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

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 Formatio n on the Alberta cratonic platform. Exshaw mud rocks represent the culminat ion of an upper Devonian transgression that commenced with deposition of op en marine carbonates of the Big Valley Formation. The transgression was abr uptly halted at the Devonian-Carboniferous boundary by a rapid eustatic sea -level fall and the consequent deposition of near shore marine to shelfal b ioturbated siltstones and sandstones of the Exshaw Formation. The siltstone s grade into laminated, organic-lean mud rocks to the north in more distal offshore shelf settings. These mudrocks reflect a period of low primary pro duction but continued anoxia of bottom waters. Continuation of anoxic condi tions resulted from development of restricted circulation in the epicontine ntal sea due to the establishment of a westward-located physical barrier. F ollowing this event was a relative sea-level rise that led to deposition of black, laminated mudrocks, of the Banff Formation. (C) 2001 Elsevier Scien ce B.V. All rights reserved.