Late Mesozoic to Early Cenozoic Foreland Sedimentation in Southwest Montana: ABSTRACT

Abstract

Stratigraphic variations in the petrology of Upper Jurassic through lower Tertiary sandstones from southwest Montana, and inferred dispersal patterns and depositional environments, reflect the tectonic evolution of a foreland basin in response to gradually increasing orogenesis. Upper Jurassic to Lower Cretaceous (Aptian) chert-litharenites were largely derived from an older folded and faulted miogeoclinal prism of terrigenous and carbonate rocks farther west in Idaho. Volcanic and plutonic sources were negligible. Deposition was on a broad coastal plain extending east toward the craton. Subordinate admixtures of sediment containing low-rank metamorphic rock fragments derived from an intrabasinal reactivated Precambrian structure (Belt arch) and regionally extensive inter alated fresh to brackish-water limestones are unique characteristics of this retro-arc sequence. Marked upward increase in the quantity of locally derived plutonic and volcanic rock fragments in Blackleaf (Albian) and younger sandstones indicates the progressively more important role of nearby comagmatic sources. A transition in time from paralic sedimentation to dominantly alluvial sedimentation, was associated with the increase in igneous detritus. Dispersal patterns were complex because of local topographic barriers. Early Tertiary sedimentation occurred in extensional fault-bounded basins in an intra-arc setting. Highly immature lithic sandstones and arkoses with a complex plutonic-volcanic-gneissoid provenance accumulated in a variety of high- and low-energy alluvial environments. End_of_Article - Last_Page 791------------