Limited Ichnologic Fidelity and Temporal Resolution in Pelagic Sediments: Paleoenvironmental and Paleoecologic Implications

Abstract

The potential of ichnofabrics to yield information on endobenthic communities, organism behaviors, and paleoenvironmental conditions is widely recognized among sedimentary geologists. However, the extreme range in both ichnologic fidelity and temporal resolution of ichnofabrics is commonly not fully appreciated. Ichnofabrics developed in slowly deposited pelagic sediments, such as the Cretaceous Demopolis Chalk (U.S. Gulf coastal plain), represent one extreme. Ichnofabrics in the Demopolis Chalk, preeminently expressed only at transitions between marls and chalks, are time-averaged composites that formed over periods exceeding 8 kyr. Ichnologic fidelity is low; distinct biogenic structures reflect only the work of elite deep-tier tracemakers. Moreover, modeling that employs densities of elite burrows and burrow systems, sedimentation rates, and conservative estimates of trace-maker life spans indicates that preserved ichnofossils likely represent less than 10% of the time recorded in associated host sediments. Limited temporal resolution and completeness of these and comparable ichnofabrics call for caution in paleoenvironmental and paleoecologic interpretations and have implications for understanding behaviors, spatial distributions (patchiness), and vertical segregation (tiering) of tracemakers in marine pelagic substrates.

True substrates: The exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces

“…level of the preserved bedding plane [for example, as in a scenario of slow sedimentation with continual overprinting and reworking by bioturbation (e.g. Savrda, 2014)]. Examples of palimpsest bedding planes are shown in Fig.…”

“…However, palimpsest bedding planes are considered to be those which either: (i) host signatures of states of D, E or S, that were demonstrably non‐contemporaneous with the synoptic topography of the bedding plane of interest, extending from above or below beds; or (ii) are completely overprinted with signatures of states of D, E or S that developed after the active lithic surface had migrated above the level of the preserved bedding plane [for example, as in a scenario of slow sedimentation with continual overprinting and reworking by bioturbation (e.g. Savrda, 2014)]. Examples of palimpsest bedding planes are shown in Fig.…”

“…The degree of overprinting of multiple generations of burrows can also be used to estimate the relative persistence of stasis (e.g. Wheatcroft, 1990; Bentley et al ., 2006; Gingras et al ., 2011; Savrda, 2014) with the caveat that bioturbation is rarely pervasive across the entire spatial extent of a depositional environment, so overprinting is not inevitable (Miller & Smail, 1997; Dashtgard, 2011; Marenco & Hagadorn, 2019).…”

Animal–substrate interactions preserved in ancient lagoonal chalk

“…Ichnodiversity in shelf and deep-sea chalk is typically relatively low. However, this low level may reflect taphonomic overprint due to the extremely high bioturbation degree in these deposits 108 , resulting in ichnofabrics of low fidelity showing a preservational bias towards elite trace fossils 144 . Intense bioturbation may have allowed preferential preservation of deep-tier trace fossils, preventing preservation of those emplaced in shallow tiers, therefore artificially decreasing ichnodiversity 145 .…”

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

“…In such settings, colonization only takes place during short periods of reduced hydrodynamic energy (i.e., short-term colonization windows) (Pollard et al 1993;Desjardins et al 2010Desjardins et al , 2012. On the other hand, under slow accretion at the sea bottom due to suspension fallout, such as in offshore and shelfal settings, the colonization window is typically continuously open (Buatois and Ma ´ngano 2011), although the temporal resolution of the ichnofabrics could be limited (Savrda 2014). All in all, the variable of time seems to be intrinsic to the concept of the colonization window.…”