Using ichnological relationships to interpret heterolithic fabrics in fluvio‐tidal settings

Abstract

In fluvio-tidal settings, the sediment is dominantly derived from the river systems. However, the importance of landward tidal transport of sediment in tidally influenced sedimentary environments is difficult to assess, particularly in the rock record. This problem is addressed using two intervals within the Lower Cretaceous McMurray Formation, each representing a distinct inclined heterolithic stratification motif. The ichnological variation between the heterolithic intervals is analyzed to determine which lithosomes are associated with brackish-water (tidally influenced) colonization windows. From this, the relative fluvial influence responsible for the deposition of the fine and coarse members can be determined. Both of the inclined heterolithic stratification fabrics studied record the deposition of fluvio-tidal point bars wherein the heterolithic bedding represents variations in river discharge. The first fabric comprises inclined heterolithic stratification in which bioturbation only occurs in mudstone beds. This fabric indicates that deposition occurred in more proximal positions within a fluvio-tidal system (i.e. the outermost inner to middle estuary or distributary channels). In this example sand deposition is interpreted to represent high-energy, freshwater dune migration within a fluvial-dominated setting, whereas mud beds reflect brackish-water suspension deposition during times of low river discharge. The second fabric, which is interpreted to have developed in more distal depositional positions (i.e. the middle estuary or seaward of the turbidity maximum in deltas), consists of inclined heterolithic stratification with laminated mudstone and bioturbated sandstone. In these inclined heterolithic stratification successions the mudstone beds were deposited under the influence of freshwater and heightened sedimentation rates, whereas bioturbated sandstone was colonized under brackish-water conditions and in the presence of tidally facilitated sediment transport. In both examples, the bioturbated lithosomes are related to colonization windows that indicate the predominance at that time of marine or tidally influenced processes over fluvial processes.

Sedimentological and ichnological variations in fluvio‐tidal translating point bars, McMurray Formation, Alberta, Canada

“…Facies in the translating point bars of the Long Lake area demonstrate significant lateral and vertical variations, interpreted to be the result of spatial and temporal changes in flow velocity, sedimentation rates and suspended-sediment concentrations. Previous work generally subdivided inclined heterolithic stratification (IHS) deposits into fluvially dominated IHS positioned landward of the turbidity maximum zone (TMZ), IHS deposits positioned centrally in the turbidity maximum zone (TMZ) and tidally dominated IHS deposits positioned seaward of the TMZ (see Dalrymple & Choi, 2007;Melnyk & Gingras, 2020). Sedimentological and ichnological descriptions that are assigned to fluvially dominated IHS are roughly similar to those of F3.…”

“…Bioturbated mud beds in IHS have been interpreted to record slow to moderate tide-dominated deposition of mud (see Melnyk & Gingras, 2020), which hosts marine nutrients and supports biogenic reworking. In F2B, bioturbated mud beds (M20-M22) (Fig.…”

“…The sand to sandy mud beds (M1–M3 and M18–M22) of F2 are interpreted to be tidally derived, whereas mud bed deposition is interpreted to be associated with seasonally elevated river discharge (e.g. Shchepetkina et al ., 2016; Melnyk & Gingras, 2020). Current ripple lamination and cross‐stratification in sand beds were formed under moderate current velocities in the lower flow regime, likely under transitional flow (TF) and turbulence‐enhanced transitional flow (TETF) near‐bed conditions (Fig.…”

Bioturbation, heavy mineral concentration, and high gamma-ray activity in the Lower Cretaceous McMurray Formation, Canada

“…Stratified sandstone and bioturbated mudstone have been interpreted to form within a fluvio-tidal system, precisely, in the inner to middle estuary or deltaic distributary channel. The deposition of sand occurred during high freshwater discharge episodes, facilitating dune migration, while mud accumulated during the periods of low discharge from tidally influenced, suspension-rich brackish water (Melnyk and Gingras, 2020). Mud flocculation and deposition were fostered along the seawater-freshwater interface (e.g., La Croix and Dashtgard, 2014).…”

Unlocking the Architecture of the Colonization Window: Ichnofabrics From Upper Cretaceous Tide-Influenced Meander-Loop Deposits

“…Ichnofabrics in deposits of meander-loop systems show a heterogeneous distribution, ranging from sparse bioturbation in point bars within the high-sinuosity channels to intense bioturbation in the deposits of the overbank areas (e.g., Ranger and Pemberton 1992; Smith et al 1993; Gingras et al 1999; Solórzano et al 2017; Gowland et al 2018; Rodriguez et al 2018; Shchepetkina et al 2019; Buatois et al 2020; Melnyk and Gingras 2020; Fietz et al 2021). This spatial heterogeneity reflects a variety of processes taking place in meander-loop systems and controlling the colonization window.…”