Ecospace utilization in early Phanerozoic deep‐marine environments: deep bioturbation in the Blakely Sandstone (Middle Ordovician), Arkansas, USA

Abstract

Ichnofabric analysis of alternating light and dark-coloured mudstone layers in the Blakely Sandstone (Middle Ordovician) at Crystal Springs Landing, Lake Ouachita (west of Hot Springs, western Arkansas, USA) reveals two equilibrium palaeoichnocoenoses. The first was emplaced under variable, but low, oxygen levels during deposition of the dark-coloured layers; small diameter transition layer burrows overprint a mixed layer ichnofabric. The transition layer infauna was tiered with abundant Chondrites representing the deeper of two shallow tiers. Light-coloured layers accumulated during prolonged intervals in which the sediments were oxygenated to a greater extent and depth. Preservation of a mixed layer ichnofabric within them is the result of limited, but deep (up to at least 400 mm), reworking subsequently in the transition layer by an equilibrium community. These transition layer trace fossils are not tiered. If representative of oxygenated sediment columns in Ordovician deep-marine environments, an extensive volume of infaunal ecospace was colonized (in this case by deposit feeders); its more efficient use subsequently, including vertical partitioning of the infaunal community into specific environmental niches (tiering), could have accommodated increases in diversity and community complexity. Changes over time in the maximum depth to which sediments were bioturbated, alone, would therefore be a poor measure of the extent of ecospace utilization.