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Factors influencing the formation of organic-rich sedimentary facies: Examples from the Devonian-carboniferous Exshaw formation, Alberta, Canada Caplan, Mark Laurence

Abstract

A unique set of palaeo-oceanographic conditions is responsible for formation of Devonian-Carboniferous organic-rich, laminated mudrocks. These conditions are poorly understood as widespread development of mudrocks is not witnessed in modern oceans. Multidisciplinary research of the organic-rich Famennian-Tournaisian Exshaw mudrocks (lithofacies B₁) and subjacent Famennian Big Valley carbonate ramp deposits of the Alberta Unstable Cratonic Platform (AUCP) indicate that eutrophication of surface waters led to carbonate ramp demise and deposition of Exshaw mudrocks. The onset of eutrophication coincided with a relative sea level rise as Exshaw and Big Valley deposits onlap paleobathymetric highs developed on the AUCP. An abrupt relative sea-level fall at the D-C boundary is evidenced by progradation of nearshore marine shoreline cross-bedded sandstones, bioturbated siltstones and organic-lean mudrocks (lithofacies B₂) over distal epicontinental shelf organic-rich mudrocks. A second transgressive event is reflected by retrogradation of the shoreface in southeastern Alberta, culminating in deposition of organic-rich, laminated mudrocks of the Banff Formation. Sedimentology, ichnology, palaeontology and geochemistry (XRF, XRD, Ϩ¹⁵Ntot and Ϩ¹³Corg isotopes, FTIR and ROCK-EVAL) have been utilized to examine the genesis of organic-rich (lithofacies B₁and organic-lean (lithofacies B₂) laminated Exshaw mudrocks. Lithofacies B₁ displays high TOC's, lighter Ϩ¹⁵Ntot high HI and lower Ox.I. (ratio of oxygen functional groups to aliphatic organic compounds) indicating high primary production and supply of labile organic matter to the sea floor. Lithofacies B₂ which sharply overlies lithofacies B₁ in the distal shelf region of north-central Alberta, displays low TOC's, heavier Ϩ¹⁵Ntot low HI and higher Ox.I. indicating low primary production and a decreased supply of more refractory organic matter to the sea floor. Both primary productivity and degree of organic matter recycling in the water column were important in governing organic matter quality and quantity. Global distribution of laminated D-C mudrocks are equated with the Hangenberg mass extinction event. The development of black mudrocks and anoxic benthic conditions were restricted to low palaeolatitudes resulting in demise of carbonate platforms. Global cooling and resultant glaciation in Gondwana enhanced oceanic circulation, upwelling efficiency, generation and burial of carbon in marine sediments. These effects led to the development of anoxic benthic environments, mass extinction and production of globally distributed organic-rich mudrocks.

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