Quaternary Alluvial Fans Research Articles

Recent seismic and gravity data from the Cenozoic Jefferson and Madison basins provide new information concerning their structural and depositional histories. Both basins are north-south elongated structural basins formed as a result of horizontal extension after Laramide horizontal thrusting. Each basin is bounded on the east side by a sinuous faulted steep mountain front, and large west-sloping alluvial fans extend almost completely across both basins. Gravity data show that each basin in the subsurface is asymmetric with a large steep west-dipping fault on the east flank, and one or more east-dipping fault(s) of smaller magnitude on the west flank. The deep axis of each basin runs parallel to the east mountain front and lies east of the surface geographic central axis. Jefferson basin has two deep, closed, structural lows (one east of Silver Star and one east of Twin Bridges), which are separated by a structural arch. Sediment depth on the arch exceeds 3,000 m (10,000 ft). Madison basin is shallow on its north end (approximately 2,100 m, 7,100 ft) where it is terminated by the prominent northwest-southeast Spanish Peaks structural trend, and progressively becomes deeper (4,500 m, 5,000 ft or more) south of Ennis, Montana. Seismic data confirm or support the gravity data. Seismic also shows the folded and thrusted rocks of the east mountain footwall block dipping steeply westward to where they gradually disappear beneath the thick Tertiary sediments. Tertiary strata lying directly against the large west-dipping basin fault show dip reversal caused by drag-folding during basin subsidence. Downthrown rollover type anticlines are thus present on the east side of the basins. Numerous small faults, many antithetic, cut the deeper strata and diminish in throw upward. Strata seen in the seismic sections can be subdivided into a lower set which forms the bulk of the basin fill (possibly equivalent to the Formation, late Eocene to early Miocene); a thinner middle set unconformably overlying the lower set (equivalent to the Sixmile Creek Formation, Miocene and Pliocene); and an upper set composed of west-dipping Quaternary alluvial fan deposits. Each set thickens toward the east basin-bounding fault. In the lower Renova set, lacustrine intervals are indicated by their consistent lateral seismic character, whereas fluvial intervals appear to terminate abruptly. End_of_Article - Last_Page 1352------------

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