Palaeoenvironmental and palaeoceanographic controls on black, laminated mudrock deposition: example from Devonian–Carboniferous strata, Alberta, Canada

Abstract

Abstract Sedimentological, palaeontological and ichnological analyses indicate that heightened episodes of equatorial upwelling, in tandem with an expanded and intensified oxygen minimum zone, promoted deposition of upper Famennian to lower Tournaisian laminated, organic-rich mud rocks of the Exshaw Formation on the Alberta cratonic platform. Exshaw mud rocks represent the culmination of an upper Devonian transgression that commenced with deposition of open marine carbonates of the Big Valley Formation. The transgression was abruptly halted at the Devonian–Carboniferous boundary by a rapid eustatic sea-level fall and the consequent deposition of near shore marine to shelfal bioturbated siltstones and sandstones of the Exshaw Formation. The siltstones grade into laminated, organic-lean mud rocks to the north in more distal offshore shelf settings. These mudrocks reflect a period of low primary production but continued anoxia of bottom waters. Continuation of anoxic conditions resulted from development of restricted circulation in the epicontinental sea due to the establishment of a westward-located physical barrier. Following this event was a relative sea-level rise that led to deposition of black, laminated mudrocks of the Banff Formation.