Relation of Unconformities, Tectonics, and Sea Level Changes, Cretaceous of Western Interior, United States and Canada: ABSTRACT

Abstract

Intrabasin tectonics have influenced patterns of deposition and geographic distribution of major unconformities within the Cretaceous of the Western Interior. Eight major regional to subregional unconformities have been identified. Five of these have been related by previous workers to sea level changes and to well-documented regressive-transgressive cycles. New studies of recurrent movement on basement-controlled fault blocks suggest a synchronous relation among fault block movement, sea level changes, and unconformities. Which fault blocks moved on the basin floor, and when, can be explained by stress fields generated by direction and rates of plate motion. Unconformities associated with north-northwest fault trends are caused by more westerly movement, and those associated with east-northeast trends by more northerly plate motion. Expansion of the Gulf of Mexico, the Atlantic Ocean, or the Arctic Ocean during these plate motions may account for associated sea level changes. The 81 to 82 m.y. unconformity and shoreline regression in the Western Interior and synchronous volcanic events on the northern margin of the Gulf of Mexico illustr te the relations. Uncertainty exists in dating many of the unconformities. However, by use of the time scale of Obradovich and Cobban, the approximate dates for unconformities are estimated as follows: (1) late Neocomian to early Aptain, >100 m.y.; (2) late Aptian-early Albian, 100 m.y. ±; (3) Albian, 96 to 97 m.y.; (4) early Cenomanian, 93 m.y. ±; (5) Turonian to early Coniacian, 87 to 88 m.y.; (6) late Coniacian-early Santonian, 81 to 82 m.y.; (7) late Campanian 71 to 74 m.y.; and (8) late Maestrichtian, 64 to 69 m.y. Several billion barrels of oil have been found in sandstones associated with unconformities in the Cretaceous. Future stratigraphic trap exploration will be guided by a knowledge of tectonic influence on sedimentation during sea level changes and how these factors controlled distribution of source rock, migration patterns, reservoir rock, and seal. End_of_Article - Last_Page 1006------------