MIXED SILICICLASTIC-CARBONATE DEPOSITION ON A LOWER CAMBRIAN COASTLINE: HOTA / PEYTO FORMATION, GOG GROUP, ALBERTA, CANADA

The Gog Group (Lower Cambrian) comprises a thick succession (up to 3000 metres) of predominantly clastic rock. It is subdivided into four main formations (McNaughton, Mural, Mahto & Hota / Peyto fms), the upper two of which are carbonate-rich. This paper is focused on mixed siliciclastic-carbonate strata in the upper units of the Gog Formation (the Hota-Peyto Member) in the Whirlpool Point area, west of the Kootenay Plains & north of the David Thompson Highway, Alberta, Canada. Sandstone units in the study interval are dominantly fine to medium-grained with medium & coarse-grained beds & laminae. Large-scale cross-beds; herringbone cross-strata; bi-directional current ripples & numerous reactivation surfaces characterize clastic units. Channelized scours, up to 3 metres deep, locally incise clastic sandstone beds occur interbedded with carbonate-rich sandstone beds. These beds are typically fine-grained to medium-grained with abundant out-sized carbonate grains (up to 1 mm x 3 mm) & range in the overall proportion of carbonate grains to clastic grains from ~<1:2 to >2:1. The carbonate grains observed in the study area include locally abundant oncoids (up to several cm across), ooids & fragmentary / abraded skeletal debris (including mollusc, archaeocyathid & possible brachiopod fragments). Carbonate-rich beds also typically exhibit large-scale cross-stratification, along with bi-directional current ripples & reactivation surfaces. Both clastic-dominated & carbonate-dominated end-members are interpreted to have been deposited in tidally influenced intertidal to shallow subtidal depositional settings. The scale of the bedforms, the occurrence of numerous reactivation surfaces, & the scale of localized channel units indicates a mesotidal or macrotidal setting. Mixing of carbonate & clastic sediments within the study interval is interpreted to be a function of variable sediment delivery to the coastline. A high proportion of clastic sediment indicates deposition proximal to a fluvial point source. Increased proportion of carbonate sediment likely occurred when the clastic depocentre shifted away from the study area, allowing development of carbonate banks, & increased preservation of reworked abraded / fragmentary skeletal debris in cross-stratified sandstone units.