2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 2
Presentation Time: 1:50 PM

Eolian Silt in Marine Carbonate of the Middle Pennsylvanian Bird Spring Formation: Evidence for High-Frequency Climate-Controlled Sedimentation in Western Equatorial Pangaea

STAGNER, Alice, Geological and Reservoir Engineering Technology, ConocoPhillips, 600 North Dairy Ashford, PR-3036, Houston, TX 77079 and SOREGHAN, Gerilyn S., Geology and Geophysics, University of Oklahoma, 100 E Boyd St, Suite 710, Norman, OK 73019, alice.f.stagner@conocophillips.com

The Pennsylvanian Bird Spring Formation (Arrow Canyon, Nevada) developed in a carbonate shelf setting in western equatorial Pangaea, far from fluvial-deltaic systems. Glacioeustatic-driven sequences (cyclothems) consist of ~3-9 m thick, asymmetric high frequency cycles (HFC) of carbonate mud-, wacke/pack- and grainstone, with discrete occurrences of siliciclastic silt, bounded by flooding and/or exposure surfaces. We studied a ~24 m succession of the Middle Desmoinesian, focusing on the proportion, character, and stratigraphic distribution of the siliciclastic fraction. Owing to the lack of proximal fluvial-deltaic feeders, the silt is of inferred eolian origin and thus offers a record of aridity, wind strength and atmospheric circulation.

Through a series of physical and chemical treatments, we isolated eolian-sourced material, designated herein as the silicate mineral fraction (SMF), from closely (10 cm) spaced samples. Following this extraction, we measured amount (weight %) and grain size of the SMF, and assessed composition and microtextures using a combination of petrography, electron microscopy and microprobe analysis. These data indicate the SMF in Bird Spring strata contains two end-member components: 1) a dominantly quartzofeldspathic, ≤4F -sized component of inferred eolian origin, and 2) a doubly terminated quartz component of mostly ≤3F size and authigenic origin. The source of silica for the authigenic component is under investigation. However, we infer original eolian transport of very fine-grained siliceous dust into the shelf and subsequent remobilization of silica as a likely origin, owing to the absence of siliceous biogenic sources in these generally shallow ramp facies.  SMF flux decreases upward within glacioeustatic cycles of the Bird Spring Formation, from ~33% immediately above sequence boundaries (i.e., lowstand to early transgressive), decreasing to <1% in shallow marine, grainy facies (highstand intervals). This pattern suggests that wind strength and aridity increased during glacial to incipient interglacial periods, and decreased during interglacial periods.