Abstract
Black shales are usually interpreted to require anoxic bottom waters and deeper water sedimentation. There has long been a debate about whether the Devonian Cleveland Shale Member of the Ohio Shale (CSM) was deposited in shallow- or deep-water depositional environments. This study looked at the CSM at 3 stratigraphic sections and 5 well cores in northeastern Ohio. The CSM mostly consists of sapropelite (interbedded carbonaceous black mudstones and gray calcareous claystones). The black and gray “shales” are rhythmically bedded at micro- ( ≥35 km, while hyperpycnites are too thin ( ≤50 m depth).
Highlights
Black shales are defined as mudrocks that have >3% organic carbon, a mole fraction of Fe2+ > 0.8, and a Munsell Color Value of N1 [1]
The Devonian black shales in the Appalachian basin became major targets for conventional oil and natural gas resources starting in the early 1900s [3], and the Lower Paleozoic total petroleum system including these Devonian black shales had produced about 600 million barrels of oil equivalent (MBBOE) and over 1.5 trillion ft3 of natural gas by the 1990s [4]
Thick sequences of black shales have been explained using: 1) restricted circulation models where organic matter deposition consumed dissolved oxygen from bottom waters, leading to the reduction of sulfate and production of hydrogen sulfide; 2) open ocean models where organic matter produced in the photic zone was largely recycled, producing an oxygen-minimum zone in the underlying water column which facilitated preservation of a portion of the sedimentary organic matter falling to the sediment interface; and 3) continental shelf models where there was limited recycling of organic matter that was produced in the photic zone because of shorter water residence times prior to settling to the sea floor [6] [7]
Summary
The major controls on black shale accumulation were considered to be: the availability of abundant organic matter, types of organic matter, water chemistry, and preservation of organic matter both in the water column and in the sediments (facilitated by rapid deposition and oxygen-deficient conditions) [6]. The Ohio Shale was one of the widespread, middle-to-late Devonian organic-rich shales (including the correlative Antrim Shale in Michigan and the Chattanooga Shale in Kentucky and Tennessee) found in the Appalachian basin in eastern North America (Figure 1) [16] [17]. The Chagrin Shale Member consists of fossiliferous gray shales that form an eastward-thickening wedge ranging from 0 - 400 m thick in northern Ohio. Because of the benthic fossil content, the unit has been interpreted to represent an interruption of anoxic bottom water conditions of the underlying Huron Shale Member and overlying Cleveland Shale Member [15]
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