Paleobathymetric implications of microborings in Tertiary strata of Alaska, USA

Microboring algae, bacteria and fungi that lived under non-tropical conditions, are abundantly present in four richly fossiliferous marine strata of Paleocene to Miocene age in Alaska. By comparing their boring-trace communities with endolithic guide ichnocoenoses characterizing various photic zones, which have been previously defined mainly in tropical and subtropical environments, the Alaska strata can be placed in bathymetric zones as follows: the lower Upper Paleocene Prince Creek Formation in the dysphotic zone, the Lower Oligocene Stepovak Formation in the shallow euphotic zone (shallow euphotic zone III), and the Upper Miocene Tachilni Formation in the deep euphotic zone. These interpretations agree very well with bathymetric reconstructions inferred from other criteria. A fourth formation, the lower Middle Miocene Narrow Cape Formation, contains only heterotrophic microborers as if it had been deposited in the aphotic zone, in contrast to previous reconstructions. In this formation the periostracum of the mollusc shells is exceptionally well-preserved. This may explain the absence of boring algae, because only heterotrophs can penetrate through organic shell layers. The absolute depth ranges of these ichnocoenoses zones are inferred from their ranges in comparable latitudes of the modern North Atlantic. The most important features of bathymetrically controlled tropical and subtropical guide ichnocoenoses are also present at the non-tropical Alaskan sites, although some boring algae that are widespread in tropical realms are absent.