The Middle Montney — A Dirty Old Carbonate Formation With a Secret Past?

Abstract

In sedimentary research, the ultimate goal is to identify paleodepositional environments, levels of sediment compaction, and diagenetic history from the time of sediment deposition, through burial, until sampling. The Lower Triassic Montney Formation of the Western Canadian Sedimentary Basin is generally interpreted as a sequence of interbedded sand-, siltstones and shales deposited in a shallow, clastic ramp setting that deepened to the west. Although geographically localized coquina units are recognized within the middle member, an overall siliciclastic depositional model is assumed. Here we present new evidence from the Middle Montney of British Columbia illustrating that the depositional model needs to be revised. Ubiquitous early diagenetic concretions containing abundant carbonate material and moderately abraded, likely monospecific, epifaunal bivalve shell fragments indicate a limited amount of sediment transport and reworking. These concretions contain minimal siliciclastic material; and in some, albite feldspars are completely absent. Based on observations of sediment drape, the Middle Montney Formation has lost ∼35-65% of its initial sediment thickness (dissolution, compaction). We argue that the original Middle Montney sediment represented classical carbonate sediments (bioclastic carbonate muds) prior to burial and onset of diagenetic alteration. During burial, subsequent pore fluid migration and mineral alteration, calcite/aragonite and biogenic silica (>80% of original sediment composition) were transformed into dolomite, quartz, feldspar, and clay minerals. This mineral composition of the Middle Montney, has led to incorrect classification as siliciclastic deposits. Rather than accumulation through depositional processes, high levels of compaction/dissolution and mineral neoformation during burial concentrated the organics, detrital minerals, and authigenic silicates and dolomite observed in the Middle Montney. These marked diagenetic alterations and textural transformations are the result of burial diagenesis under the influence of hydrothermal fluids. This presentation dissects the Middle Montney Member in detail using petrographic, mineralogical and scanning electron microscopy evidence. A new depositional and diagenetic model of a Middle Montney carbonate-dominated shelf margin is also presented. This new model may have significant exploration implications as industry continues to search for new targets within the Montney Formation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016