Paleoecology of microbial-rich Late Frasnian carbonate slope buildups, Western Canadian Sedimentary Basin

Abstract

The late Frasnian Nisku Formation in subsurface Western Canada hosts the last known episode of carbonate buildup in the Cynthia-Winterburn basin below the Frasnian-Famennian boundary. The Zeta Lake Member is composed of dozens of carbonate buildups around the basin, although many compose a NE-SW oriented structure informally known as the Pembina Pinnacle Reef Trend. Four paleoecological communities are present in these late Devonian reefal environs: coral-pelmetazoan-microbial, coral-stromatoporoid-bivalve, coral-stromatoporoid-microbial, and stromatoporoid-microbial. Paleoecological communities differ principally by function and substrate colonization. The majority of buildups accreted in two growth phases under similar oligophotic to mesophotic conditions but different hydrodynamic regimes. Phase I accumulated during transgression and highstand as a mud-mound core with a low diversity coral motif. Phase II accumulated during falling sea level as a reef. Framework facies are composed of i) sub-cm scale rugose colonies with polymuds that approximate a bank meadow, or ii) carbonate crusts with synsedimentary cement and microbial biolithites with relatively minor contribution from metazoans that mimic interior reef strata. Fine-grained sediment that infills boundstones is grouped into four mud microfacies. Mud 1 and Mud 2 are reworked biogenic, resembling firmgrounds, with Mud 1 being microbial-enriched and low diversity while Mud 2 has highest skeletal diversity and terrigenous siliciclastics. Muds 3 and 4 are interpreted organic framework that could support stromatactis and create shelter cavities. Generally, Mud 3 has tubular calcimicrobes and is found in all coral-spongiostromate facies, whereas Mud 4 has dendritic calcimicrobes and increases within stromatoporoid facies relative to coral facies. These microfacies confirm a microbial-origin for fine-grained carbonate within late Frasnian buildups. Hyperspectral analysis in the short-wave infrared range is used to characterize four key buildup components: micrite, microbial carbonate (Renalcis), skeletal carbonate and pore-filling cement. Segmentation by threshold method reveals that the absorption feature strength of water and calcite are sufficient to distinguish target components. Renalcis has very high raw reflectance attributed to very fine crystal size and is readily distinguishable from other carbonate components. This study demonstrates the feasibility of short-wave infrared reflectance spectroscopy to isolate and potentially quantify microbial carbonate in compositionally similar ancient reef strata.