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

We examine the basin geometry and sedimentary patterns in the Muddy Creek h alf graben of south-west Montana by integrating geological mapping, structu ral and basin analysis, Ar-40/Ar-39 geochronology, biostratigraphy and refl ection seismic data. The half graben formed in late Middle Eocene to early Oligocene (?) time at the breakaway of a regional, WSW-dipping detachment s ystem. Although the structure of the half graben is that of a supradetachme nt basin, facies patterns and basin architecture do not conform to a recent model for extensional basins above detachment faults. The border fault, th e Muddy Creek fault system, consists of three en echelon, left-stepping nor mal faults separated by two relay ramps. The fault steepens southward towar d each en echelon step, ranges in dip from 8 to 60 degrees near the surface , but flattens at depths between 0 and 3 km. A broad ENE-plunging displacem ent-gradient syncline defines the central part of the half graben and is fl anked 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 adjace nt anticlines. These facies patterns suggest that folding was coeval with e xtension and sedimentation in the half graben. Pre-extensional volcanic roc ks and interbedded conglomerate filled a major ESE-trending palaeovalley al ong the future axis of the Muddy Creek half graben. Synextensional sediment ary deposits include lacustrine and paludal shale, mudstone and sandstone p onded in the centre of the half graben, and a narrow (typically <1.5 km wid e) fringe of coarse alluvial-fan and fan-delta conglomerate and sandstone d erived from the footwall. Angular unconformities and rock-slide deposits oc cur only locally within the syntectonic sequence. These facies patterns agr ee well with the half-graben depositional model of Leeder & Gawthorpe but n ot with a more recent supradetachment basin model of Friedmann & Burbank de spite the demonstrably low dip-angle of the basin-bounding normal fault. Th ese data show that it may not be possible to differentiate between supradet achment basins and half graben with steeper border faults using the archite cture of the associated basin-fill deposits.