Testing models of rift basins: structure and stratigraphy of an Eocene–Oligocene supradetachment basin, Muddy Creek half graben, south‐west Montana

Abstract

We examine the basin geometry and sedimentary patterns in the Muddy Creek half graben of south‐west Montana by integrating geological mapping, structural and basin analysis, 40Ar/39Ar geochronology, biostratigraphy and reflection seismic data. The half graben formed in late Middle Eocene to early Oligocene (?) time at the breakaway of a regional, WSW‐dipping detachment system. Although the structure of the half graben is that of a supradetachment basin, facies patterns and basin architecture do not conform to a recent model for extensional basins above detachment faults. The border fault, the Muddy Creek fault system, consists of three en echelon, left‐stepping normal faults separated by two relay ramps. The fault steepens southward toward each en echelon step, ranges in dip from 8 to 60° near the surface, but flattens at depths between 0 and 3 km. A broad ENE‐plunging displacement‐gradient syncline defines the central part of the half graben and is flanked by narrow SE‐and NE‐plunging anticlines to the north and south. Fine‐grained deposits of the syntectonic basin‐fill are thickest in the central syncline and interfinger with footwall‐derived conglomerate near the adjacent anticlines. These facies patterns suggest that folding was coeval with extension and sedimentation in the half graben. Pre‐extensional volcanic rocks and interbedded conglomerate filled a major ESE‐trending palaeovalley along the future axis of the Muddy Creek half graben. Synextensional sedimentary deposits include lacustrine and paludal shale, mudstone and sandstone ponded in the centre of the half graben, and a narrow (typically <1.5 km wide) fringe of coarse alluvial‐fan and fan‐delta conglomerate and sandstone derived from the footwall. Angular unconformities and rock‐slide deposits occur only locally within the syntectonic sequence. These facies patterns agree well with the half‐graben depositional model of Leeder & Gawthorpe but not with a more recent supradetachment basin model of Friedmann & Burbank despite the demonstrably low dip‐angle of the basin‐bounding normal fault. These data show that it may not be possible to differentiate between supradetachment basins and half graben with steeper border faults using the architecture of the associated basin‐fill deposits.