Grain-Orientation and Paleocurrent Systems in Medina Formation: ABSTRACT

Abstract

Several sedimentological parameters were examined to define the paleoenvironments of the Formation (Lower Silurian; outcrops along the Niagara Escarpment in Ontario and New York State). The diverse sedimentological parameters change regionally as well as vertically in a quasi-regular manner. The trends of such changes are similar in all the properties examined. The consistency within the basin of the variations of the variables suggests a deltaic paleoenvironment for the sediments. Two of the sedimentological parameters, grain orientation and directional sedimentary structures, were studied in the Thorold and Grimsby Sandstone Members of the Formation. Determination of grain orientation yielded a complex pattern of directions hardly explainable except by a mixed environment model. It was found that the total sampled population of the grain orientations of the Thorold and Grimsby Sandstones was nonhomogeneous. Utilizing a trial and error methodology homogeneous subpopulations were distinguished: 1. Subpopulations related to the type of sedimentary structure present in the sampled bed: (a) cross-bedded (subpopulation); (b) laminated (subpopulation); and (c) massive (subpopulation). 2. Subpopulations related to the stratigraphic location of the samples: (a) Thorold plus upper third of the Grimsby; and (b) lower two thirds of the Grimsby. 3. Subpopulations related to the geographic location of the samples: (a) east of the Niagara River (New York region); and (b) west of the Niagara River (Ontario region). The various subpopulations evidently are interrelated and they are partly duplicated in the behavior of the azimuths of the cross-beds and other directional sedimentary structures. The directional data obtained from the Thorold and Grimsby Sandstones suggest the existence of two major paleocurrent systems: 1. A first system, trending northwest, was measured in the azimuths of cross-beds and in the cross-bedded grain orientation subpopulation of primarily the New York region. 2. A second system, trending northeast, was measured in the azimuths of cross-beds in the Ontario region and in the laminated grain orientation subpopulation. The characteristics of the paleocurrent systems and the results obtained from other sedimentary properties which better define the subenvironments composing the Medina delta, suggest that the first paleocurrent system is related to distributary channel complexes and the second to marine currents. End_of_Article - Last_Page 540------------