TESTING THE GRAIN VERTICALITY METHOD IN DISTINGUISHING EOLIAN FROM MARINE CARBONATES

Le Guern and Davaud (2005) introduced grain verticality criterion as a new petrographic method for distinguishing eolian from marine carbonate facies. Using the image analysis of thin sections from Pleistocene eolianites and modern nearshore sediments, Le Guern and Davaud conclude that water lain carbonate facies have more unimodal grain orientation and flatter rose diagrams than eolian facies.

We tested their method using the image analysis of thin sections from eolian and marine facies of Bahamian Pleistocene and Holocene rocks, the Jurassic Sundance Formation, and the Mississippian Ste. Genevieve Formation. Our preliminary results show that Bahamian Pleistocene eolianites have a large number of elongated (length/width ratio > 2:1) grains, a bimodal grain orientation, and have a higher occurrence of vertical (60-90 degree inclination) relative to horizontal (0-20 degree inclination) grains, which supports Le Guern and Davaud conclusions. Likewise, Bahamian Holocene beachrock exhibits a large percentage of elongated grains, a more unimodal grain orientation, and a flatter rose diagram; further validating the proposed method. Eolianites of the Sundance formation have a polymodal grain orientation, a small number of elongated grains; however, the grains are more vertical than horizontal. Its marine facies have flatter rose diagram, a higher amount of elongated grains, and a higher occurrence of horizontal relative to vertical grains. This observation generally supports Le Guern and Davaud method; however, it is not as conclusive as the Bahamian examples. The Ste. Genevieve eolianites have more elongated grains than the Sundance but less than the Bahamian eolianites. However, of the eolian facies studied, the Ste. Genevieve exhibits the flattest rose diagrams and has a lower occurrence of vertical grains than horizontal. Its marine facies is in accordance with the other marine facies. Deviation of Ste. Genevieve eolianites from the expected grain verticality pattern may be consequence of: several episodes of eolian dune development which were interrupted by marine transgressions and the reworking of the dunes and makes it unique from the other units; and/or less frequent precipitation and a drier climate during the Mississippian could also be a controlling factor of its more unimodal grain orientation.