WHITE RIVER GROUP EQUIVALENTS IN MONTANA

White River Group equivalents in Montana have previously been known by a plethora of lithostratgraphic names and were placed within the lower Renova Formation. More recently these units have been correctly identified as laterally equivalent basin deposits and are informally known as Cenozoic Sequence 2. Sequence 2 is bounded by regional unconformities and is age constrained by biostratigraphic and radiometric data. These data yield an age range of about 37 Ma to about 30 Ma for Sequence 2.

An oxic paleosol consistently marks the unconformity where Sequence 2 rests upon pre-Tertiary or Sequence 1 (about 55 Ma to about 43 Ma in age) rocks. Sequence 2 contains mudstone, siltstone, sandstone, and conglomerate, although the coarser-grained lithologies are typically lensoidal. Fluvial depositional systems dominate Sequence 2. The sandstones are typically ribbon sandstones suggesting a preponderance of fixed channel systems. Mudstone makes up as much as 80% of the Sequence 2 fluvial system rocks. Other minor depositional components of Sequence 2 include debris flow units that contain conglomerate lenses and some sheets, and lacustrine lenses of shale and carbonate. Calcic pedocomplexes typically cap the sequence where it occurs in mid-basin to basin-margin settings.

The hiatus between Sequence 2 and overlying sequences in some areas is only approximately 3 Ma, however, in many areas, it is often at least approximately 14 Ma. This is the real “mid-Tertiary” unconformity, in that its maximum age is tied to the top of Sequence 2 at approximately 30 Ma. This mid-Tertiary unconformity is angular in a number of locations, but in some locations is simply a disconformity.