CORRELATION USING A SEDIMENTOLOGICAL MODEL OF PART OF HAMILTON GROUP (MIDDLE DEVONIAN) NEW YORK STATE

The Cooksburg Sandstone Member of the Moscow Formation is proposed and divided into 6 informal units. It is recognized to the north and south of the Catskill Front and also in the subsurface. The member is a tongue of 45 ft of shallow marine sandstones which are partially the lithological equivalent of the Portland Point Limestone of Cooper (1930). It records a transgression in which there were two phases of sealevel rise, an initial slow rise (phase I) followed by a rapid rise (phase II), occupying a brief interval of time in the Catskill depositional regression. During both these phases there was deposition of shallow marine sands and development of barlagoon couples. Immediately following the deposition of the Cooksburg Member, a further barrier bar-lagoon couple developed in the nearshore area. It is suggested that the Portland Point Limestone developed as a consequence of a decrease of suspended terrigenous detritus reaching the shelf-like area where the limestone formed. Detailed correlation (determination of time equivalence) of the Portland Point with nearshore shallow marine strata depends on the particular sedimentological hypothesis employed. Model I correlation is based on a decrease of suspended material which was caused by flood plain alluviation induced by the rapid rise of sealevel (phase II) deduced from the Cooksburg Member; the Portland Point is correlated with the Cooksburg. Model II suggests that the decrease of suspended material was caused by entrapment in the lagoon which followed the Cooksburg; the Portland Point is correlated with the barrier bar-lagoon couple. These two models are discussed with regard to the thickness trends they necessitate, the detailed stratigraphy of the units involved, and evidence from the Holocene transgression. It is concluded that both models are applicable. The Portland Point was initiated as a response to decrease of fines induced by the Cooksburg sealevel rise, and the effect was continued, but with increasing escape of sediment to the shelf area, by the lagoonal phase. The lagoon was a less effective sediment trap, and in areas nearer to shore Portland Point conditions ended before the destruction of the barrier bar-lagoon couple. While the base of the Portland Point is close to a time plane, the top is diachronous, becoming younger away from shore. An estimate is made of the precision of the correlation (order of 103 yrs) and the duration represented by phase II deposits of the Cooksburg plus the overlying barrier bar-lagoon couple (order of 105 yrs). This precision is better than that generally obtainable through use of fossils. The correlation of the Cooksburg with the Catskill redbeds is presented, and their sedimentologic relationship discussed.