SHIFTS IN FAUNAL ABUNDANCE AND SKELETAL CARBONATE GRAIN CONTENT OF NORMAL MARINE SOFT SUBSTRATES ASSOCIATED WITH THE LATE DEVONIAN MASS EXTINCTION, WESTERN UNITED STATES

As part of an ongoing effort to understand the scope and nature of the ecologic changes in the soft sediment realm associated with the Late Devonian mass extinction, carbonate sediments of Late Devonian (Frasnian and Famennian) and Early Mississippian age were examined in the field and laboratory.

The materials for this study are from the Great Basin and Rocky Mountain regions of the Western United States, which provide an excellent opportunity for examining the ecologic and sedimentologic changes associated with the Late Devonian mass extinction. In these regions outcrops are plentiful and the rock units record carbonate deposition across the shelf during this critical interval.

Field data from this project have demonstrated a large-scale shift in the dominant fauna composing the communities in the pre-extinction Devonian to those of the post-extinction Devonian and Early Mississippian. However, field data do not allow the direct measurement of faunal abundance, for this we present laboratory analyses that include point counts and percent area estimates (extrapolated to volume) of the fauna composing the rock.

To measure faunal abundance, over 200 thin sections and acetate peels were analyzed using a 1 sq. cm grid, within which point counts and percent area estimates were made for each fossil group (e.g. pelmatozoans, brachiopods, etc.). The results of this study confirm the field evidence of ecologic change and demonstrate a fundamental shift in the origin of carbonate grains from the pre- to post-extinction shelf systems.

The primary signal from this data set shows that pre-extinction communities are dominated by brachiopods, which consistently contribute to the substrate approximately twice the skeletal material of any other fossil group. Thus, much of the sediment is of brachiopod origin. This domination ceases in the post-extinction Late Devonian when pelmatozoans come to dominate.

Pelmatozoan domination in the Early Mississippian is exceptionally well demonstrated by the fine-grained component of Mississippian rocks where they account for most of the macrofossils as well as the majority of the matrix and contribute to the sediment between 2 to 6 times the volume of skeletal material of other faunal groups.