On top of the Onondaga Formation is a series of rocks consisting primarily of shales and sandstones called the Hamilton Group. These rocks formed from sediments which had started eroding of the rising Acadian Mountains to the east.
The lowest (earliest) formation in the Hamilton Group is the Marcellus Shale. This unit is named for an outcrop in the town of Marcellus in the Finger Lakes region of New York but it's quite widespread and extends southward into the Virginias.
Distribution of the Marcellus Shale in the subsurface
Here in New York, the Marcellus crops out at the surface along its northern and eastern edges along the Mohawk and Hudson River Valleys.
Where the Marcellus Shale outcrops at the surface in New York
Black shales like the Marcellus form in a reducing (oxygen depleted) environment. We know this because they contain a high organic content (organic material will break down in the presence of oxygen) - which gives them their black color - and also contain minerals like pyrite (FeS2) and uraninite (UO2) which only form in chemically-reducing (oxygen-poor) environments (sulfate-reducing bacteria play a huge role in mineral formation in this type of setting. In the black shale layers, there are generally few fossils and mostly of planktonic (floating) or nektonic (swimming) organisms which lived higher up in the more oxygenated water column. There are few benthic (bottom-dwelling) fossils.
Catskill Geologist Bob Titus of Hartwick College has called the environment in which these fossil-poor black shales formed the "Poison Seas."
These types of environments are called euxinic - literally meaning "like the Black Sea" which was called Pontus Euxinus by the Romans.
Salt water flows into the Black Sea from the Straits of Bosporus which connects to the Mediterranean Sea and fresh water enters through rivers such as the Danube, Dniester, Dniepr and Don. This mixing of different density waters in a deep basin leads to stratification with an upper oxidized layer and a lower anoxic (oxygen depleted or chemically-reducing) layer. Black sediments accumulate on the seafloor here which are the sedimentary precursors of black shales.
Closeup of weathered Marcellus Shale
There are basically two models for the formation of these widespread black shales in the middle of the Devonian Period. The first is that loading of the crust from Acadian mountain building basically buckled the once shallow sea on the continent side of the mountain belt downward. This downwarped basin was quite deep, had restricted circulation with the open ocean, and became stratified much like the Black Sea is today. The black shales eventually were covered with coarser sediments forming the Catskill Delta system.
A different model holds that the sea was relatively shallow at the time but seasonally anoxic due to spring blooms of phytoplankton that depleted the waters of oxygen as they died and decomposed during the summer and fall months.
There's still a lot to learn about how things worked some 380 million years ago here in the Hudson Valley!