Lithological analysis of the Lower Devonian vertebrate-bearing beds at the MOTH locality, N.W.T., Canada: insights to taphonomy and depositional setting
Sedimentary and faunal characteristics of the Lower Devonian Man On The Hill (MOTH) vertebrate-bearing succession, Mackenzie Mountains, Northwest Territories, Canada provide insights to the paleoenvironmental conditions. Based on sedimentological and petrographic analyses of a 6-m vertical section of the vertebrate-bearing strata, three depositional scenarios for the MOTH locality are proposed, the favoured interpretation being that the vertebrate-bearing interval was deposited on the outer margin of a carbonate platform within an intra-shelf topographic low below storm-wave base. This interpretation is supported by the presence of fine, alternating laminae of light grey argillaceous limestone, or calcareous shale, and dark grey silt to sand-rich calcareous shale, as observed in thin section, and the characterization of the lithology for the vertebrate-bearing strata as an interlaminated argillaceous limestone and calcareous shale. An abundant, low-diversity association of cryptic trace fossils, previously unrecognized at MOTH, is described. These trace fossils and the presence of pyrite suggest that the intra-shelf topographic sag at MOTH had restricted circulation and was generally hypoxic. Based on the instability of such a restricted environment, a cause of death for the cyathaspids Dinaspidella and Nahanniaspis is unclear. However, turbidite deposition and seasonal turnover can be eliminated as possible causes of death, as vertebrates were not located frequently enough within light or dark grey layers and minimal signs of skeletal abrasion or fracturing were observed in Dinaspidella and Nahanniaspis. The high preservation potential of the fossils at MOTH likely was enhanced by the hypoxic nature of the bottom waters within the intra-shelf topographic low below storm-wave base.