Obrution Deposits: their formation & fidelity The Fidelity of Fossil Assemblages: An Exercise at Recognizing Good vs. Poor Spatial and Temporal Fidelity

Assemblage Types Biocoenosis: All members of the community are represented. Thanatocoenosis: All fossils present were members of the community, but some members are missing. Taphocoenosis: Some fossils present were not members of the community – time averaged (poor temporal fidelity) or transported (poor spatial fidelity) assemblage Assemblage Types: 1. Biocoenosis: All members of the community are represented. 2. Thanatocoenosis: All fossils present were members of the community, but some members are missing. 3. Taphocoenosis: Some fossils present were not members of the community – time averaged (poor temporal fidelity) or transported (poor spatial fidelity) assemblage

Problem: Case of Unusual Preservation Find a strangely preserved fossil assemblage in Hamilton Group rocks (Middle Devonian, 350 Ma) from western New York State. Found in thin limestone beds. A large interior seaway sat on the interior of North America at this time. Your task: Determine which type of assemblage is represented. The following slides and associated notes will guide you through the problem. Problem: Case of Unusual Preservation Find a strangely preserved fossil assemblage in Hamilton Group rocks (Middle Devonian, 350 Ma) from western New York State. Found in thin limestone beds. A large interior seaway sat on the interior of North America at this time. Your task: Determine which type of assemblage is represented. The following slides and associated notes will guide you through the problem.

Shown is an outcrop of the Centerfield Member of the Middle Devonian Hamilton Group at Brown’s Creek in Genesee County, western New York State. The Centerfield is composed of shales, mudstones, and limestones. The red arrow points out the mud-rich rocks (shales and mudstones), and the blue arrows denote the thin limestones. It is within the thin limestones that the unusually preserved fossil assemblages occur. The mud rocks exhibit more typical fossil preservation (shown in a subsequent slide).

This is a stratigraphic section showing the stratigraphy of the Centerfield Member. The Brown’s Creek outcrop exposes the lower half of the section, including sampling intervals (or horizons) 1-19. The thin limestones containing the unusual fossil assemblage are represented by horizons 11 and 13.

This and all subsequent slides show various styles of preservation within the thin-bedded limestones. Shown is a proetid trilobite (Phylum Arthropoda) preserved in the orientation seen here. The cephalon or head is at a right angle to the thorax and pygidium (the main body and tail). The cephalon rests on the upper surface of the limestone with the remainder of the body positioned vertically down into the limestone. A trilobite thorax is composed of numerous articulated segments.

This bivalve mollusc’s shell (belonging the Class Bivalvia, Phylum Mollusca) was originally composed of aragonite, but is preserved as recrystallized calcite. Both valves are articulated and the shells have not been laterally compressed. The specimen is positioned in the same orientation it was found within the limestone; the umbo (beak) is oriented up in the limestone. The matrix of the limestone is seen adjacent to the fossil.

The obrution mechanism is described here The obrution mechanism is described here. A storm event higher up on the continental shelf in shallower water causes erosion and resuspension of fine-grained sediments (muds). These suspended muds then are transported downslope to deeper water by offshore storm currents (storm ebb tidal flows of sorts) and these sediments smother an entire benthic community quickly with little disturbance. Smothering leads to anoxic (low oxygen) conditions and decay which generates a microreducing environment within the sediments. This ultimately causes carbonate to come out of solution as an early cement. A limestone is formed soon after burial. This combination of sudden burial and early lithification leads to the excellent preservation and high fidelity. If this mechanism can be applied to this Hamilton Group fossil assemblage, then the assemblage is best classified as a thanatocoenosis.

Stratigraphers have been able to trace these Hamilton Group limestones in western New York State (represented by cartoon inset “e”) across the state into central and eastern New York where the erosive effects of storm events can be seen (in cartoon insets “a & b”).