CONTRASTING DEPOSITIONAL SETTINGS OF EPICONTINENTAL ORGANIC RICH FACIES: SHALLOW VS. DEEP WATER BLACK SHALES

There is no single answer to the question of black shale bathymetry: “shallow or deep”? In fact, there are two nearly opposite extremes of black shale in Paleozoic epeiric sea deposits. There are undisputed very shallow water (<10 meters) black shales. These shales have very distinct associations that give an unambiguous interpretation. In many cases, these shales are thin (<1 m) and rather localized. However, some may be very widespread and apparently record unusual shallow lagoonal settings associated with initial transgressions. These organic-rich shales are most frequently interbedded rhythmically with micritic carbonates that show a suite of features that include stromatolites, flat pebble conglomerates, even desiccation cracks. Associated fossil assemblages include carbonized dasyclad algae ostracodes, especially leperditians, gastropods, nautiloids, lingulids, and stromatoporoids and corals. These dark shales tend to grade laterally and vertically, in transgressive successions, into offshore shoal facies or into peritidal facies in shallowing upward successions.

In contrast, more widespread black shales (e.g., Devonian Marcellus, Geneseo) are very distinct. Many such black shales are associated with sea level highstands and, as such, it is likely these facies should correspond to deeper water deposits in transgressive successions. A number of independent lines of data indicate that most of these shales record offshore, deeper water settings, perhaps rarely exceeding 100 m. Late Middle Devonian black shale facies in New York grade concentrically into gray, more fossiliferous facies that pass further upramp into siltstones with diverse biofacies. The dark shales thus appear in basin center positions and are deeper than typical offshore marine biofacies. There is also evidence from microendoliths in brachiopods from these “deeper water” facies, still record distal photic zone, as the distinctive cyanobacterial traces are similar to those that occur in the deep euphotic-dysphotic, not the shallow euphotic. Thus, we argue that these shales are relatively deep water facies, recording water depths of tens, but not hundreds of meters. Portions of black shales deposited during maximum highstand, but relative to some earlier stratified basin models, these are moderately shallow depths.