COMPARISON OF DEEP AND SHALLOW MARINE BLACK SHALES: THE VACA MUERTA, NEUQUEN BASIN, ARGENTINA AND THE MIDDLE AND UPPER DEVONIAN BLACK SHALES, NORTHERN APPALACHIAN BASIN, USA (Invited Presentation)

A commonly applied model for marine black shales is that they form in deep, quiet, permanently anoxic basins. A closer examination of the record shows that they also can form in shallow marine conditions with moving bottom currents and in waters that may be seasonally or periodically anoxic but not permanently anoxic.

Most of the organic-rich facies (>2%TOC) in the Late Jurassic-Early Cretaceous Vaca Muerta Formation of the back-arc Neuquén Basin, Argentina were deposited in a basinal setting in relatively deep water (>125 meters). There are clinoforms visible on seismic data that show reflectors that can be traced from a shallow carbonate platform down the slope into a basin where the organic-rich shales are preserved. The lowermost and most organic-rich part of the Vaca Muerta immediately overlies a subaerial unconformity and was likely deposited in shallower conditions. There are no other unconformities found within the organic-rich interval.

Conversely, in the Middle and Upper Devonian strata of the northern Appalachian Foreland Basin, USA there are numerous organic-rich black shales that were deposited in relatively shallow marine conditions. There are no clinoforms visible on seismic data, outcrops or log correlations. The shales become more enriched in TOC moving from east to west up onto the flanks of the Algonquin-Findlay Arch where they thin and onlap erosional unconformities that occur throughout the section, typically just beneath the most organic-rich interval. In the Middle Devonian Marcellus Shale, shallow marine carbonates are interbedded with high-TOC black shale. It simply can’t have been very deep, no more than a few tens of meters. Many other black shales overlie unconformities or have internal unconformities and may have been deposited in similarly shallow conditions.

Both the Vaca Muerta and the Devonian black shales of the Appalachian Basin have in situ benthic fossils and ripple cross-lamination, even in the most organic-rich sections demonstrating that the bottom waters were neither quiet nor permanently anoxic. Each black shale needs to be evaluated based on it’s own characteristics.