Laterally-mixed versus temporally-segregated origins for mixed systems: Contemporaneous icehouse sedimentation on the Pennsylvanian (Desmoinesian-Missourian) Sacramento shelf
Abstract
Carbonate and siliciclastic sediments are often thought to be negatively correlated, but contemporaneous deposition of these "mixed" systems is common where favorable climatic conditions exist adjacent to terrigenous source terrains. Most examples of ancient mixed systems are "vertically mixed" (temporally segregated) reciprocal deposits, while well-constrained examples of "laterally mixed" (temporally coeval) systems are rare. Either lateral mixing was truly rare in the past or it is under-represented due to model-driven interpretation bias. Categorizing systems as laterally-mixed versus temporally-segregated results in drastically different predictions of facies distributions and interpretations of eustatic, climatic, and tectonic forcing.
This study of >75km of nearly continuous Desmoinesian/Missourian outcrop in the Sacramento Mts (Oro Grande Basin) documents contemporaneous carbonate and siliciclastic deposition next to the actively uplifting granitic Pedernal massif. Twenty-seven measured sections, one continuous core, fusulinid biostratigraphy, outcrop photopan mapping, paleocurrent analysis, and extensive hand sample and petrographic observations constrain this laterally mixed system. A wave swept carbonate shelf (Bug Scuffle Mbr) dominated by nutrient tolerant, filter feeding heterotrophs and limited photoautotrophs is bisected by a tide-dominated siliciclastic fan delta (Gobbler Fm), offering new understanding of sediment mixing during the transition to Pennsylvanian icehouse. This unambiguous example of an along-strike shift from in situ carbonate sedimentation to point-sourced siliciclastics in an ancient setting contrasts common depositional models for icehouse mixed systems in which large swings in sea level segregate reciprocal deposits. The results of this study suggest that laterally-mixed systems are likely more common in the rock record than currently recognized and can develop even during periods high-frequency, high-amplitude sea level changes.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP33B..05R
- Keywords:
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- 1039 Alteration and weathering processes;
- GEOCHEMISTRY;
- 1051 Sedimentary geochemistry;
- GEOCHEMISTRY;
- 4217 Coastal processes;
- OCEANOGRAPHY: GENERAL;
- 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY