PALEOBATHYMETRY OF WIDESPREAD DEVONIAN BLACK SHALES: GOING DEEPER, BUT NOT THAT DEEP

In the interpretation of black shale bathymetry, context is everything. Undoubtedly there are very shallow water (<10 meters) black shales and they have distinct associations, e.g., with stromatolites, flat pebble conglomerate, even desiccation cracks. However, these are not features seen in the more widespread Devonian shales (e.g., Devonian Marcellus, Geneseo, Rhinestreet). Both stagnation and hyperwarming models predict an association of many such black shales with sea level highstands and, as such, it is likely these facies should correspond to deeper water deposits than those below in transgressive successions. A number of independent lines of evidence indicate that most of these shales record offshore, deeper water settings, but perhaps rarely exceeding 100 m. Counter to some assertions that Middle Devonian Marcellus back shales pinch out onto subaerial unconformities, these shales actually correlate to relatively deeper water carbonates on the Findlay Arch. Drill cores through pinnacle reefs in NY extend, locally, 30 meters upward into the black Marcellus shale and, as they appear to have been drowned, provide some contraint on the depths of accumulation of those shales. The top limestones of the pinnacles are corroded but show no evidence of vadose solution or karsting and, indeed, do not even have shallow Hamilton biofacies. Likewise, Middle and Upper Devonian dark shales in Iowa basinal areas correlate up-ramp into the shallow subtidal fillings of drowned, subaerial valleys, incised as much as 100 m into slightly older carbonates. Also, late Middle Devonian black shale facies in New York actually grade concentrically into gray, more fossiliferous shale facies that pass further upramp into siltstones with diverse biofacies, and which onlap more than 15 km eastward over terrestrial strata in the Catskills region. The dark shales thus appear in basin center positions and are deeper than typical offshore marine biofacies. Conversely, there is also evidence from microendoliths in brachiopods from these “deeper water” facies, still record a 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 were deposited during maximum highstand, but relative to some earlier stratified basin models, these are moderately shallow depths.