PALEOENVIRONMENTAL IMPLICATIONS OF AN ORDOVICIAN TRACE FOSSIL ASSEMBLAGE, DAUPHIN FORMATION, SOUTH-CENTRAL PENNSYLVANIA

The first marine ichnofossil assemblages reported, herein, from the Dauphin Formation in south-central Pennsylvania were collected from several localities in allochthonous rocks in the Hamburg sequence. While graptolites and conodonts from the Dauphin Fm. and elsewhere in the sequence have assisted in constraining ages, from Late Cambrian to Middle Ordovician, the Dauphin lebensspuren provide a previously unstudied record of paleoecological activity at the sediment-water interface of a trench slope. Heteractinellid sponge spicules, mostly disarticulated, also are common. The assemblage yields valuable paleoenvironmental information that helps in interpreting the succession of depositional environments in the sequence and regional Taconic orogenic models.

The ichnofossil assemblage includes more than six different forms: a nereitid meandering/looping horizontal trace; multiple asymmetrical and symmetrical branching forms (Chondrites); an unbranched solitary two-furrowed trail; abundant straight horizontal burrows (~2 mm wide); straight and meandering non-branching traces (5-10 mm wide) (Planolites); and sediment and fecal pellet infilled burrows (Tomaculum sp.). Well-defined vertical burrows are absent in the assemblage.

These fossils are representatives of the well-known Nereites ichnofacies, a diverse association of traces thought to be produced by deposit feeders or scavengers. This ichnofacies typically characterizes deep ocean/turbidite depositional settings and is consistent with other sedimentological data gathered from the same outcrops. These data indicate that sediments are typically bathyal to abyssal, deposited in quiet, oxic to dysoxic waters.

Marine trace fossils are important facies indicators because they are not reworked into younger strata, indicate water depth, and show rapid response to changes in the local environment. These fossils therefore represent a valuable source of paleoenvironmental data for incorporation within Taconic orogenic models of Pennsylvania and other mid-Atlantic states.