2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 7
Presentation Time: 10:15 AM


TAPANILA, Leif and ANDERSON, Julia R., Department of Geosciences, Idaho State University, Campus Box 8072, Pocatello, ID 83209-8072, ltapanila@hotmail.com

The Upper Guilmette Formation in southeast Nevada preserves a remarkable suite of burrowed carbonate and mixed siliciclastic rocks that were deposited following the Frasnian Alamo Impact Event (AIE). Ongoing field mapping in the Pahranagat and Irish Ranges focuses on these ‘aftermath' deposits to document the recovery of faunas and the reconfiguration of a passive carbonate shelf after the AIE.

Trace fossils are important in recognizing pioneer colonization of event surfaces. The carbonate breccia and graded beds that form a hummocky surface demarking the end of AIE deposition were rapidly colonized by burrowers and borers that produced permanent dwelling and equilibrium structures. Thalassinoides and Teichichnus both recur throughout the studied area as the pioneering benthos in graded limestones, whereas Trypanites borings are restricted to the tops of truncated megaclasts and stromatoporoid boulder clasts, which formed patches of localized rockground seafloor following the AIE.

Early deposition following the impact was sediment starved, producing several firmground horizons evidenced by sharp-walled Thalassinoides with Fe-stained surfaces infilled with bioclastic grainstone. With increased carbonate production and the development of bioherms to the north in the Irish Range (containing abundant endolithic trace fossils), interbedded bioturbated and barren nodular limestones in the northern Pahranagat suggest restricted circulation of bottom waters. Further south in the Pahranagat, a rich invertebrate fauna including abundant Zoophycos suggests well-oxygenated, open marine circulation. Increased siliciclastics upsection include quartz sand shoals with abundant Skolithos, and scoured channel surfaces bored with Trypanites indicating hardground formation.

Ichnofabric indices range from 0 to 5 in the studied area, and a wide variety of ethologies and assemblages of trace fossils are represented.