THE MASHIPICONG MBR. OF THE UPPER SILURIAN OF SOUTHEASTERN NY AND NORTHWESTERN NJ: A SHALLOW WATER BLACK SHALE

The Mashipacong Mbr. of the Rondout Fm. of northwestern NJ and southeastern NY, sandwiched between the Whiteport Mbr. of the Rondout Fm. below and the Thacher Mbr. of the Manlius above, was defined by Epstein et al., 1967, and consists primarily of medium-dark-gray shale and calcareous shale interbedded with light- to medium-dark-gray very fine-grained, thin-bedded limestone and argillaceous limestone, often exhibiting a ‘curly’ bedding. The unit ranges in thickness between a little over 2m in SE-NY to over nearly 5m in central NW-NJ. The northern-most positively identified outcrop of the Mashipacong Mbr. is at Cuddebackville, NY. North of here, the unit is not identified as part of the Rondout and is most likely correlative to the lowermost beds of the overlying Manlius Fm., the ‘Curley Beds’ of Logie (1933), ranging from 0 to >0.5m in thickness. Epstein et al. assigned this interval in NJ to the Rondout based upon similarity of lithology to the Rondout Fm. below as opposed to that of the overlying Manlius Fm. This assignment differs from the historical inclusion of beds of a similar lithology to the Mashipacong Mbr. that comprise the lowermost portion of the Manlius Fm. in eastern NY. It is herein suggested that the Mashipacong Mbr. be transferred to the Manlius Fm. in its type area of NW- NJ, as well as the extension of the name northward into NY to at least the Thacher Park area southwest of Albany.

The Whiteport Mbr. is a very fine-grained, laminated, white to buff weathering dolostone, often preserving abundant desiccation cracks and likely represents a supratidal environment. The Thacher Mbr. of the Manlius Fm. is dominated by blue-gray, flaggy-bedded, fine-grained fossiliferous limestones that display cyclic repeated shallowing-upward meter-scale sequences representing shallow sub-tidal to open water lagoonal environments. The lithology of the intervening Mashipacong Mbr. in both NJ and NY is dominated by dark-gray, often laminated shales or argillaceous limestones and represents a unique, very shallow-water depositional environment, likely reflecting restricted circulation in a lagoonal environment. This lithology is a prime example of a time-specific facies that is preserved over a wide geographic area and may represent a short-lived interval of restricted water circulation and warming within the basin.