|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 151-170|
Bureau of Economic Geology, Austin, Texas
Sedimentary cycles in the Eocene of the northern Gulf Coast are the result of repeated strand transgressions and regressions, occurring within a structural and depositional basin similar to the present Gulf. Sedimentary patterns within these cycles are defined in terms of lithology, faunal content, sedimentary structure, and stratigraphic relationship. Details of cycles are shown best in Claibornian sequences because of well-defined alternations of transgressive and regressive units, and because of diversity of depositional units within individual cycles.
As observed in outcrop, Eocene cycles consist of two general phases--marine transgressive and nonmarine regressive. Rocks of both phases generally overlie regional disconformities. Transgressive phase includes open marine, glauconitic sands and marls overlain by restricted or marginal marine clays. Locally, open marine clays, representing a third or inundative phase, are interbedded with or overlie basal glauconitic units; this phase is developed best in gulfward facies. Regressive phase consists of a basal areanaceous unit representing fluvial to marginal marine deposition, and an upper argillaceous-carbonaceous unit consisting mostly of lagoonal, paludal, and flood-plain deposits.
Cyclic deposition is probably the most general feature of the Gulf Coast Eocene sequence. When defined in terms of vertical and lateral sedimentary patterns, these cyclic features provide a valid and workable basis for classification of the Gulf Coast Eocene sequence.
[Note: Publication authorized by Director, Bureau of Economic Geology, The University of Texas, Austin, Texas.]
Eocene deposits of most of the northern Gulf Coast area are a thick and very complex sequence of continental, deltaic, and marine sediments. Eocene rocks are predominantly arenaceous and argillaceous in the northwestern part of the Gulf Coast area and largely calcareous and glauconitic in the eastern part of the province; gulfward facies generally are argillaceous. The arenaceous-argillaceous sequences are largely nonmarine or deltaic and represent maj or regressions; fossiliferous, calcareous-argillaceous facies of the eastern part of the province extend into the western deltaic and nonmarine sequence and represent principal marine, transgressions. The alternation of transgressive and regressive units, with recurring patterns of sedimentation, constitute depositional cycles.
Bornhauser (1947), Ellisor (1929), Fisk (1938,1940), Huner (1939), Lowman (1949), Mullen (1940, 1941), Murray (1955, 1961), Murray and Thomas (1945), Murray and Wilbert (1950), Stenzel (1939, 1941, 1952a, 1952b, 1952c), Stenzel and Turner (1940), Thomas (1942), and Wendlandt and Knebel (1929) have discussed or mentioned cyclic depositional sequences within the Eocene as well as other series in the Gulf Coast Cenozoic. Causes of depositional cycles are not well known but are explained commonly as the result of (1) eustatic change of sea level, (2) shifting of sites of major deltaic accumulations, (3) differential uplift and subsidence, and (4) variations in source and rates of sedimentation.
Sedimentary cycles are known from most all Cenozoic sequences of the Gulf Coast region. Essential features of Cenozoic cycles, however, are shown in Eocene rocks of east Texas, Louisiana, Mississippi, and Alabama. Area referred to in this paper is generally that part of the Eocene outcrop east from the San Marcos Arch in Texas to the Alabama River in Alabama. Stratigraphic units mentioned in this paper and a tentative classification of outcropping Eocene cyclic deposits of the northern Gulf Coast are given as Table 1.
Acknowledgments--I express appreciation to the following persons for reading the manuscript and offering helpful suggestions and criticisms: P. T. Flawn, D. E. Owen, P. U. Rodda, and Josephine Casey, Bureau of Economic Geology, The University of Texas, and A. J. Scott, Department of Geology, The University of Texas. I have drawn freely from the numerous publications dealing with Gulf Coast geology.
In simplest terms, the Eocene sequence of the northern Gulf Coast region consists of alternations of predominantly sand and clay units. Such rock types as glauconite, marl, and lignite are significant but constitute quantitatively little of the total section. Eocene rocks of the northern Gulf Coast are laterally and vertically variable; however, they can be classed into a few distinct rock groups that in some cases represent distinct depositional units. These include:
Sedimentary structures and faunal assemblages characterize certain of the above units; likewise, stratigraphic relationships between certain of these units are distinct and persistent.
By considering the Eocene sedimentary complex in terms of a few, generally distinct, lithologic and paleontologic units, inferred depositional environments can be contrasted. Relationships of these units in vertical sequence constitute persistent sedimentary patterns which, through recurrence within the Eocene section, define sedimentary cycles. Units consisting largely (up to 90 percent) of glauconite (no. 1 above) are distinctive but, unfortunately, are not well understood in terms of the depositional environments they represent. Sand units (no. 4 above) though occupying a definite position in the Gulf Coast Eocene sequence, formed under locally varying conditions; mostly, however, they are fluvial deposits locally modified in marginal marine environments. The main purpose in this paper is to show position and relationship of these units (primarily as rock units) in cyclic successions developed in varying facies, mainly in outcrop sections but to some extent in subsurface sections. Units are, in part, designated by such modifiers as openmarine, fluvial, restricted-marine largely for convenience in discussion, though there is no implication that their origin is well understood.
Among the most distinct units in the Gulf Coast Eocene are glauconitic sands and associated glauconitic marls. These are quantitatively insignificant but very important in regional correlation and vertical pattern analysis. Amount of glauconite varies from sparse (approximately 5 percent) in marginal facies to as much as 90 percent in gulfward facies. Typical Eocene units include Seguin, Marthaville, Newby, Weches, and Wheelock of Texas; lower parts of the Cane River and Cook Mountain of Louisiana; Winona and Archusa-Potterchitto of Mississippi; and Moodys Branch in most of the northern Gulf Coast. Glauconitic units in the Alabama Sabinian include the Bashi and Ostrea thirsae beds. A general description of the glauconitic units of the northern Gulf Coast Eocene is as follows:
Glauconite, medium to coarse grained, pelletoid, light green gray to dark green, weathers red brown. Some quartz sand, medium to fine grained, poorly sorted, increases in marginal sections with decrease in glauconite. Locally argillaceous; some marl or argillaceous limestone, concretionary, glauconitic. Unit massive to poorly bedded, locally thin and even bedded; typically not cross-bedded except in marginal or up-dip limits; bedding features locally obscured or obliterated by burrowing organisms; burrows filled with glauconitic material similar to surrounding rock, common especially in lower parts of units; few clay partings; ironstone concretions common; biostromes, especially of oysters and other mollusks; commonly with basal conglomerate of phosphorite pebbles, shell fragments, shark teeth, teleost otoliths.
Glauconitic sands and marls have a diverse molluscan fauna with gastropods and pelecypods predominating. Foraminifera likewise are diverse and abundant; they include polymorphinids, nonionids, buliminids, textulariids, miliolids, and rotaliids (Howe, 1939). Corals, mostly scleractinian, bryozoans, ostracodes, shark teeth, and teleost otoliths are common.
Associated with marine glauconitic sands and marls are calcareous, fossiliferous, nonlignitic, nonglauconitic or only slightly glauconitic, light to buff-gray clays. These grade laterally and vertically into glauconitic sands and marls, or where locally very calcareous, they may grade to indurated marls or argillaceous limestones. Faunas of these clays are diverse and similar to those of associated glauconitic units, with abundant mollusks, foraminifers, and corals. Examples of calcareous, open marine clays are parts of the Wheelock and Weches of Texas; lower Cook Mountain and lower Cane River of Louisiana; Shubuta, North Creek, and parts of the Archusa-Potterchitto and Winona of Mississippi. Fossiliferous, calcareous clays and glauconitic sediments are common in much of the Alabama Claibornian and Jacksonian.
Chocolate-brown clays containing a restricted marine fauna form readily recognizable units in the Gulf Coast Eocene. [Note: Restricted marine is used in this paper merely to contrast with open marine, and includes a variety of enclosed to partly enclosed, marginal marine environments.] These clays are laminated, soft, and unctuous to locally silty; commonly they are montmorillonitic or muscovitic, and locally they contain selenite crystals, clay-ironstone concretions, iron sulfide nodules, and thin glauconitic lenses.
Representative restricted marine clays in the northern Gulf Coast Eocene include the Marquez, Therrill, parts of the Hooper, Landrum, Stone City, Caddell, Whitsett, and Manning of Texas; upper parts of the Cane River and Cook Mountain of Louisiana; Zilpha and Gordon Creek of Mississippi. Restricted marine clays commonly contain pteropods, small thin-shelled pelecypods, corbulids, venericarids, Ammobaculites, Haplophragmoides, Trochammina, and Trochamminoides; they commonly have a high pollen count. Fossils are not abundant, and depauperate or dwarfed forms are characteristic. Chocolate-brown clays generally overlie and are transitional to glauconitic units; commonly they are overlain disconformably by fluvial to marginal marine sands. Presence of glauconitic lenses indicates local open marine conditions of deposition. These glauconitic lenses contain a similar but generally less diverse fauna than main glauconitic units.
Developed as basal units of the regressive part of Eocene cycles are sands formed as fluvial to marginal marine or deltaic deposits. They include parts or all of the following units in the northern Gulf Coast Eocene: Simsboro, Carrizo, parts of the Queen City, Sparta, Bryan, Cockfield, Koscuisko, and Meridian. A summary description of a typical rock unit is as follows:
Sand, light colored, weathers light gray to variegated; quartzose, clean, with minor amounts of dark chert and heavy minerals, locally ferruginous; subrounded to locally well rounded; poorly cemented, matrix of fine-grained silt or clay, locally indurated with quartzitic or limonitic cement; massive to crossbedded or lenticular; fine to medium grained, grains locally coarse and polished; clay pebbles or boulders at base, subrounded to angular, with clay, light gray, silty or arenaceous; finely divided fragments of lignite, carbonaceous material, and silicified wood common; thin partings of clay and silt weather commonly to indurated layers of flaky partings; limonitic and sideritic concretions common,
On basis of sedimentary structure, lack of marine fossils, and lack of glauconite in other than trace amounts, these sands commonly are interpreted as fluvial to marginal marine with deposition by streams at or near coastal margins. Locally, such sands were reworked as beach deposits or deposited as channel sands on deltas, or as blanket sands on and around delta slopes. Clays associated with these sands either as matrix or interbeds commonly are kaolinitic in fluvial facies but contain some montmorillonite in marginal marine facies. Low-relief disconformities, locally representing a buried-hill topography, generally are found beneath fluvial facies of these sands (Stenzel, 1951a; Hayes, 1951). Although formed under slightly varying conditions of deposition, sands of the types described above occupy a definite position in Eocene sedimentary cycles and are here considered as common units.
Typically developed as the upper part of nonmarine phases of Eocene cycles are clays and silts representing lagoonal, marsh, paludal, flood-plain, or fluvial deposition. These sedimentary units overlie and to some extent are interbedded with fluvial or marginal marine sands. The nonmarine phase of Eocene cycles consists primarily of a lower arenaceous unit and an upper argillaceous unit. Nonmarine clays and silts generally are light to dark gray but may be variegated. Xyloid lignite, either as thin beds or as finely divided fragments, is common. These clays and silts are generally laminated but commonly some of the clays are massive. Clays vary in composition from chiefly kaolinitic in lower Eocene cycles to montmorillonitic in upper Eocene cycles; montmorillonite reflects late Eocene volcanic activity, especially in the western Gulf Coast region. Fossils are scarce, except for silicified or lignitized plant detritus, and locally include fresh-water pelecypods and gastropods and brackish- or fresh-water foraminifers and ostracodes.
Fluvial channels and ancient soil horizons are common features within these nonmarine clay and silt sequences; channels are filled with sand similar to sands of the lower part of the nonmarine phase. Sideritic or limonitic concretions, as well as boulder-size concretions of sideritic or calcareous siltstone, are common.
Deposits included in this group represent some of the thickest outcrop sequences in the northern Gulf Coast Eocene. They represent a variety of nonmarine and deltaic environments and are much less amenable to categorization than other Eocene units. Although generally forming upper parts of nonmarine phases of Eocene cycles, clays and silts of this type locally occur within principally marine phases where they can be separated from restricted marine clays and silts mainly by lack of marine fossils.
Units in the northern Gulf Coast Eocene that consist partly of nonmarine clays and silts include: Calvert Bluff, Pendleton, upper Carrizo, Queen City, Stone City, upper Cook Mountain, upper Yegua of Texas, and upper Cockfield of Louisiana; Hatchetigbee, upper Meridian, upper Koscuisko, and upper Yegua of Mississippi. A few nonmarine clays and silts occur in the Sabinian of Alabama (Hatchetigbee) but are not common in the remainder of the Alabama Eocene. Much of the undifferentiated Sabinian strata in the Tyler Basin of Texas, north-central Mississippi, and the Mississippi Embayment of Kentucky and Tennessee consist of nonmarine clays and silts interbedded with fluvial sands.
Figure 1--Typical Eocene sedimentary cycle and pattern, northern Gulf Coast region.
Angular unconformities are not common in the Gulf Coast Eocene section; they generally occur only in extreme marginal areas, such as parts of the Mississippi Embayment where there is overstepping by certain stratigraphic units of the main, outer depositional border of the Coastal Plain. Gulfward or toward the central parts of the Gulf Coast depositional basin, angular unconformities grade into regional disconformities which in turn grade to conformity basinward.
Two types of disconformities, one developed beneath basal transgressive units, and the other beneath basal regressive units, are especially significant in regional correlation and classification of Eocene rocks and in the delineation of sedimentary patterns.
Erosional disconformities generally marking the base of nonmarine sequences are the result of subaerial channeling. Examples of such disconformities occur beneath the Carrizo, Meridian, Simsboro, parts of the Tuscahoma, Sparta, Koscuisko, and certain Sabinian sands of Mississippi. In local sections erosional relief is slight, generally as little as 1 to 3 inches though locally as much as 15 feet; however, regionally, a buried-hill topography, such as that beneath the Carrizo (Stenzel, 1951) and the Sparta (Hayes, 1951), was developed; local relief of these surfaces is approximately 50 feet.
One of the most common and most significant types of disconformities in the Gulf Coast Eocene is that developed beneath marine transgressive units, generally glauconitic sands and marls. Examples of marine transgressive disconformities are at the base of the Seguin, Marthaville, Newby, lower Tallahatta, Weches, Winona, basal Cane River, Wheelock, Milams, Archusa, lower Lisbon, Gosport, and Moodys Branch. Minor and local marine disconformities commonly occur at the base of glauconitic lentils within other units. An example of a minor marine transgressive disconformity is that beneath the Hurricane Lentil of the Landrum (Stenzel, 1940b).
The following physical features generally are associated with transgressive marine disconformities: (1) uneven erosional surface, (2) basal conglomerates, mostly of local origin, (3) basal glauconitic sand or marl, and (4) burrows in basal glauconites extending downward into underlying units. Erosional surfaces developed with marine transgressive units generally are of very low relief. Filled burrows extending into underlying units form a very uneven contact. In addition, local, broad, shallow channels and scour swales cut at time of transgression are developed. Absence of glauconites and burrows in basal units within these channels or swales suggests relatively rapid cutting and filling.
Basal conglomerates of phosphorite pebbles, clay pebbles, shell fragments, otoliths, and shark teeth are common in basal marine transgressive units. Similar basal conglomerates are common to marine transgressive units in the Upper Cretaceous (Gulfian) of the Gulf Coast.
Basal units of transgressive marine parts of Eocene cycles generally are glauconitic, consisting mostly of sand-size glauconite pellets in the form of fecal or pseudo fecal pellets. These basal units vary in composition regionally along both strike and dip. Marginal units are only slightly glauconitic and commonly devoid of fossils or at least without diverse faunal assemblages; mid-dip or gulfward sections are highly glauconitic and fossiliferous; sections farther gulfward become progressively more calcareous, and glauconitic marls are common. Stenzel (1953) has shown in detail facies changes in the Newby Member of the RekIaw Formation across the Tyler Basin in east Texas.
Fossil burrows commonly are associated with transgressive marine units and characterize underlying marine disconformities. Burrows are 0.5 to 2.0 inches in diameter and generally about 2 to 3 inches long, but locally much longer. Burrows form simple, tubular or branching and connecting networks and are filled with glauconitic material similar to the enclosing rock. Burrows were developed by animals boring or digging into the substrate for either food or shelter and are attributable to crustaceans, mollusks, and holothurians.
Transition of rock units by alternation or interfingering is a common feature in the Gulf Coast Eocene sequence. Such boundaries are important in describing relationships of rock units but are of relatively little importance in regional correlation and delineation of Eocene sedimentary cycles.
Transition by gradation or blending differs from alternation or interfingering in that it is a single, nonoscillatory lithic change. As they are more persistent regionally, blend contacts are more nearly isochronous than arbitrarily picked boundaries in alternating or interfingering sequences. Blend contacts occur between slowly deposited, open marine glauconitic sands or marls and restricted marine clays. Best example of this type of contact is that between the Weches (Winona) and Therrill (Zilpha). Locally, blend contacts are shown between the Newby and Marquez, the Wheelock and basal Landrum, and equivalent units within the Gulf Coast. That the succession of rock types is persistent regionally and nonoscillatory is unique. As can be shown from both physical and biostratigraphic evidence, richly glauconitic, marine transgressive units represent slow-rate deposition or "condensed" sections (Stenzel, 1952a). Under such uniform and stable conditions of sedimentation, an influx of different material would result in greater contrast and distinction, of both rock type and depositional environment, than if such an influx succeeded less uniform conditions of deposition. In the latter case, transition probably would be through alternation and interfingering. This is shown in comparing regional relationships of contacts with regional change in facies of superjacent rock units. In up-dip sections or locally along strike where richly glauconitic units grade to sparsely glauconitic units, there is a coincident change from blend to alternating contact between these and overlying units.
The extent to which depositional cycles and patterns can be recognized in the Eocene sequence of the northern Gulf Coast depends on the degree and frequency of alternation of regionally persistent marine and nonmarine units. Claibornian strata represent maximum in range and succession of depositional environments and display features of Eocene cyclic sedimentation most clearly. From features shown within this sequence of rocks, a general cycle of sedimentation can be constructed (Fig. 1). Vertical patterns shown within Claibornian cycles are recognized within the Sabinian of east-central Texas (principally nonmarine regressive) and the Jacksonian and Claibornian of Alabama (principally marine transgressive) but generally within a more restricted range of depositional environments; commonly only the marine or nonmarine phase is developed. Lateral changes result in relationships within different sequences similar to vertical relationships within the entire Eocene section. For example, prominence of marine deposits in the Sabinian of Alabama results in cycles more apparent than in other areas of Sabinian rocks where marine deposits are few or absent; the Alabama Sabinian is similar to the Claibornian of the northwestern Gulf.
A fundamental pattern exists in all Eocene cycles; it may, in mid-dip or gulfward sections, for example, involve a change from richly glauconitic, fossiliferous sand to restricted marine clays, contrasted to a change from sparsely glauconitic, slightly fossiliferous sand to lagoonal clays and silts in equivalent, marginal or up-dip sections. Absolute vertical pattern may vary, but relative vertical change is consistent within a general depositional regime. Cyclicity or vertical pattern is most difficult to discern in sections that are almost completely marine as less variety in depositional environments is shown. An example of such a section is the Claibornian of Alabama where patterns and cycles are delineated mostly by recurrence of submarine-developed disconformities underlying predominantly glauconitic units; similar features occur in gulfward or down-dip sections of the Eocene rocks. In the latter case, the Eocene sequence grades gulfward to nearly uniform argillaceous facies without disconformities; cycles correlative to up- or mid-dip cycles can be determined generally by vertical variation in faunal assemblages.
Vertical patterns are recognized within limits of discernment of rock or deposition units. Regional persistence of a fundamental vertical pattern through a variety of sedimentary environments and sequences and the repetition of this pattern throughout the section provide the basis for recognition of Gulf Coast Eocene sedimentary cycles.
Sedimentary cycles within alternating marine (transgressive) and nonmarine (regressive) sequences are shown well in the Claibornian of Texas, Louisiana, and Mississippi, and in the Sabinian of Alabama.
Fluvial sands (such as the Sparta, Yegua, and locally the Queen City) form the basal units and arenaceous parts of nonmarine phases of Texas Claibornian cycles (Fig. 2). These sands are medium grained, cross-bedded, and contain clay pebbles in their lower parts; generally, the basal contact is a regional disconformity, commonly associated with a buried-hill topography. The sands grade vertically into deltaic, lagoonal, or nonmarine, lignitic clays and silts forming the upper, argillaceous part of the nonmarine phase. Uppermost parts of this phase commonly include local lenses of glauconitic sand with a marine fauna and are represented by such units as the Stockdale, Stone City, Eaton, and Creola. Rock transition within the nonmarine phase typically is through alternation or interfingering, though local intraformational disconformities occur as the result of channeling. Each of the nonmarine phases of Texas Claibornian cycles has a maximum thickness of about 1,000 feet in outcrop sections.
Figure 2--Texas Claibornian cycles and sedimentary pattern.
A marine phase overlies the predominantly nonmarine phase with regional disconformity developed by basal transgression of a glauconitic unit. Basal glauconitic sands and marls and fossiliferous clay units include the Newby, Weches, and Wheelock. Remainder of the marine phase in Texas Claibornian cycles is essentially regressive marine, consisting generally of restricted marine, chocolate-colored clays. Units occupying this position in Eocene cycles commonly are referred to as the inundative phase; in certain mid-dip or gulfward sections, these units are inundative, represented by open marine, fossiliferous clays, though the restricted marine clays generally found in outcrop are a part of the regressive marine rather than the inundative phase. Locally, thin silt or sand lenses are present within restricted marine clays, and typically thin, glauconitic lentils such as the Hurricane Lentil of the Landrum are developed. Fauna of the upper argillaceous part of the marine phase generally is sparse and consists of marginal marine mollusks and foraminifers, though the fauna of included glauconitic lenses is similar to that of basal glauconitic units. Upper parts of marine phases of Texas Claibornian cycles are the Marquez, Therrill, and Landrum. Total thickness of individual marine phases of Texas Claibornian cycles at outcrop generally is less than 100 feet.
Glauconitic, fossiliferous lentils that occur in the uppermost part of nonmarine argillaceous phases of cycles are not invariably developed on a persistent, regional basis but occur with enough regularity and in the same position in sedimentary cycles to constitute a pattern (Fig. 3). These units are characteristic of the Texas Claibornian both in outcrop and in subsurface gulfward sections where they include such units as the Discocyclina advena marl, preceding main Weches transgression, and the Nonionella cockfieldensis and other fossiliferous intervals of the upper Yegua, preceding the main transgression of the basal Jacksonian. In a sense these units can be considered as initial marine transgressive units in that they precede main marine transgressions. They are less persistent and have a less diverse, though similar, faunal assemblage than main, transgressive, glauconitic units.
Figure 3--Comparison of initial marine transgressive units, Eocene sedimentary cycles, northern Gulf Coast region. A larger version of this figure is available.
Glauconitic lenses occur within chocolate-colored, restricted marine clays of the upper or regressive marine, argillaceous part of Texas Claibornian cycles. Locally these lenses form distinct stratigraphic units such as the Harwood Lentil of the Marquez in the area of the San Marcos Arch. They represent minor marine transgressions within regressive marine sequences and locally occur above basal disconformities similar to disconformities beneath main transgressive units.
A few exceptions occur to the general pattern outlined above. Patterns of sedimentation within the Queen City Formation are mostly of a type different from the remainder of the Claibornian in Texas; patterns within the Queen City Formation are similar to patterns in Sabinian rocks of the southern flank of the Sabine Uplift. Also within the upper part of the Cook Mountain Formation, largely a regressive marine unit, a thin, fluvial or marginal marine sand unit (Spiller Member) and glauconitic sand lentils as noted above, such as the Hurricane and Serbin, are developed locally. These suggest a tendency toward development of local cycles.
The Claibornian section of central and northwestern Louisiana is similar to the Claibornian of east Texas except the Queen City is absent (east from Nacogdoches County, Texas) and the Reklaw is either absent or included with the Weches as the Cane River Formation. The Louisiana Claibornian section is slightly thinner than the Texas Claibornian; in addition, marine phases of cycles are more prominent, and generally nonmarine phases are relatively thinner and include a greater proportion of marginal marine deposits. Two cycles are recognized in the Claibornian outcrop of Louisiana (Fig. 4): Cane River-Sparta and Cook Mountain-Cockfield (Yegua). Basal units of the Cane River and Cook Mountain consist of glauconitic sands and marls like basal glauconitic units in the Texas Claibornian; these units overlie regional marine disconformities and include local intercalations of open marine clays. Forming the upper part of the marine phase are chocolate-colored, silty clays of the upper Cane River and upper Cook Mountain. Glauconitic lenses are common within these clays and generally more prevalent than in analogous units in Texas Claibornian cycles. Regressive or predominantly nonmarine phases of Louisiana Claibornian cycles are represented by the Sparta and Cockfield (Yegua). Both consist of a basal, cross-bedded to massive, fluvial or deltaic sand, overlying a regional nonmarine disconformity in the Sparta but apparently not in the Cockfield. Upper parts of regressive phases are argillaceous and consist mostly of lagoonal silts and clays. Throughout these units, sparsely glauconitic sands are present, especially in the Sparta (Stenzel and others, 1957). Initial marine transgressive, glauconitic sands, represented by the Dodson and Creola, respectively, form reasonably distinct units in the upper parts of regressive units and are in all respects similar to initial marine transgressive units in Texas Claibornian cycles.
Figure 4--Louisiana Claibornian cycles and sedimentary pattern, outcrop, west-central Louisiana. Based in part of data from Andersen (1960) and Hunter (1939).
Gulfward or down-dip modification of Claibornian cycles has been shown by Fisk (1940) and Holland and others (1952) in studies of the subsurface Claibornian of central Louisiana (Fig. 5). Gulfward change is shown in greater prominence of marine units, decrease in number of disconformities, and development of a nearly symmetrical depositional cycle. Marine phases of cycles (Cane River and Cook Mountain) consist of basal glauconitic marls and are overlain by gray, fossiliferous, open marine clays that form a distinct inundative phase. The marine phase is terminated by a glauconitic marl similar to, though less distinct than, the basal unit. Fisk recognized distinct facies within the generally nonmarine and deltaic, regressive parts of Claibornian cycles. Fluvial, channel sands which mark the basal part of the regressive phase in most outcrop cycles are median in position gulfward and do not overlie regional disconformities. From this median position in gulfward cycles, fluvial sands grade upward and downward through deltaic, blanket sands to brackish, gray-brown, silty clays which form the upper and lower parts of the regressive phase. Farther gulfward, fluvial sands grade laterally to deltaic blanket sands which in turn grade to brackish silts and clays. At this point, Claibornian cycles include a transgressive marine phase of glauconitic marl and open marine clay, and a regressive marine phase of silts and clays.
Figure 5--Gulfward or mid-dip modification of Claibornian cycles and sedimentary pattern, subsurface, central Louisiana (modified from Fisk, 1940). A larger verison of this figure is available.
The Claibornian of central and eastern Mississippi is similar to the Claibornian of Louisiana and Texas and shows cycles and depositional patterns, especially in the middle and upper parts of the section, remarkably like those of the northwestern Gulf Claibornian. Two cycles, Winona-Zilpha-Koscuisko and Wautubbee-Cockfield, are distinct; sedimentary patterns within the Tallahatta Formation (Lower Claibornian) are less distinct than typical Claibornian sedimentary patterns. Glauconitic sands and marls, including the Winona and Archusa-Potterchitto of the Wautubbee, are basal marine transgressive units. These units are similar to the Weches, Newby, and Wheelock of the Texas Claibornian. Regional marine disconformities occur beneath these units. Upper parts of the marine phase of Mississippi Claibornian cycles, including the Zilpha and Gordon Creek, consist of restricted marine clays with local lenses of glauconitic, fossiliferous sands. Nonmarine regressive phases are marked by basal, fine- to medium-grained, cross-bedded to massive, fluvial sands that generally grade upward to lignitic silts and clays. Nonmarine erosional disconformities mark the bases of these regressive phases which are similar to nonmarine, regressive units in the northwestern Gulf Claibornian; regressive sand units include the Meridian, Koscuisko, and Cockfield. Initial marine transgressive units, characteristic of the uppermost part of the regressive phases in the northwestern Gulf Claibornian, have not been reported generally in Mississippi Claibornian cycles, though Stenzel (1940a) has reported glauconitic lentils of the Creola in the uppermost part of the Cockfield of Mississippi.
Eastward to Alabama, nonmarine regressive units are absent and the entire Claibornian is essentially a marine, calcareous-glauconitic sequence analogous to transgressive and inundative marine units of the Claibornian of the northwestern Gulf. Northward in Mississippi, marine units decrease in prominence, though cycles equivalent to cycles in central and eastern Mississippi are recognizable.
The Tallahatta, Lower Claibornian of Mississippi, represents a deviation from or modification of the general Claibornian sedimentary pattern. It consists of two units: a lower and eastern facies called the Basic and an upper and northern facies called the Neshoba. Lower part of the Basic generally is glauconitic with the remainder a siliceous, fossiliferous claystone. Northward in Mississippi, the Basic grades both vertically and laterally to the Neshoba, a nonglauconitic to sparingly glauconitic, nonfossiliferous, fine-grained, crossbedded to massive sand. The Neshoba represents marine to locally nonmarine deposition. Analogy of sedimentary pattern within this sequence to patterns of typical Claibornian cycles is not clear, though the Basic probably is equivalent to the marine transgressive part and the Neshoba to the nonmarine regressive part of Claibornian cycles.
Fairly uniform glauconite-carbonate shelf deposition occurred within the eastern Gulf region during much of the Eocene (Toulmin, 1955; La Moreaux and Toulmin, 1959). The only significant exception occurred during early Eocene. In most of the Gulf Coast, Sabinian rocks are predominantly regressive, but in Alabama, alternation of western Gulf regressive deposits and eastern Gulf carbonate deposits within the Sabinian results in sedimentary cycles similar to Claibornian cycles of the northern and western Gulf (Fig. 6). The Alabama Sabinian differs from the northwest Gulf Claibornian in having transgressive phases generally as prominent as regressive phases and, in general, showing less distinct and less persistent sedimentary pattern. A lower cycle (Fig. 6), the Tuscahoma-Nanafalia, consists of a basal, coarse-grained, crossbedded sand with clay-pebble conglomerate (Gravel Creek) similar to basal fluvial sands of Claibornian cycles. This basal sand is overlain by a glauconitic sand and marl unit (Ostrea thirsae beds) that is analogous to basal marine transgressive units of Claibornian cycles and overlain in turn by very fossiliferous, glauconitic clay (Grampian Hills) representing inundative deposition. A basal marl of the Tuscahoma apparently is regressive as it is conformable to underlying inundative clays. The Tuscahoma represents generally nonmarine or deltaic, regressive deposition with a basal fluvial sand grading vertically to lignitic, fine-grained sands, silts, and clays. Two glauconitic, fossiliferous marls, the Bells Landing and Greggs Landing, represent minor marine transgressions within this regressive phase. An upper Sabinian cycle consists of a basal glauconitic marl, Bashi, overlain by carbonaceous and lignitic, fine-grained sands, silts, and clays (Hatchetigbee ) similar to the upper part of the Tuscahoma-Nanafalia cycle. This upper cycle lacks an inundative phase at the outcrop and is comparable to Claibornian cycles; it has a thin, marine, transgressive phase and a relatively thick, nonmarine to brackish, regressive phase.
Figure 6--Alabama Sabinian cycles and sedimentary pattern.
Cycles developed within thick, mostly regressive deltaic sequences are of two general types. Where marine influence is minor, cycles show patterns of deposition analogous to Claibornian patterns. Within depositional sequences containing no marine sediments, cycles are developed that show only the sedimentary pattern of the regressive phase of Claibornian cycles.
Minor Marine Influence
Gulf Coast Eocene sequences showing patterns and cycles generally comparable to those of the Claibornian type occur in the Sabinian sequence of east-central Texas, on the southern flank of the Sabine Uplift, and in east-central Mississippi. The Sabinian of east-central Texas (Fig. 7) includes a basal, locally glauconitic sand containing abundant oysters (Seguin Formation as restricted by Beckman and Turner, 1943) representing marine transgression. A regional marine disconformity occurs at the base of the Seguin. This basal unit is analogous to marine transgressive units in Claibornian cycles though of a more marginal marine aspect. Forming the upper part of a generally marine to brackish phase are lignitic clays containing local lenses of sparsely glauconitic sand (Hooper). That part of the east-central Texas Sabinian analogous to the regressive phase of Claibornian cycles consists of a basal, fluvial to locally marginal marine sand (Simsboro) very similar to basal sands of Claibornian regressive phases, overlain by a thick sequence of predominantly lignitic silts and clays (Calvert Bluff). The Calvert Bluff is similar to upper argillaceous parts of Claibornian regressive phases. A marine transgression, apparently minor, is represented in the uppermost Sabinian by the Sabinetown Formation, which consists of a basal glauconitic, fossiliferous sand, overlain by restricted marine to lagoonal clays and silts. The Sabinetown occupies a position in the Sabinian comparable to initial marine transgressive units in Claibornian cycles but differs significantly in that it does not precede a major marine transgression. It is comparable to the Bashi-Hatchetigbee sequence in the Sabinian of Alabama and possibly ranks as a distinct, though minor, marine cycle. The Carrizo, considered by different authors as lowermost Claibornian or uppermost Sabinian, is here included tentatively within the Sabinian as the regressive part of a Sabinetown-Carrizo cycle. The Carrizo is a fluvial to marginal marine sand, similar to regressive sands of the Claibornian. The upper part generally is argillaceous and locally includes initial marine transgressive units (Fig. 3).
Figure 7--Comparison of east-central Texas Sabinian (Wilcox Group) and representative east Texas Claibornian (Yegua-Cook Mountain) sedimentary cycles.
The Sabinian of the southern flank of the Sabine Uplift of Texas and Louisiana is similar to that of east-central Texas and has comparable marine cycles: Marthaville (= Seguin and Hooper) and Sabinetown. The regressive phase within the Sabinian of the Sabine Uplift, represented by the Pendleton (= Simsboro and Calvert Bluff), differs from the Sabinian of east-central Texas in containing several minor, marine, transgressive units (fossiliferous, glauconitic sands). The Pendleton is a more gulfward facies than equivalent strata in east-central Texas with marine influence great enough to alter the typical sedimentary pattern of regressive phases. Murray and Thomas (1945) defined several sedimentary cycles within the Sabinian of the Sabine Uplift, but recent work by Andersen (1960) has failed to support these earlier conclusions. The Queen City Formation (Claibornian) and the Fayette Group (Jacksonian, as defined by Murray, 1961) of Texas consist predominantly of regressive nonmarine (deltaic) deposits but include several local marine units. Sedimentary patterns shown within these sequences, as within the Pendleton, apparently are intermediate to typical regressive and typical transgressive phases. Alternations of marine and nonmarine units are not sufficiently persistent nor distinct, or perhaps not sufficiently known, to allow recognition of definite patterns of sedimentation.
No Marine Influence
Totally nonmarine sections, as the Sabinian of the northern part of the Tyler Basin in east Texas, of northern Mississippi, and of other regions of the upper Mississippi Embayment, generally are poorly known in terms of vertical sequence. In areas where this type of Eocene sequence has been studied (Mellen and McCutcheon, 1939; Attaya, 1951), complete cycles, consisting of units analogous to the regressive phase of typical Claibornian cycles, are developed. These cycles consist (Fig. 8) of a basal, fine- to medium-grained, highly cross-bedded, fluvial sand, conglomeratic in the lower part, and overlying a regional, nonmarine disconformity. Basal sands are similar to basal regressive sands in other Eocene cycles. Forming the upper part of these nonmarine cycles are argillaceouslignitic, paludal to lagoonal sequences consisting of three fairly distinct units: a lower unit of even-bedded, silty clays and silts; a middle unit of lignites, underclays, and silty, lignitic clays; and an upper unit of laminated silty clays and massive to laminated silts. This upper argillaceous phase of nonmarine Sabinian cycles is comparable to the argillaceous parts of regressive phases in other Eocene cycles.
Figure 8--Units of sedimentary cycles developed in regressive, nonmarine sequences; Sabinian (Wilcox Group) of northern Mississippi. Based on data from Mellen and McCutcheon (1939) and Attaya (1951).
Gulf Coast Eocene sequences that are mostly or entirely marine include the Claibornian of Alabama and the Jacksonian of Alabama, Louisiana, parts of Mississippi, and extreme east Texas.
Jacksonian and Claibornian sequences of Alabama consist mostly of glauconitic marls and sands and open marine clays, with a few restricted or marginal marine units. The entire section is analogous lithologically to rocks forming transgressive marine units of typical Claibornian cycles. Transgressive marine disconformities occur within the section and are at least partly equivalent to disconformities within the Claibornian and Jacksonian elsewhere in the Gulf Coast. Sedimentary patterns are not distinct, but generally a concentration of glauconitic sediments occurs just above disconformities, and restricted or marginal marine sediments, where present, occur below disconformities. Thus, cycles may be recognized with the lower part transgressive marine and the upper part regressive marine; open marine or inundative sediments are generally in the middle and lower parts of cycles. Five cyclic units are shown within the Alabama Claibornian sequence (Tallahatta, lower Lisbon, middle Lisbon, upper Lisbon, and Gosport); the Jacksonian sequence consists of one main cycle of deposition.
Jacksonian rocks of central Mississippi, northwestern Louisiana, and extreme east Texas are, as in Alabama, mostly marine but in western sections show an increase in marginal marine deposits and a decrease in glauconitic sands and marls. Fluvial and deltaic sands are few or absent. Generally, marginal marine deposits contain lentils of glauconitic sediments and open marine clays.
A regional transgressive marine disconformity occurs at the base of extensive, glauconitic sands and marls (Moodys Branch). Overlying this basal unit is a predominantly clay sequence (Yazoo) which includes inundative clays in the lower part succeeded by a regressive marine carbonate lentil (Tullos, Union Church, and equivalent beds). In Louisiana the upper part of this sequence (Verda) is regressive marginal marine with minor amounts of glauconitic and open marine sediments. Danville Landing Formation (generally assigned to late Jacksonian in Louisiana) represents marine transgression and the beginning of a second Jacksonian cycle. The upper part of this cycle is regressive and consists of a unit similar to the Verda (Forest Hills); based on lateral relationship of this unit to faunally defined calcareous units in Mississippi and the east Gulf Coast, the upper part of this cycle generally is assigned to the Vicksburgian.
Depositional patterns in Jacksonian cycles of the northern Gulf Coast, typified by sequences in Louisiana, are intermediate to those in Jacksonian-Claibornian outcrop cycles in the eastern Gulf and Claibornian outcrop cycles in the northwestern Gulf Coast area. Depositional pattern and sequence in Jacksonian cycles of the northern Gulf are similar to down-dip or gulfward Claibornian cycles in the same area.
Patterns within Jacksonian cycles of extreme east Texas, south of the Sabine Uplift, are similar to equivalent sequences eastward but include greater amounts of marginal and restricted marine sediments; glauconitic sands and marls are common, but inundative, open marine clays are uncommon. A lower cycle consists of basal transgressive glauconitic sand and marl (Moodys Branch) overlain by restricted marine clays and interbedded glauconitic lenses (Caddell). An upper cycle contains basal glauconitic sediments (part of the Wellborn) locally overlying a marine erosional disconformity, and a regressive sequence of restricted marine and lagoonal to deltaic clays and silts (Manning and Whitsett). Westward in east-central Texas, Jacksonian rocks are predominantly arenaceous (Fayette Group) and represent mainly regressive deposition with several minor, marine transgressions (Renick, 1936). Similar lateral relationships are shown in marginal facies of Jacksonian of Arkansas and other parts of the Mississippi Embayment.
Cyclic features of deposition probably are the most basic element of the Gulf Coast Eocene sequence. Similar cyclic features characterize the remainder of the Gulf Coast Cenozoic (Lowman, 1949; and others) and also have been described and utilized in classification of Texas Cretaceous rocks (Lozo and Stricklin, 1956). Provided cyclic units are delineated in terms of succession of rock and depositional units, they can be used meaningfully in a vertical division of Eocene strata. Fundamental patterns of sedimentation are persistent, and analogous patterns can be recognized through varying facies so that cyclic division provides a basis, or at least a support to other bases, for provincial correlation. In utilization of cyclic features as a basis of classification, it is desirable that such features be defined in terms of a sedimentary pattern common to all cycles within the section and that such features not be understood as simply alternations of rock or depositional units. Further, analogous features in terms of sedimentary pattern and vertical sequence of different depositional cycles should be recognized and recognized on a comparable order of magnitude.
Commonly recognized divisions of the Gulf Coast Eocene, i. e., Wilcox, Claiborne, and Jackson, generally are understood as time-rock divisions and are based on general fauna and flora content and regional features of strand transgressions and regressions. These divisions are commonly referred to as "groups," though, as pointed out by Murray and Wilbert (1950) and Murray (1961), all are not lithic entities. Lowman (1949) objected to use of "time-stratigraphy" in Gulf Coast stratigraphy and suggested that divisions be defined in terms of regional transgressive and regressive features. General characteristics of Eocene faunas and floras are known but not sufficiently to permit proper delineation of time-rock units; further, evolutionary patterns of Eocene species have been studied in detail in only a very few cases (Stenzel, 1949; Smith, 1906, 1907; Fisher, Rodda, and Dietrich, 1964).
On basis of cyclic deposition, Lozo and Stricklin (1956) have established "tectonic-sedimentary lithogenetic" entities within the Texas Comanchean which they formally call divisions. Young (1963) has proposed such units throughout the Texas Cretaceous. Similar units are demonstrable within the Gulf Coast Eocene; such cyclic units as those indicated by Roman numerals on Table 1 could be called divisions in the concept of Lozo and Stricklin, though several problems are posed in attempting to apply such a classification uniformly throughout the Gulf Coast Eocene.
Cyclic deposition seems to be the most fundamental and basic element in the history of the Gulf Coast Eocene, and when carefully defined throughout the Gulf Coast, in terms of rock and paleontologic units, should provide a logical basis for classification.
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