PRELIMINARY INTERPRETATION OF A 2D SEISMIC LINE IN SW MONTANA—APPLYING SEISMIC STRATIGRAPHY PRINCIPLES TO INFER GEOLOGIC SETTING

The 2D seismic line, EC-CEL-CZM-C, is a 113-mile long seismic section in southwestern Montana. The seismic section was shot in 1974, reprocessed in 1999, and the southern 51.33 miles of the line were purchased by the USGS in 2013. A preliminary interpretation of this seismic section led to identification of three broad seismic-stratigraphic domains: 1) shallow, flat-lying continuous reflectors, 2) a deeper domain containing flat-lying units with interspersed, turbidite-like deposits, and 3) an even deeper domain containing high-amplitude reflectors arranged in sigmoidal or oblique tangential geometries. The shallow domain is relatively undisrupted in the northern part of the section, but contains numerous steeply-dipping, north-facing, south-vergent reverse faults that have produced rollover anticlines, in particular one coincident with the Nye Bowler Lineament. The turbidite-like deposits in the middle-depth domain form stacked, vertically aggrading packages that approximate shingling, channels, channel complexes, and channel complex sets. This middle unit is interpreted to be a part of the lower Belt Group deposited during the Mesoproterozoic in the Helena Embayment, an area in which a marine environment transgressed over fluvial deposits. The deepest domain contains sigmoidal and oblique tangential reflectors, which are interpreted to be stacked ramp-flat-ramp geometries of numerous duplex structures from shortened Archean rocks.