Mississippian Shelf Margin and Carbonate Platform from Montana to Nevada: ABSTRACT

Abstract

The Kinderhookian to middle Meramecian history of a carbonate platform and shelf margin, extending from Nevada to Montana, is documented through four time-rock correlation charts and five successive maps that are synchronized by foraminiferan, conodont, and coral zonations. The platform was bordered on the west by a starved basin, a flysch trough, and orogenic highlands. The history of platform development is an integral part of the sedimentary cycle of the deep-water Deseret starved basin. Antler orogenic activity produced epeirogenic movements on the craton, which affected sea level and caused episodic progradation and retreat of the carbonate shelf margin. The sequential history is: (1) in earliest Mississippian time a narrow, northeast-trending seaway bordered by low oastal plains received mostly fine clastic sediments; (2) during late Kinderhookian time, a carbonate platform and shelf margin formed as a result of eastward expansion of the seaway; (3) during early Osagean time, the shelf margin retreated and a broad, gentle (less than 0°5^prime) clinoform ramp developed; (4) during middle Osagean time, lowering of the basin and craton and rise of sea level changed the pattern and sedimentary regime of the carbonate platform. Progradation of the shelf margin over the former ramp resulted in maximum expansion of the platform concurrent with maximum deepening of the starved basin. The foreslope attained a maximum steepness of 5°; (5) in middle Meramecian time, uplift of the craton and lowering of sea level caused shoaling of the carbonate plat orm and development of a sabkha landward. With increased uplift a karst plain developed over most of the former carbonate platform, and some cratonic sands were transported westward by streams into the basin. Meanwhile, filling of the flysch trough allowed eastward spillover of distal-flysch sediments to almost completely fill the basin. End_of_Article - Last_Page 716------------