Detailed facies analysis of Cenomanian–Turonian organic‐rich mudstones: Implications for depositional controls on source rocks

• Main Authors: Emma L. Percy, Per K. Pedersen
• Format: Article
• Language: English
• Published: Wiley 2020-06-01
• Series: The Depositional Record
• Subjects: Cenomanian–Turonian; mudstone; sedimentology; source rock
• Online Access: https://doi.org/10.1002/dep2.106

Abstract Understanding mudstone depositional processes, including both traction transport and dynamic mechanisms, requires a re‐assessment of how these processes relate to the accumulation and preservation of organic carbon in fine‐grained successions. This relationship was addressed using facies described in detail from a Cenomanian–Turonian aged, organic‐rich, marine mudstone‐dominated succession from the Albertan Western Interior Seaway. All facies exhibit a high degree of heterogeneity at the millimetre‐to‐decimetre scale, contain evidence of bioturbation and display similar ranges of sedimentary structures including wave ripples and current ripples, sharp‐based graded beds and starved ripples. Described facies vary in grain size as well as relative abundances of biogenic versus siliciclastic grains. Siliciclastic grains are dominantly composed of quartz silt as well as silt‐sized to sand‐sized clay mineral–rich aggregates while the biogenic grains are dominantly composed of silt‐sized to sand‐sized calcareous pellets, bivalve fragments and foraminifera. Although the succession is relatively clay mineral‐rich, the majority of clay minerals occur as mud aggregates that were transported by traction processes. The sedimentary structures present are diagnostic of sea floor reworking by waves and currents, indicating that the basin was relatively shallow (i.e. not hundreds of metres deep) with frequent reworking of the sea floor sediment with suspension settling deposits being rare. The abundance of bioturbation and in situ bivalves indicates that there was sufficient oxygen at the sea floor and that widespread bottom water anoxia was not persistent at the time of deposition of the organic‐rich interval of interest. The accumulation of organic‐rich sediment appears to be a preservational phenomenon caused by sediment being buried more rapidly than the organic carbon can oxidize in the surficial layers due to episodic inputs of siliciclastic material during large storm events.