HOW DEEP WAS IT? WHEN A BENTHONIC FORAMINIFERAL BIOFACIES MODEL LEADS US ASTRAY

Organisms are typically sensitive and thus adapted to a number of environmental factors, many of which may correlate with water depth. Therefore, fossil macro- and microfossil benthos have been widely used as paleobathymetric indicators, commonly based on applying modern depth ranges to interpretations of the rock record. Yet, it has been also recognized that 1) organisms may evolve and adapt over geologic time, and 2) water depth is only one, and in fact generally not an independent, factor controlling the living range of benthonic foraminifera and other benthos. The classic benthonic foraminiferal biofacies (BFB) model of Ingle (1980), for example, was evaluated by Lagoe (1988) to show that some “lower bathyal” foraminifera were present in outer-shelf to shelf-edge depths in the Eocene Tejon Formation of California, based on both stratigraphic and sedimentologic data.

Other examples of Paleogene shelf and deltaic strata bearing anomalously “deep” (middle to lower bathyal, 500 to >2000 m) foraminifera include the Oligocene Blakeley and Lincoln Creek formations of Washington and the Eocene Coaledo Formation of Oregon. The lower member of the latter, a well-studied delta complex, was recently re-interpreted by McDougall (2023)* as a submarine fan deposit, based on a slightly revised (from Ingle, and including Lagoe’s work) BFB model. However, virtually all indicators of depositional environment except for marine microfossils as interpreted by McDougall (2023) -- physical sedimentary structures, molluscan macrofauna, trace fossils, and not least the lateral presence of coal -- indicate the lower Coaledo Formation is deltaic (estuarine - neritic - upper bathyal). Indeed, the lower Coaledo Formation may be considered a type example of stacked parasequences in a prograding delta system and does NOT exhibit any characteristics of a submarine fan. We argue that, with McDougall’s data, the lower Coaledo is a prime case for revising the current BFB model. Without further amendments to the model, there will continue to be a legacy of confusion and anomalously deep paleowater depths unreconciled with sedimentologic and stratigraphic context, resulting in incorrect paleogeographic reconstructions.

*McDougall, K., 2023. Micropaleontological evidence of a submarine fan in the lower Coaledo Formation, southwestern Oregon, USA: J. Foram. Res. 53(4), 311-337.