Kansas Geological Survey, Subsurface Geology 12, p. 73-76
Raymond M. Coveney, Jr.1, W Lynn Watney2, and Christopher G. Maples2
1University of Missouri-Kansas City
2Kansas Geological Survey
One of the most popular models for deriving metals for enriched black shales is accumulation from contemporary sea water during slow sedimentation in deep anoxic basins. The mechanism is simple, plausible, and actualistic in that it can be demonstrated for modern sediments (Holland, 1979). Ancient anoxic seas may have been similar to the present day Baltic Sea and the Black Sea where the most enriched metal values typically occur in offshore basins (Pilipchuk and Volkov, 1974). Although there are problems with exact analogies, the anoxic-basin model generally works so well that stratigraphic intervals of black shale are often equated with deposition in deep-water, sediment-starved conditions and the shales are referred to as condensed (thin, long duration) sections in sequence stratigraphic literature. However, tremendous amounts of metals (table 1) occur in prominent Pennsylvanian black shales that accumulated rapidly (e.g., the Mecca Shale Member) near a shoreline of siliciclastic deltas and peat swamps that were undergoing marine inundation. Hence metal-rich black shales do not always represent slow rates of sedimentation nor long-term sediment-starved conditions, thus they are not always condensed sections.
Table 1--Values for selected heavy elements and sulfur-isotopes in Mecca-type, Heebner-type, and transitional shales. Middle Pennsylvanian Mecca-type shales were deposited near an ancient shoreline characterized by abundant peat swamps and deltaic siliciclastic sediments. They contain large amounts of molybdenum (Mo). Upper Pennsylvanian Heebner-type shales, which contain less Mo, formed offshore, probably in relatively deep waters. Values for Mo in transitional Middle Pennsylvanian shales are intermediate.
Sulfur-isotope values (means and standard deviations for hand-picked grains of pyrite and sphalerite) are listed as per mil deviations relative to the Ca on Diablo meteorite troilite (CDT) standard (from Coveney and Shaffer, 1988). Values for Mecca-type shale and transitional shale are respectively for the Mecca Shale Member of the Illinois basin and the unnamed shale in the Verdigris Formation of Missouri, Kansas, and Oklahoma. Note that sulfides in the offshore (transitional) shales of the Verdigris are more depleted in 34S than the nearshore Mecca. Typically euxinic shales are very depleted in 34S typically yielding 34S values near -20 to 40 per mil (Ohmoto and Rye, 1979). Hence Mecca-type shales differ isotopically from euxinic beds which are more closely approximated by Heebner-type and transitional beds.
Note: Values for Mecca-type shales from Coveney and Glascock (1989) [40 samples of M. Penn. shales from Indiana, Kentucky, and Illinois]; P2O5 and Corg from Coveney et al. (1987). Values for transitional shales from Coveney and Glascock (1989) [16 offshore M. Penn. samples from Iowa, Missouri, Kansas, and Oklahoma]; P2O5 and Corg from Coveney et al. (1987). Values for Heebner-type shales from Coveney and Glascock (1989), (six U. Penn. black shales from Kansas and Missouri; Corg estimated from loss on ignition at 600°C [1112°F]). Isotope values for black (euxinic) shale from Ohmoto and Rye (1979).
|ppm (g/T)||wt. %||34S (per mil)|
Thin, black Pennsylvanian marine shales are almost invariably enriched in organic matter and heavy metals, such as zinc and uranium. Based on three factors (depositional environment, organic matter type, and inorganic geochemistry), Pennsylvanian marine black shales form two distinct, but intergradational varieties (fig. 1): 1) Mecca-type (molybdenum-rich) shales that formed near the ancient (deltaic) shoreline and are enriched in terrestrial organic matter, very enriched in Mo, V, U and Se, but only slightly phosphatic and 2) Heebner-type (molybdenum-poor) shales that formed offshore and contain lesser amounts of terrestrial organic matter, lesser amounts of Mo, V, U and Se, but abundant phosphate. Mecca-type shales yield sulfur-isotope values that are more variable but distinctly richer in 34S than Heebner-type shales and shales from euxinic environments (table 1). Some "transitional" Middle Pennsylvanian shales from the western midcontinent craton have characteristics intermediate between those of Mecca-type and Heebner-type shales.
Figure 1--Paleogeographic settings of high- and low-stand conditions and resulting stratal succession in Middle and Late Pennsylvanian.
Heavy-metal concentration and organic-matter accumulation in black shales have been attributed to bottom stagnation due to upwelling onto platforms along continental shelves (Brongersma-Saunders, 1969) and to salinity stratification from freshwater runoff (Demaison and Moore, 1980). In both models the bottom stagnation provides a trap for organic matter under long-term, high-stand, sea-level conditions. Studies of Quaternary sapropels of similar composition are beginning to reveal significant information about types, rates, and duration of processes leading to formation of black shales deposited during glacioeustasy. For example, the most Mo-rich Black Sea sediments formed over a very limited time between 6,000 and 1,600 yrs B.P. (Calvert et al., 1987) near the centers of topographic basins (Pilipchuk and Volkov, 1974). Even allowing for errors in radiometric dates, the formation of Old Black Sea sapropel (~30 cm [12 inches] thick) containing up to 245 ppm Mo and averaging 115 ppm Mo (Pilipchuk and Volkov, 1974) took no more than 4,400 years.
Calculations for the Mecca Shale imply that Mo enrichments would have taken at least 150,000 years of sedimentation (Coveney and Glascock, 1989) if they derived Mo solely from Pennsylvanian oceans containing as much Mo as today's sea water. However it is possible, as Zangerl and Richardson (1963) suggested, that the shales and metals were deposited several orders of magnitude more quickly and derived their metals from submarine hydrothermal venting as suggested by Coveney and Glascock (1989).
Significantly, organic matter in metal-rich sediments from the Black Sea is fully marine but accumulated under ephemeral conditions of increased productivity during the relatively rapid transition from Pleistocene lacustrine conditions to the more fully marine conditions of today. Calvert et al. (1987) infer that the most fully developed sapropel (and most metal-rich) sediments were deposited during marine flooding (due to glacial melting) rather than under static euxinic conditions. Calvert et al. (1987) note that increased marine production rates caused by influx of water from the Mediterranean Sea may have been more important than the presence of anoxic waters in the development of Old Black Sea sapropel. Inflowing ocean waters may have added significant amounts of Mo during dynamic flux conditions to the"stagnant" Black Sea basin (Pilipchuk and Volkov, 1974). Other sites of significant sapropel accumulation during Quaternary flooding episodes have been identified on several continental shelves preceding sea-level high stand. These include the eastern Mediterranean and fringes of the shelves near the Amazon and Mississippi River deltas. While water was relatively deep (>100 m [330 ft]), causal mechanisms appear to include enhanced organic productivity and bottom stratification due to increased freshwater runoff or presence of basinal brines. In general, flooding at the high rates and magnitude caused by Quaternary glacioeustasy, coupled with climate perturbations, led to sapropel development.
Evidence for nearshore and exaerobic conditions in midcontinent Pennsylvanian shales come from independent analyses. Molybdenum and some other metals are most abundant near the ancient shoreline in beds containing abundant terrestrial-type organic matter as inferred by pyrolysis gas-chromatography and Rock-Eval analyses (Coveney et al., 1987). Quinby-Hunt et al. (1988) note that dark shales with similar appearances can form under oxic as well as under anoxic conditions and that metal ratios (V:Fe:Mn) reflect depositional environments as defined by oxidation potential and organic productivity. For example, occasionally high Mn concentrations in Pennsylvanian shales imply periodic oxic conditions. From projected rates of burial of fish fossils, Zangerl and Richardson (1963) inferred that the extensive Middle Pennsylvanian Mecca and Logan quarry shales of Indiana were deposited relatively quickly in shallow water as epeiric seas transgressed rapidly across the midwestern United States accumulating debris from coastal peat swamps. Mecca-type shales contain abundant terrestrial -type organic matter (Coveney et al., 1987), erratic sulfur-isotope values (Coveney and Shaffer, 1988) and thin intercalated oxic zones (Maples, 1986), all of which fit rapid deposition near the ancient shoreline and fluctuating water conditions (fig. 1). Heebner-type shales of the Middle and Upper Pennsylvanian (fig. 1) may have accumulated more slowly offshore in deeper waters, but in some cases these beds are associated with local facies transitions suggestive of shallow water or vertically limited anoxic bottom water or slower rates of transgression (e.g., the Eudora shale, Heckel, 1975). In addition, some offshore beds contain significant amounts of terrestrial-type organic matter concentrated at basal positions in the black shale suggesting flooding conditions with a significant component of freshwater runoff (Wenger and Baker, 1986) preserved by rapid burial.
Any model of origin for metal-rich Pennsylvanian shales or their host cyclothems must take into account the differences between Mecca-type and Heebner-type shales. The presence of enriched metal values in near-shore shales, such as the Mecca, invalidates the common assumption that metal-rich shales necessarily reflect slow deposition of a compressed section in a starved basin filled with anoxic waters. The presence of Mecca-type shales in addition to Heebner-type shales in Pennsylvanian cyclothems poses complications in modeling of sedimentation of the late Paleozoic of the Midwest. Data that can provide information on timing and environmental conditions of sapropel formation in glacioeustatic-derived Quaternary depositional sequences need to be examined more systematically worldwide to further quantify parameters in their formation. Moreover, interdisciplinary studies based on regional microstratigraphic sampling of individual black-shale events are needed through varied settings of Pennsylvanian depositional sequences.
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Zangerl, R. . and Richardson, E. S., 1963, The paleoecologic history of two Pennsylvanian black shales: Chicago Natural History Museum, Fieldiana Geology Memoir, v. 4., 352 p.
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Web version May 12, 2010. Original publication date 1989.