Interpreting Heterolithic Fabrics Using Ichnological Relationships: Case Study From the McMurray Formation, Alberta, Canada

Abstract

In tidal-fluvial settings, the source of the sediment is typically considered to be the river systems. However, the importance of net landward tidal transport of sediment in these environments is difficult to assess. This problem is addressed using three well-preserved intervals within the Lower Cretaceous McMurray Formation, each representing a distinct heterolithic fabric. The ichnological variation between the seasonally-influenced heterolithic intervals is analyzed to determine which lithosomes are associated with brackish water (tidally influenced) colonization windows. From this, the relative fluvial influence between the fine and coarse members can be determined. Two of the fabrics studied are from fluvio-tidal bar deposits with the third coming from an interpreted prodelta setting. In all three cases, the heterolithic bedding represents variations in fluvial inflow and coarsely represents seasonal variations. The first fabric presented in this study comprises inclined heterolithic stratification wherein bioturbation is associated with the mud beds. In this example, sand deposition is interpreted to represent high-energy bedform migration within a fluvially dominated setting, whereas the mud beds reflect brackish-water suspension deposition during times of low fluvial inflow. The second fabric consists of inclined heterolithic stratification wherein the mud is laminated and unburrowed and sandstones contain bioturbation. In this case the mud beds are deposited under the influence of fresh water and heightened sedimentation rates while sandstone deposition reflects less energetic conditions and the resumption of tidal winnowing and landward transport of sand. The third heterolithic fabric is characterized by mud-dominated wavy bedding with burrowing limited to thin (<2cm) sandstone interbeds and bioturbated mud tops. In this interval, river-derived prodeltaic mud is punctuated by sand interbeds that result from a reduction in river inflow and the ensuing dominance of marine processes. In all three cases, biogenic reworking is related to colonization windows that indicate the predominance at that time of marine- or tidally-influenced processes over fluvial processes.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019