A Multidisciplinary approach to facies analysis and paleoenvironmental reconstruction of the Cretaceous Second White Specks Formation, Eastern Margin of the Western Interior Seaway, Canada

Abstract

The Cenomanian-Turonian strata of the Second White Specks (2WS) Formation on the cratonic margin of the Western Interior Seaway (WIS) are comprised of organic-rich, carbonate-siliciclastic successions. This study focuses on a carbonate-siliciclastic succession of the 2WS in two cored wells located on the eastern margin of the Cretaceous Interior Seaway. Based on sedimentary facies and organic/inorganic geochemistry representing distal and proximal sections of the 2WS, it was possible to interpret factors, including productivity, preservation, and dilution of the Organic Matter (OM). The results revealed eight subfacies and three main facies associations, including deep shelf calcareous mudstones and marlstones, and shallow shelf bioclastic limestones. The carbonaceous mudstones have high type II immature kerogen content and the bioclastic limestones have low organic content dominated by type III and degraded type II kerogen. The 2WS mineralogy primarily consists of calcite, clay minerals, and quartz, and is characterized by a low degree of physical and chemical diagenesis. Based on mineralogical analysis, carbonate is the dominant component, and its proportion is inversely associated with terrigenous silica content. There is a general increase in carbonates and a decrease in siliciclastics from the distal to the proximal sections within the study area. Lateral changes in physical and chemical properties of similar facies associations are mainly controlled by changes in water depth. The 2WS was deposited during a long-lived transgression comprised of higher order parasequences. Core logging data from multiple drillholes show that these parasequences have a general coarsening upward trend with an upward shift from pelagic (i.e., planktonic foraminifera and coccoliths) to benthic (i.e., Inoceramus) carbonate productivity. During deposition of the mudstone/marlstone facies association, a clear and oxygenated water column allowed for a high photosynthetic activity to occur within the photic zone, which is reflected in OM enrichment and higher micronutrient content, such as Zn and Ni, predominance of fecal aggregates, and planktonic foraminifera. In the mudstone/marlstone facies, oxygen deficient pore water conditions, which are reflected in the high redox-sensitive trace element concentrations such as V, contributed to the high degree of OM preservation. The 2WS along the eastern margin of the WIS was deposited on a low gradient carbonate ramp environment. The oxygenated broad ramp accommodated the bioclastic limestones, which are characterized by high benthic carbonate dilution rate that resulted in lower OM preservation potential. The shallow shelf setting was characterized by restricted wave energy and minimal fluvial input from the adjacent craton under temperate climatic conditions. This study contributes to our understanding about paleo-oceanography and hydrocarbon exploration in analogous epeiric seas and mixed siliciclastic‑carbonate marine settings around the world.