road to Kehlstein tea house. B, rock dominated by dasycladacean algae, but containing also echinoderm debris (note ooidally overgrown grain at upper left) and mollusks (pleurotomarian snail); note composite ooid at bottom. From Grimming Range, near Mitterndorf (east of area shown in Fig. 1).
The biota is varied. The oncoids show a vesicular structure and irregular overlapping growth zones; in places they reveal a dense network of filaments, indicating blue green algae as the most likely contributor. Like oncoids in the late Paleozoic of North America some of these show an association with encrusting Foraminifera (Fig. 27B). Coarser borings in them record the action of algae or small invertebrates. The occurrence of algal mats in tidal flats and oncoids in subtidal sediments in the Recent setting has been described by Ginsburg (1960).
Other algae are represented by rhodophytes? (solenoporids), codiaceans, and dasycladaceans (Fig. 28B). The invertebrates include sponges, corals, bryozoans, brachiopods, gastropods, pelecypods, rare ammonites, ostracodes, and echinoderms. The biota is richest near the reef (and may include much which is carried in from the reefs), and becomes progressively impoverished toward the dolomitic ultra-back-reef facies of the Hauptdolomit. Corals, for example, are rare in the Lofer facies, and calcareous sponges are probably only carried in as detritus. Snails locally form dense accumulations (Fig. 29), and may reach large size. But the most conspicuous fossils in member C are the megalodont pelecypods, which occur in almost every bed.
Figure 29--Accumulation of gastropods (Zygopleura variabilis) in subtidal member; Tennengebirge (after Zapfe, 1962b).
Figure 30--Colony of megalodonts in growth position as seen on bedding parallel surface; from photograph by author, Tennengebirge (Wieselstein). Probably mainly Conchodus infraliasicus; very large, thick shells may belong to Dicerocardium.
The megalodonts are pachyodont clams, presumably ancestral to the Jurassic Diceras and to the chamids and rudistids. They include the genera Megalodus and Conchodus (Zapfe, 1957), as well as a variety of other forms, one of which may be Dicerocardium Stoppani, whereas the others are undescribed. These shells are all relatively very large (ranging in diameter from 10 to 40 cm), and include thin-shelled as well as thick-shelled species.
The megalodonts are occasionally found in shell-bed concentrates of scattered valves, but more commonly they occur as bivalved specimens in growth position (Fig. 30, 31, 32, 33). Some such specimens are restricted to definite clam horizons only one individual thick, representing perhaps a single generation only; elsewhere they occur scattered through much of member C.
Figure 31--Colony of megalodont pelecypods (Conchodus infraliasicus) in growth position, in bedding plane parallel polished slab of Vienna Natural Hisstory Museum; from Tennengebirge (after Zapfe, 1957).
Figure 32--Megalodont pelecypods in life position (in vertical section); field sketch, Dachstein.
Figure 33--Giant megalodonts in vertical section; from photograph by author, Dachstein section.
According to Zapfe (1957) Megalodus and Conchodus lived partly buried in the sediment, with beaks down and hinge essentially vertical. The larger, more exotic forms are seen so frequently with their valves open toward the top that I am inclined to believe that they, like the Recent Tridacna, gained or supplemented their food by "farming" symbiotic algae in their mantle tissues.
The megalodont limestone may be in transitional contact with the underlying loferites of member B, or may succeed them abruptly and even disconformably, with an erosional relief of 50 cm, and a basal conglomerate of loferite chips (Fig. 23).
The megalodont limestone evidently represents deposition below low-tide line, in a vast lagoon, not more than a few meters deep, in which normal salinities existed along the reef, but abnormal ones progressively restricted the fauna in the back-reef direction. Agitation was sufficient to stir the arenitic and oncoidal bottom, but not enough to cause widespread shifting of sediment and resedimentation of the large clams.
In many places the beds of the cyclothems are cut by dikes of limestone, which vary in width from the order of a centimeter up to a meter or more, and which may be traced laterally and vertically for meters or tens of meters. The dikes normally consist mainly of mechanically introduced lime mud or sand, but many also contain chemically precipitated radiaxial calcite, and some branch out into veins filled only by such precipitates. Because these dikes obviously represent submarine fissures filled by sediment, I have here applied the British term neptunian dikes to them. In the uppermost part of the Dachstein Formation most of the dikes are of early Jurassic age, consisting of red encrinites and of pink- to light-red ammonite-bearing calcilutites. They appear to represent the earliest sediments of the mid-Liassic transgression over a karst surface with deeper fissures.
In the middle parts of the formation, in which most of my observations were made, no such Jurassic dikes were recognized. The dikes are filled here with homogeneous to pelleted lime mud, which ranges from mauve colors unlike any seen in the bedded sediment to brick-red and ochreous-brown limestone identical with that of member A of the Lofer cycle, found in cavities below the unconformity. The Triassic age of some of these dikes is further documented by the mass occurrence, in some of them, of Triassic rhynchonellid brachiopods such as Halorella and Halorelloides.
Origin--The dikes have not been systematically studied, and many important observations are lacking. The upper termination, depth of penetration, and lateral extent of any one dike are not known. Gray dikes may be much more common than appears at first sight, since the red ones are so much more conspicuous. No studies of orientation have been made. Three modes of origin appear possible: (1) tectonic fracturing, (2) mudcracking on a very large scale, and (3) solution along joints during periods of emergence.
Some dikes appear to be due to tectonic fracturing; at the Eisriesenwelt caves, above Werfen (Tennengebirge), for example, dikes show stratigraphic offsets on the order of 40 cm, and send off irregular branch veins which disrupt the massive limestone into an angular breccia. These dikes also show brecciation of their internal deposits by repeated movement.
The other possibilities envision an origin of the cracks during the interval of emergence, which is in accord with the prevalence of fillings resembling member A of the cyclothem. Some of the Jurassic dikes, such as a prominent one on the trail between Riemann house and Ingolstätter house in the Steinernes Meer, show evidence of solution fluting on their walls, and are thus clearly modified by erosion. Nothing of this sort has been observed in the Triassic dikes examined to date. The other possibility, that some of the dikes are due to large-scale mudcracking, has been discussed above, under the heading of shrinkage structures.
Zebra dikes--In the Dachstein range (Krippenstein) and in the Tennengebirge (especially at the Eisriesenwelt ice caves above Werfen), neptunian dikes of mauve-colored calcilutite show extensive sheet-crack development, producing zebra limestone. The big zebra dike at the Eisriesenwelt (Fig. 15, 16F) has the sheet cracks developed normal to the dike walls, and essentially parallel to the bedding of the country rock. A dike at the Krippenstein shows sheet cracking parallel to the dike walls. In general, the sheet cracks are simply filled by radiaxial calcite, grown inward from both walls, but some of the cracks in the Eisriesenwelt dike contain a last, internal filling of pinkish calcilutite (not illustrated).
Clearly this sheet cracking represents a form of dewatering of the lime muds; whether this occurred under water or during one of the emergent intervals of the eustatic cycle remains to be determined. The zebra structure has been found only in gray to mauve dikes, not in red ones (which are otherwise closely associated).
Brachiopod dikes--At widely separated localities (Dachstein Range, Tennengebirge, Steinernes Meer) the Dachstein Limestone contains red dikes replete with rhynchonellid brachiopods. Such brachiopod accumulations are not known from the normal, bedded limestones of the Lofer facies, but may also occur in the red-cavity fills of member A. A dike generally carries only one species of brachiopod; thus, Leuchs (1928) reported an accumulation of Rhynchonella juvavica in a cavity fill (or neptunian dike?) of the Breithorn (Steinernes Meer), and the writer has seen a dike of small rhynchonellids in the Steinernes Meer on the trail from Riemannhaus to Kärlinger Haus, an accumulation of Halorella in a red dike near the Scheiblingskogel (Tennengebirge; Fig. 24A, 24B), and an accumulation of Halorelloides in a similar dike only a few meters distant.
Walther (1885) observed such accumulations in the Dachstein Range, and wrote (author's transl.): "Almost all of these animals are oriented in parallel in the fashion in which living brachiopods are attached by their pedicles, and the first glance tells us that they lived in a cavity and were covered and buried by an inflow of mud." This observation may apply to the specimen illustrated in Figures 15A and 15B. Figure 15C shows another type of accumulation, in which the brachiopod shells evidently lodged in the bottom of a fissure and were then buried by an alternation of radiaxial calcite spar precipitation and the influx of red lime mud.
It thus appears that during the transgression of the seas, in the earliest stage of the Lofer cyclothem, some of the fissures destined to become neptunian dikes were open to the sea, and that brachiopods grew upon the walls. Possibly these fissures served as tidal channels. Some shells of the dead brachiopods accumulated at the bottom of the fissures and were there buried, whereas other live brachiopods were buried and preserved in situ.
Some remaining problems--It seems odd that no sediments resembling the calcarenites of the megalodont beds have as yet been found in the dikes, and that nothing resembling member B--no intraclasts of algal loferites, in fact, no dolomite of any kind--has been encountered.
In summary, the Lofer cyclothems appear to record the following sequence of events:
(1) Retreat of the sea, exposing a lagoonal floor some tens of kilometers wide to desiccation and weathering in a fairly arid environment. Prism cracks on a half-meter scale were formed in places, and giant prism cracks may have opened deep fissures which later became neptunian dikes. Sheet cracks up to several meters long developed in the sediment as drying progressed to deeper levels. Aragonite shells were partly to completely leached away, leaving molds. Aridity and perhaps the porous and only semiconsolidated nature of the sediment prevented the development of a karst topography, but solution occurred at the surface and brought about the accumulation of a weathering mantle, high in ferric iron. At the same time, solution near the water table produced irregular cavities, and solution may have enlarged sheet cracks as well.
(2) Return of the sea brought first a washing away of the weathering mantle, which was partly trapped in the solution cavities and open fissures to produce red- and brown-cavity fills and neptunian dikes. Brachiopods occupied the walls of certain of these fissures which acted as tidal channels and maintained near-normal salinities.
(3) A period of intertidal regimen resulted in the growth of extensive algal mats, alternating with areas of muds inhabited by gastropods. Normal tidal range may have been extended by special seasonal factors such as monsoonal seiches. Periodic desiccation of the sediment produced small-scale mudcracks, sheet cracks, and shrinkage pores. Dolomite was formed, in part possibly as a primary sediment, in part as a penecontemporaneous replacement of aragonite or calcite, near the surface. Internal sedimentation, in part mechanical, in part chemical, occurred within the sediment.
(4) Continued rise of sea level brought the area under continuous cover by the sea--i. e., into the shallow neritic realm. A diversified biota now lived near the reefs, but decreased in back-reef direction in response to deviation from normal salinity. A most characteristic element of the fauna were the megalodont pelecypods, belonging to a number of genera.
(5) Eustatic drop in sea level brought a brief period of intertidal conditions, and a return to exposure and erosion, thus starting a new cycle.
Of the three rhythms expressed in the Lofer facies, I have thus far described two-the millimeter lamination of the loferites, and the Lofer cyclothem. The third rhythm, a megacyclic grouping of cyclothems, emerges from further analysis, and will be discussed below.
The length of the Triassic Period has been estimated at 45 million years by Holmes (1960), and at 49 million years by Kulp (1961). If the six stages of the Triassic were of equal duration, then the Dachstein Formation, representing the Norian and Rhaetian Stages, was deposited over a time span on the order of 15 million years. The thickness of the formation ranges from 1,000 to 1,500 meters; hence, we may conclude that the mean overall rate of sedimentation, and the mean overall rate of tectonic subsidence, were on the order of 70 to 100 meters per million years, or 0.07-0.10 mm per year. Since the overall average includes times of nondeposition and erosion, and various types of limestones which were formed at varying rates, it is clear that some of the individual limestone beds were deposited at rates very much greater than this overall average.
The algal-mat sediments and the pellet loferites exhibit a millimeter lamination, due in part to variations in texture, in part to the alignment of shrinkage pores and microsheet cracks. Schwarzacher (1948) measured 611 laminae, and arrived at the frequency distribution of lamina thickness illustrated in Figure 34. The mean thickness is 1.3 mm, the modal thickness 1.1 mm. Sander had suggested that this lamination represents an annual varving, and Schwarzacher came to the same conclusion. Rates of algal-mat growth reported by Ginsburg and others (1954) from southern Florida are of the same order of magnitude.
Figure 34--Frequency distribution of thickness of laminations in loferites, from Loferer Steinberge (after Schwarzacher, 1948).
It thus seems reasonable to consider the millimeter laminations as annual deposits, formed at 10 to 20 times the mean overall rate of Dachstein sedimentation, in response to some seasonal factor. This may be sought in such occurrences as seasonal highs and lows of water level (as occurs in the Rann of Cutch) or to seasonal rainfall causing changes in salinity and thereby in the composition of the algal flora, or in seasonal stirring of the waters by storms and consequent supply of carbonate mud.
Length--If the Dachstein Formation was deposited over a time span of 15 million years, and if it contains around 300 cyclothems as estimated here, then the mean time span of a cyclothem is on the order of 50,000 years. This is obviously a very rough figure; it is neither close enough to the magic 21,000 year equinoxial cycle, to which Sander was inclined to attribute the cyclothems, to support it, nor far enough off this mark to refute it. It is, however, close to the 41,000 year cycle of variation in obliquity, the angle between the earth's equatorial plane and the plane of the ecliptic.
Oscillation--The cyclothems resulted from a relative variation in sea level. The amplitude of this oscillation is not clear; but because the deepest water deposits show no evidence of having been formed more than a few meters below sea level, and because cavities were formed as much as 10 meters below the unconformity in some cyclothems, it appears that the oscillation amplitude ranged from a minimum of a few meters in some cycles to a minimum of 10 to 15 meters in others.
Assuming a regular oscillation pattern, one can set up three extreme cases (Fig. 35): One in which the area was generally high and dry, but was periodically submerged for a brief interval; one in which periods of emergence and submergence of about equal duration alternated; and one in which a normally submerged condition was periodically interrupted by a brief interval of emergence. Of these alternatives the last appears the most likely; the disconformity is not a deeply eroded karst surface. In places it retains shrinkage features (prism cracks) which must have been formed at the top of the emerging sediment, proving that not much material was removed. We are thus forced to conclude that the interval of emergence was brief.
Figure 35--Possible patterns of relative sea-level oscillation. General upward trend of curves shows relative rise in sea level, which permitted accumulation of sediments; it is mainly an expression of tectonic regional subsidence. Superimposed on this general trend are cyclic oscillations of relative sea level, for which three possible patterns are shown: (A) sea level rises periodically, for brief interval, above its normal level; (B) sea level oscillates gradually between high and low points; (C) sea level drops periodically, for a brief interval, to levels below normal. C appears to fit Lofer cyclothems.
The duration of the intertidal state may be roughly calculated as follows. Algal-mat sediments accumulated at a mean rate of 1.3 mm per year. If the associated intertidal sediments accumulated at the same rate, the mean thickness of intertidal sediments per cycle (75 cm) represents a time span of 577 years. One can double or triple this figure without occupying much of the total time available per cycle. It thus appears that the normal state of the area was the subtidal one, represented by the megalodont limestones, and that the periods of emergence were brief interruptions.
Regression--In most cyclothems the regressive phase is thicker and more completely recorded by sediments than is the transgressive phase (Wells, 1956; Fischer, 1961). In the Lofer cyclothems the reverse is true; there is little record of regression. This is probably due to a combination of factors:
(1) We lack, at present, any way of recognizing the actual peak of the transgression, i. e. the precise level in the cyclothem at which the deepest submergence occurred. This lies somewhere within the megalodont limestone member. Recognition of sea-level changes in this sequence are dependent upon recognition of the disconformity and of intertidal sediments.
(2) My generalization (1961) that transgressive sequences are thinned by an internal disconformity, along which the advancing offshore high-energy zone has eroded and reworked the marginal low-energy sediments applies only to areas of loose sediment and shifting barrier beaches. In the Dachstein setting, the high-energy zone at the edge of the deeper water was held in place by the upward growth of the barrier reefs. The lagoonal area remained protected, and could accumulate a comparatively complete record of the transgression. Yet, as mentioned above, a small disconformity is locally found between the intertidal loferites and member C.
(3) Presumably intertidal sediments were formed during the regression, and in some cycles they are found. Such regressive intertidal sediments, formed during a falling sea level, were probably thinner than the equivalent ones built during the transgressive phase of the cycle. Also, they were the first sediments to be attacked by erosion during the emergent interval. Their usual absence is therefore not surprising.
Dictator--As noted above, the main movement in the area was tectonic subsidence at the mean rate of 70 to 100 meters per million years. Superimposed on this is an oscillation of relative sea level, with an amplitude of perhaps up to 15 meters, and a period of perhaps between 20,000 and 100,000 years.
This oscillation, which is responsible for the Lofer cycle, must have a cause or dictator (Sander, 1936). There are two possibilities (Fig. 36): (1) either absolute sea level remained fixed throughout, and the relative oscillation was caused by local tectonic movements--a periodic up-and-down bob on the normal subsidence pattern; or (2) the oscillation was produced by a periodic, brief fall and return of sea level. Such variation in absolute sea level could in turn be due either to far-off tectonic causes (rhythmic changes in volume of ocean basins), or to factors which periodically removed an appreciable amount of water from the sea, such as the periodic growth of a small ice cap. Although the possibility of a local tectonic dictator cannot be ruled out, I am inclined to favor a eustatic one, as being a simpler explanation. If the control was eustatic, then similar cyclothems should be observable in very shallow marine sediments of equivalent age, in other parts of the world. If it were eustatic and climatically induced, then it might be reflected in non-marine sediments as well. It is tempting to think of the cyclothems described by Van Houten (1962) from the age-equivalent, non-marine Lockatong Formation of the eastern United States as such an indirectly related cyclothem going back to the same basic cause. The 41,000 year cycle in the earth's obliquity must bring about a rhythmic variation from somewhat more seasonal to somewhat more uniform climates, and may represent such a basic cause or dictator.
Figure 36--Cyclothems dictated by tectonic or eustatic controls. A, objective relative sea-level patterns in time, as recorded in Lofer cyclothems. B, and C, different interpretations of cause. In both, tectonic subsidence is held responsible for overall trend (rise in relative sea level). Oscillations superimposed on this trend are attributed in A to periodic tectonic uplift and return, in B to eustatic oscillation; this appears the more probable answer.
Schwarzacher (1954) noted that cyclothems in the Loferer Steinberge tended to be grouped into sets of five, each such set forming a cliff separated from the next by a shelf (Fig. 37). Such megacyclic grouping is not immediately apparent from our stratigraphic sections; but when the individual cyclothems of the Steinernes Meer section are taken apart and plotted (Fig. 38), there emerges a strong suggestion of a spatial rhythm, a repeated variation pattern in the thickness of successive cyclothems. In this diagram the horizontal axis represents time, and the individual cyclothems are evenly spaced along it on the assumption that the cycle had a fixed period. The vertical axis is used to plot vertical movements. Thus, the mean subsidence vector represents the mean overall subsidence which occurred during the sedimentation of the section. In effect, it is the trace which a particle at the base of cycle 1 would have described between its origin at sea level at time 0, and its burial to a depth of 120 meters at time 20, if subsidence had proceeded at a constant rate. The individual cyclothems 1.20 are plotted at their proper stratigraphic elevation above this vector. They are then seen to fall into a wavy band, in groups of six to eight.
Figure 37--Topographic expression of cyclothems and megacycles in Loferer Steinberge. After photograph by Schwarzacher (1954).
Figure 38--Megacycles. Basic assumption: Lofer cyclothems have uniform period. Twenty cyclothems of Steinernes Meer section are shown as stratigraphic column at right. Toward left, they are spread out evenly along time dimension of diagram. Mean subsidence vector connects sea level at time 0 with base of stratigraphic column at time 20. Each cyclothem is plotted at proper stratigraphic height above this vector. Cyclothems plot in undulating line, in sets of six to eight, which may be taken as megacycles. This megacyclic pattern is attributed to variations in rate of subsidence, and suggested actual subsidence curve is drawn alongside mean subsidence vector. A larger version of this figure is available.
Three possible causes for such a megacyclic grouping come to mind. One is that a long-range eustatic fluctuation is superimposed over the short-term oscillation here assumed. Another is the possibility that our basic assumption of a uniform period in the cyclothem rhythm is in error, and that the length of the cycles fluctuated in a rhythmic manner. A third, and to me the most probable one, is that the mean subsidence rate expressed in the straight subsidence vector, does not represent the actual subsidence pattern, and that the megacyclic grouping has resulted from rhythmic variations in the rate of subsidence. When the wavy pattern of the cyclothems is transferred to the subsidence vector, we can plot a suggested "actual subsidence curve" according to which the region sank by repeated downwarps, which appear to follow a common pattern. The period of these would appear to be between 120,000 and 800,000 years.
Bottom mud: same as m2.
Loferite (here defined): carbonate sediment riddled by shrinkage pores. Partly synonymous with "birdseye limestone."
m1, m2: m1 represents a primary muddy sediment, deposited as a bed. m2 represents internal sedimentation of mud, within cavities developed in the m1. Further generations and subgenerations may be recognized in some rocks. Synonymous with Sander's k1, k2, d1 and d2.
Neptunian dikes: bodies of sediment cross-cutting beds in the manner of dikes, formed by sedimentation in submarine fissures.
Prism cracks (here defined): network of mudcracks essentially normal to bedding.
Radiaxial calcite (Bathurst, 1959, radiaxial fibrous mosaic): calcite cavity linings composed of subparallel individual grains elongated normal to the wall, generally associated in super-grains showing undulose extinction and curved cleavage.
s1 (here defined): the first, generally radiaxial and cloudy calcite lining of cavities.
s2 (here defined): a later generation of calcite, showing an equant mosaic and greater purity.
Sheet cracks (here defined, borrowed from Dr. A. Lees' use of the term sheet spar for spar-filled sheet cracks): planar cracks, commonly parallel to bedding, and in these rocks attributed to shrinkage of the sediment due to dewatering. Parallel sets of sheet cracks produce zebra limestone.
Shrinkage pores (here defined): irregular pores formed in muddy sediment by shrinkage (desiccation). Partly synonymous with "birdseye" when spar-filled. Grades into microsheet cracks and small prism cracks (fenestra of Tebbutt, Conley and Boyd).
Stratofabric (here defined): arrangement of strata in any body of stratified rock, from the dimensions of a thin section to those of a sedimentary basin.Zebra limestone (here defined): limestone banded by parallel sheet cracks.
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