Stratal Architecture and Stratigraphic Evolution of Ancient Mixed Siliciclastic-Carbonate Slope Deposits of the First Isaac Carbonate, Neoproterozoic Windermere Supergroup, Southern Canadian Cordillera

Abstract

The first Isaac carbonate (FIC) is a thick (~195 m) lithologically-distinctive and mappable mixed siliciclastic-carbonate deep-water slope succession that represents a key stratigraphic marker within the Neoproterozoic Windermere turbidite system, southern Canadian Cordillera. Detailed analysis of facies, stratal architecture and stacking patterns at two study areas separated by about 20 km in a dip direction provides useful insight into the interplay between carbonate sediment production-export, siliciclastic sediment supply linked to changes of relative sea level, and slope gradient. The FIC consists of three areally-extensive, mostly fine-grained calciturbidite-rich units intercalated with siliciclastic and mixed siliciclastic-carbonate, sheet-like and channelized units, plus local calcidebrites. Being sandwiched between siliciclastic-dominated slope deposits, the FIC is interpreted to record a major, long-term eustatic rise. However stratigraphically upward changes in grain size, mineralogy and architectural style are interpreted to result from changes in sediment supply controlled by short-term eustatic changes that modulated the stratigraphic response to the long-term trend. During the early stage of the long-term eustatic rise, the carbonate platform was becoming established and was connected to an inherited low-angle siliciclastic continental slope. Decameter-thick and up to kilometer-wide point-sourced slope channels were preferentially formed during short-term episodes of significantly lowered relative sea level. Channels were filled mostly with carbonate-cemented, quartzose and mixed carbonate-siliciclastic coarse-grained sandstone and conglomerate containing carbonate clasts derived from both interior and margin of the platform. Later during the long-term eustatic rise, the carbonate platform had expanded and prograded basinward, and as a result of early seafloor diagenesis, fronted a steeper submarine slope. Under these conditions episodes of short-term relative sea level change were characterized by an absence of coarse clastics and large channel complexes. Instead alternating m-thick packages of fine-grained siliciclastic turbidites or calciturbidites exported from a line sediment source dominate. Coarse clastic sediment was restricted to shallow, scoured-based features (cm- and dm-thick, few 100 m wide) filled mostly with medium- and coarse-grained palimpsest siliciclastic sand whose transport was facilitated by the steeper slope.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017