Name: The Gilboa Formation
Location: New York State, USA
Age: Devonian - 380 Million Years Old
GeologyThe geology of Gilboa centres around the thick layer of black and gray shale that occurs within the Panther Mountain Formation of the Catskill cladistic wedge. The area was a tropical savannah climate withdry and wet seasons according to palaeogeographic studies.
The grey sandstones of the area formed some 360-400 million years ago when the Catskill region was a shallow sea filling up with clay and silt washing off the high mountains to the east. The sandstones have now been uplifted high above sea level, but the layers formed in a shallow sea are still visible. Streams have eroded deep valleys into this high plateau, forming the Catskill Mountains. Continental and valley glaciers have also repeatedly covered the Catskills from 10,000 to two million years ago.
The CatskillsThe Middle Devonian began approximately 385 million years ago as a shallow sea covered most of New York State, lying west of the Hudson River. During this time calcareous sediments were deposited that are now the Onondaga Limestone, although this deposition ceased abruptly after sudden deepening of the basin and a new influx of sediments. This arose in part due to the Acadian Orogeny resulting from the continent Avalon colliding with the North American proto-continent's eastern coast. This subsequently lead to the forming of the mountains in what is now New England and the Canadian Maritimes. Sediment produced by the erosion of the newly elevated mountains was carried westward by the river systems and the coarser sediments would be deposited forming deltas where these rivers entered the sea occupying the Appalachian Basin. The increasingly finer sediments were deposited at ever greater distances from the coast to the east so that the overall result was the deposition of a massive wedge of sediments, thickest and coarsest in the east, and thinnest and finest in the west of the state. This complex is named the Catskill Delta.
TaphonomyThe Gilboa fossils consist of organically preserved cuticle found within the black shale deposits. The area is thought to have been a swamp and so an anoxic environment for excellent organic preservation of organic material, as indicated by the silty clay matrix. The fauna was probably transported to the site of deposition covered by layers of algal debris during flood episodes. It is not clear whether rapid preservation was by entombment in clays or organic material.
Beneath the fossilised trees is a layer of dark shales where marine fossils are sometimes exposed. These trees probably inhabited an area adjacent to the shores of an ancient sea that continued westward from the Catskill region. The stumps generally occur in a red shale which would have been a red clay soil, typical of warm and usually tropical climates. The stumps are deposited in lighter coloured, thinly laminated sandstones. These were flood deposits from subsiding floodwaters of the rivers which once formed the great Catskill Delta. The fossils are casts of the original trees. The flood deposited several feet of sand which buried the lower reaches of the tree trunks and then compacted tightly around the trunks to form a mould of each tree. The sand moulds emptied with the decay of the wood.