SEDIMENTOLOGY OF THE UPPERMOST COEYMANS FORMATION AND LOWER KALKBERG FORMATION (LOCHKOVIAN, HELDERBERG GROUP) ON I-88 NEAR COBLESKILL, NY

Along I-88 near Cobleskill, NY, the uppermost 4.5 m of the Ravena Mbr. of the Coeymans Fm. comprise fine to coarse biosparites with abundant holdfasts and plates of the cystoid Lepocrinites gebhardi. Disarticulated brachiopods and bryozoans are abundant with less common tabulate corals and trilobites. Vaguely nodular bedding suggests extensive bioturbation. However, remnant ripples and cross-stratification, winnowed textures and a predominance of benthic filter feeders indicate frequent wave or current agitation on shallow portions of the Helderberg ramp.

The Coeymans is separated from the Kalkberg Fm. by the Punch Kill Unconformity (PKU). The PKU comprises a condensed zone of nodular, well-sorted, echinoderm biosparites capped by an irregular, pyritized surface which provide a record of sediment starvation and stratigraphic condensation prior to Kalkberg deposition.

The PKU is overlain by a distinctive bed of bioturbated calcisilt, which is followed by nearly 1.5 m of cm to dm-scale, shaly to fossiliferous micrites with cm interbeds of poorly washed biosparites to packed biomicrites. Brachiopods dominate and many are in life position. Diverse bryozoans, trilobites, rugose corals, echinoderm debris and rare orthocones are also present. The shaly micrites represent the background sediment on a well-ventilated section of the ramp which was below normal wave base. Coarser beds are shell lags concentrated by storms that episodically winnowed mud from the lower ramp. This unit is abruptly terminated by an episode of dysoxia associated with rapid deepening represented by dark, pyritic, shaly micrite with a single interbed of fossiliferous micrite. Volcanic contributions in the lower shale are evidenced by rare euhedra of zircon and apatite.

The remaining ~2.5 m of the Kalkberg comprise thick beds of fine, echinoderm-dominated biosparites to packed biomicrites, with large, tuberous, cirri-bearing holdfasts of Mariacrinus stoloniferous. Bryozoans, brachiopods and small rugose corals are common. Most of the fauna is disarticulated, but some crinoids show minimal dispersal of ossicles. These strata accumulated on an intermediate portion of the Helderberg ramp, which was deeper than the upper Coeymans Fm. but shallower than underlying lower Kalkberg.