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ABSTRACT: Ichno-Sedimentological Record of Short-Term Climate-Controlled Redox Events and Cycles in Organic-Rich Strata

Charles E. Savrda, David J. Bottjer, Katherine Ozalas

Reduced rates of biochemical degradation of organic matter in oxygen-depleted marine settings generally result in the accumulation of laminated strata with high hydrocarbon source potential. Periods of improved oxygenation, during which the quantity and quality of organic matter are effectively reduced, are reflected by interbedded bioturbated intervals. Such benthic redox excursions may reflect variable paleo-oceanographic responses to climatic events or cycles. The potential role of climate in the short-term modulation of source rock potential is exemplified by bioturbated intervals within three predominantly laminated, organic-rich units. The Jurassic Posidonia Shale (Germany)

contains bioturbated beds whose ichnologic characteristics reflect a spectrum from short, low-magnitude redox events to longer episodes of greater magnitude. The character and distribution of these event beds appear to be controlled by sea level mediated variations in the frequency and intensity of storm-induced basin turnover. Bioturbated beds of the Upper Cretaceous Niobrara Formation (Colorado) are characterized by four oxygen-related ichnocoenoses, the distribution of which reflects cyclic variations in redox conditions. Relationships between paleo-oxygenation and organic-carbon and carbonate contents, and estimated cycle periodicities, suggest that redox variations were controlled by wet-dry climatic cycles modulated by the Milankovitch cycle of axial precession. Bioturbated beds within slope and basinal facies of the Miocene Monterey Formation (California) are variable in character, reflecting differences in duration and magnitude of associated oxygenation episodes, and may be in response to short-term variations in wind-stress-induced upwelling and/or ice-volume-controlled eustatic sea level changes.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990