Abstract

High resolution chemostratigraphy of the Montney Formation across Alberta identifies three unique and distinct chemostratigraphic units, referred to as CS1-3, with several subunits (i.e. CS1a, 1b, 3a, 3b). The lowermost two units (CS1-2) are mostly monotonous, finely laminated dolomitic “siltstones” with low geochemical (e.g. element, ratio, factor) variability except for CS1b, where both sedimentary and geochemical variability is present. The overlying units (i.e. CS3a-3b) contains dolomitic siltstones with sandy beds that show abundant sedimentary variability, pervasive bioturbation and abundant geochemical variability. This study focuses on the lowermost units (CS1a, 1b, 2) of the Montney Formation in Alberta, deposited during the Triassic Induan Period and proposes a new depositional model that largely contradicts the current depositional models of a basinal turbidite system. Evidence for a new model are derived from geochemical, petrographic and SEM data. Geochemical data (i.e. Si/Ti, As/Zr ratios, etc) suggest that the lower chemostratigraphic units (CS1-2) had minimal terrestrial influx and geostatistical analysis infers three distinct sources during the time of deposition, i.e. 1) marine benthic carbonates; 2) planktonic siliceous ooze; and 3) terrestrial material. Petrographic/SEM analyses of CS1-2 corroborate the geochemical findings revealing the dominance of authigenic minerals of quartz, dolomite, feldspars and pyrite, with few detrital grains of quartz, mica, feldspars and clay. Hence, the depositional model for the Lower Triassic Montney Formation in Alberta proposes a back-arc, semi-restricted shallow carbonate shelf with prolific benthic/planktonic shell productivity due to limited species diversity following the Permian-Triassic extinction event and limited terrestrial influx (varies locally). High benthic biogenic productivity suggests oxic bottom water conditions and possibly elevated salinity (to limit species diversity) that led to excess NaCl in the pore waters after deposition. Trace elements and total organic carbon (TOC) contents support oxic to sub-oxic sea water conditions, contradicting previous hypotheses of bottom water anoxia. Mineralogical, geochemical and organic carbon data also suggest limited influx of terrestrial and volcanic material. Since the majority of the CS1-2 sediments examined in Alberta display finely laminated dolomitic siltstones, diagenetic processes that generate dolomitic quartz-feldspar laminae from initial bedded shell debris are elaborated upon. A new proposal for the diagenetic pathway is based on findings from isolated carbonate concretions that demonstrate how in-situ porous biogenic debris (metastable aragonite, high Mg-calcite, biogenic silica and rare detrital and organic matter) can transform diagenetically during burial to “pseudo-laminated dolomitic siltstones”.