DIGSITE GEOLOGY. The structural geology on the ranch, while not terribly complex, does complicate finding the dinosaur bone-bearing layers. Mapping of the ranch and adjacent BLM lands revealed at least 2 major faults that have affected the areas within and near the current quarries.
Further away from the fault zones, the Jurassic dinosaur bones in the Morrison Formation are in excellent condition - sometimes compressed, but often fully-inflated with excellent bone shape and surface texture. Many bones are found coated with gypsum, sometimes up to an inch thick. Fractures within bones and the surrounding rock matrix are often filled with gypsum crystals up to an inch long. Hollow bone interiors are often filled with clay stone or calcite crystals.
Geology of Bone Layers 1:
Bone Bed Unit. The bottommost layer consists of interbedded siltstones, limestone stringers, large sandstone channel sands, small sandstone stringers and clay stones. It is rich in carbonized plant material, small concretions, rip-up clasts, large sections of ancient trees and - of course – Jurassic dinosaur bones.
Multiple flooding events are shown by the rip-up clasts of underlying layers. Higher energy indicators - such as thin sandstone layers and the rounded or eroded nature of some of the bones and bone fragments - also occur.
Articulation Bone Unit. Just above the Bone Bed Unit, the matrix or rock properties in this zone are thick-bedded gray clay stones and mudstones. There is much less small plant debris than within the underlying bone bed.
Semi-Articulation/Isolated Bone Unit. This layer consists of fine-grained siltstones, thin-bedded sandstones and a moderate to large amount of carbonized plant material and tree remains. Isolated lenses of claystones may have been created by ponding of floodwaters in depressions.
A possible depositing process would have involved stream or river overbank deposits, where flooding events breached or overtopped the channel 'levees', and bones and plant material settled out as the flowing waters decreased velocity.
