Sequence analysis of the Upper Cretaceous Two Medicine and Judith River formations, Montana; nonmarine response to the Claggett and Bearpaw marine cycles

Abstract

ABSTRACT Superb outcrop exposures and abundant subcrop data allow the accurate tracing of two stratigraphic discontinuities updip into fully nonmarine strata of the Campanian Two Medicine and Judith River formations (Western Interior foreland basin, Montana). These throughgoing discontinuities delimit and alluvial equivalents of two third-order sea-level cycles, and provide ground truth for recent conceptual models of alluvial sequence stratigraphy. An erosional disconformity interpreted to mark the boundary between regressive and transgressive alluvial deposits crops out in nonmarine strata of the Two Medicine Formation in northwestern Montana. It is embedded in relatively flat-based fluvial sandstone sheets dominated by downstream accretion elements, and is marked by several meters of internal erosional scour, a thick and laterally persistent intraclast lag facies, pervasive oxidation, and a shift from fine- to medium/coarse-grained sandstone. Physical stratigraphic and geochronometric evidence indicate that this fluvial disconformity, which can be traced throughout the outcrop belt, correlates with the widespread 80 Ma sequence boundary developed in distal parts of the Western Interior Basin. The erosional disconformity in the Two Medicine Formation reflects a negative base-level adjustment that occurred during the Telegraph Creek-Eagle regression (R7), and conforms to the standard definition of a sequence boundary. Identification of the 80 Ma sequence boundary in alluvial facies of the Two Medicine Formation is significant in that it is one of very few well-documented examples of a nonmarine sequence boundary, but unlike most others, it is not characterized by a readily apparent facies tract dislocation reflecting a basinward shift in facies (e.g., braided-stream deposits sharply juxtaposed over coastal coal-bearing facies). A second throughgoing discontinuity embedded within fully nonmarine deposits of the Judith River Formation in central Montana is interpreted to separate regressive and transgressive alluvial deposits that accumulated during the Claggett regression (R8) and subsequent Bearpaw transgression (T9). This discontinuity correlates with the erosional base of a backstepping composite sequence set of shoreface strata, and can be traced inland to the western limit of Judith River strata preserved in central Montana (~ 50 km). The Judith River discontinuity is not erosional, but rather reflects a very abrupt change in alluvial architecture, most notably an abrupt shift from a sand- to a mud-dominated section that can be traced in outcrop and subcrop throughout north-central Montana and into southern Alberta. The throughgoing discontinuity in the Judith River record does not conform to conventional definitions of a sequence boundary, and it apparently did not form in response to a fall in relative sea level. This discontinuity instead appears to record an abrupt increase in the rate of generation of accommodation in the Montana portion of the foreland basin (presumably related to flexural subsidence), and it is provisionally interpreted as the nonmarine equivalent of a third-order transgressive surface coincident with the updip correlative conformity.