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