The Bend Justifies the Leans: Interpreting Recumbent Ichnofabrics

Abstract

Abstract En echelon, bent burrow fabrics are commonly interpreted as an ethological response to high population densities or for filter feeding in conditions influenced by a dominant current direction. However, systematically bent burrows are more likely the result of simple shear on inclined depositional surfaces. A modern example of bent burrows is drawn from tidally influenced point bars situated near the mouth of the Shepody River Estuary, Canada. There the deposits are composed of interbedded silty mud and rare sandy mud beds that dip channelwards. The dipping bedsets are referred to as inclined heterolithic stratification (IHS). Throughout much of the area, the upper 5 to 20 cm of the substrate is unconsolidated and is to locally fluid. The fine sediment resists resuspension by the tidally generated currents, but it flows (plastically) down depositional slopes. Tension gashes, minor mud flows, and deformed biogenic sedimentary structures attest to consistent downslope shearing of the sedimentary substrate. Below the zone of mobile mud, the sediment is comparatively cohesive (firm). The change in cohesiveness is abrupt. Particularly at the to transition, sediment tends to creep downslope, straining the sedimentary fabric. Burrows that descend through the soft sediment into the comparatively firm substrate are noticeably bent at that contact. Burrows are predictably deformed due to down-slope sediment creep—i.e., they are deformed such that the basal part of the burrows point up dip, indicating downslope movement of the upper mobile layer. Burrow fabrics within the older, firm sediment (i.e., in the historical layers) are generally highly strained. Scrutiny of the modern and historical biogenic fabrics shows that: (1) active infauna display a preference to construct dwellings that are oriented perpendicular to bedding planes; (2) the presence of pristine, undeformed burrows in the active layers contrasts with pervasive deformation observed in the historical layers; (3) the temporal nature of the sediment creep occurs at a slower rate than the animal's ability to maintain the burrow structure, thus the strained burrows observed in the historical deposits were deformed after burrow abandonment; (4) the magnitude of strain varies between 1.5 and 2.1 to 1; (5) strains that exceed 2 to 1 essentially eradicate the biogenic fabric and represent a potential upper limit for the preservation of ichnofossils under similar conditions; (6) the recumbent-appearing ichnofossil assemblage in the historical layers does not specifically indicate a response to population dynamics or a preferred current direction.