266-2 Eolian Silt in Marine Carbonate of the Middle Pennsylvanian Bird Spring Formation: Evidence for High-Frequency Climate-Controlled Sedimentation in Western Equatorial Pangaea

See more from this Division: Topical Sessions
See more from this Session: Paleozoic Oceanographic and Climatic Changes: Evidence from Seawater Geochemistry and Sedimentology Records II

Tuesday, 7 October 2008: 1:50 PM
George R. Brown Convention Center, 361DE

Alice Stagner, School of Geology and Geophysics, University of Oklahoma, Norman, OK and Gerilyn S. Soreghan, Geology and Geophysics, University of Oklahoma, Norman, OK
Abstract:

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.

See more from this Division: Topical Sessions
See more from this Session: Paleozoic Oceanographic and Climatic Changes: Evidence from Seawater Geochemistry and Sedimentology Records II

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