ECOSPACE UTILIZATION IN EARLY PHANEROZOIC DEEP-MARINE ENVIRONMENTS: DEEP BIOTURBATION IN THE BLAKELY SANDSTONE (MIDDLE ORDOVICIAN), ARKANSAS, U.S.A

The middle part of the Blakely Sandstone (middle Ordovician) near Hot Springs, Arkansas is mudrock-dominated; alternations of dark-, and light-colored layers, each with an equilibrium palaeoichnocoenosis, represent semi-continuous accumulation of sediment under fluctuating oxygenation levels. The palaeoichnocoenosis emplaced under variable, but low, oxygen levels during deposition of the dark-colored layers comprises a mixed layer ichnofabric overprinted by small diameter transition layer burrows. The transition layer infauna was tiered with abundant Chondrites representing the deeper of two shallow tiers. Light-colored 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 400mm), 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); the more efficient use of this space 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.