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Mudstone Aggregate Origins and Depositional Interpretations of the Second White Specks and Carlile Formations in Eastern Alberta

Abstract

Mudstone aggregates form the main detrital component of mudstone-dominated strata of the Upper Cretaceous Second White Specks and Carlile Formations. The formations were deposited in very different settings within the Interior Cretaceous Seaway and are separated by an unconformity. The Second White Specks (2WS) Formation is comprised of organic rich mudstones with a large content of calcareous macro- and microfossils, and at several levels calcareous fine-grained sandstone beds. It varies from 5–7% TOC and is dominated by type II and III kerogens. The 2WS is an established target for biogenic oil and gas production but a thorough understanding of the complex stratal architecture remains challenging and ongoing. In contrast, the Carlile Formation is comprised of intensely bioturbated non-calcareous mudstones with a variable content of silt- and sand-sized silica grains, with the strata forming 20–30m tall mudstone clinoforms in contrast to the tabular stratal architecture of the 2WS strata. The mudstone aggregates are well preserved in the studied strata due to the relative shallow maximum burial of approximately 1500–2000m. The aggregates occur as silt- to sand-sized particles with different composition, with the oval shape indicating they are only slightly compacted and that they were semi-indurated at the time of deposition. Potential origin of the mudstone aggregates includes extrabasinal grains, and/or intrabasinal rip-up clasts, or crustacean micro-coprolite fragments. However, the presence of coccolith fragments within the aggregates clearly demonstrates that the majority of the mudstone aggregates are intraformational, however their variable composition from the surrounding matrix mud suggests transport a significant distance from their site of origin. Furthermore, significant abrupt vertical changes in the grain size of the mudstone aggregates and their chemical composition shows that the area of origin changes between dominantly intra-formation rip-up clasts to micro-coprolites, relating to relative sea level changes. However, as individual sequence stratigraphic units often have relative uniform composition of the mudstone aggregates, composition of the mudstone aggregates do not seem to be strongly related to water depth but rather changes in circulation within the Interior Seaway.