K-FELDSPAR RICH CLASTICS IN CAMBRO-ORDOVICIAN CARBONATE FACIES OF THE PINE PLAINS FORMATION, DUTCHESS COUNTY, S.E. NEW YORK

A detailed sedimentologic study of four meters of deformed dolomitic beds from the Cambro-Ordovician Pine Plains Formation of the Wappinger Group, East Fishkill, New York suggests deposition within tidal channel/channel bank, levee backslope/algal marsh, subtidal pond, lagoon, oolitic shoals/shallow shelf in a semi-arid environment. Siliciclastic detrital grains, ranging in size from clay to fine sand, consisting largely of K-feldspar, and minor plagioclase, quartz, and mica were found within almost all the dolomitic beds.

The distribution of the siliciclastic grains differed with each facies suggesting different deposition mechanisms. In subtidal and intertidal carbonate layers, clasts occurred as isolated, matrix-supported grains. In intertidal stromatolites and thrombolites the grains emphasize the respective laminated or clotted textures of the microbiolites. In supratidal carbonate environments, K-feldspar rich deposits occurred as clast supported 2 to 3 mm thick layers. Oxidized siliciclastic layers, 0 to 10 mm thick, found on intertidal and supratidal flats, contain clay size feldspars and silt size plagioclase and quartz grains. One 1-3 cm thick dark gray siliciclastic layer with internal graded laminae, clay size K-feldspar and silt size plagioclase appears to have blanketed one of the carbonate beds.

A possible source of these detrital siliciclastic grains could be the weathering of locally emergent Precambrian gneisses. However the small clast-size also supports wind transportation, in which case the source could have been hundreds or even thousands of kilometers away. Wind transported grains during episodic windstorms could have produced clast-supported layers on the supratidal flat and matrix-supported grains in the sub-and intertidal environment. Clasts deposited on the tidal flat and later carried in floodwaters could be the source of the oxidized siliciclastic layers. Mineralogy and trace element chemistry suggests the 1-3 cm siliciclastic layer may be a reworked water ash, possibly associated with a very late phase of Rodinian rifting or Iapetean arc volcanism.