A HIGH STAND SYSTEMS TRACT IN THE “PERITIDAL” MANLIUS FORMATION: HARDGROUNDS AND TEMPESTITES IN THE GREEN VEDDER MEMBER (MANLIUS FORMATION, HELDERBERG GROUP) OF NEW YORK STATE DELINEATE PRE-ACADIAN BASIN CONFIGURATION

The Green Vedder Member (GVM) of the Manlius Formation (Prídolí to Lochkovian) records a subtidal, highstand systems tract in a unit (Manlius Fm.) that has been nearly synonymous with peritidal deposition (Wilson, Ebert and Matteson 2011). Throughout its extent, the GVM rests on the Clockville Unconformity, which signals the preceding transgressive event. At Van Leuven Lake (VLL) in New York’s Hudson Valley, a few bored phosphatic nodules on a pervasively mud-cracked bed of laminated, dolomitic mudstone at the top of the peritidal Thacher Mbr. of the Manlius Fm. mark the Clockville Unconformity.

The GVM at VLL comprises 3.3 m of multiple, stacked hardgrounds that are developed mainly in carbonate mudstones. Coarse, skeletal packstones and grainstones fill sharp-sided pits, dissolution cavities and syndepositional fractures in the mudstones. Phosphatized stromatoporoids and nodules on some hardgrounds are further indicators of sediment starvation. Borings and small colonies of ramose bryozoa ornament some hardgrounds and phosphatized material.

Common elements of the GVM fauna at VLL include loboliths and skeletal debris of scyphocrinitids and current-aligned orthocone cephalopods. Some beds display shell pavements comprised of bivalves, high-spired gastropods, ostracodes and bryozoa. Reoriented geopetal fabrics and micritic internal sediments that contrast with surrounding grainstone indicate exhumation and redeposition of many cephalopod and gastropod conchs and some loboliths.

Sedimentologic, taphonomic and faunal evidence suggest that the GVM at VLL is stratigraphically condensed and represents a relatively shallow setting within the HST. Episodic storms scoured hardgrounds and reworked the mollusk-dominated fauna. To the west, proximal carbonate tempestites with minor hardgrounds comprise the GVM in the Schoharie Valley. Tempestites that are more distal occur in the GVM in central New York. This facies gradient indicates that the basin axis was in central New York during the latest Silurian and earliest Devonian. With the onset of the earliest phase of Acadian tectonism, the axis of the basin shifted eastward into the Hudson Valley and was accompanied by Plinian volcanism during middle Helderberg time (New Scotland Formation).