2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 176-10
Presentation Time: 10:15 AM

ESTUARINE SEDIMENTATION AT THE SOUTHERN TERMINUS OF A PRE-ALBIAN SEAWAY IN THE CRETACEOUS FORELAND BASIN OF WESTERN MONTANA - LOWER KOOTENAI FORMATION


SCHWARTZ, Robert K., Department of Geology, Allegheny College, Meadville, PA 16335 and VUKE, Susan M., Montana Bureau of Mines and Geology, Montana Tech, 1300 West Park Street, Butte, MT 59701, bschwart@allegheny.edu

The Lower Cretaceous Sunburst member of the Kootenai Formation near Great Falls, Montana, marks the southern marine terminus of a quartzose transgressive-to-highstand system tract within the alluvial-dominated foreland of western Montana. Tidal facies define a lobate, northward opening, estuarine complex that developed along and south of the eastern flank of the Sweetgrass Arch, at the southern end of the pre-Kootenai Whitlash paleovalley tract. Northward opening of the estuary is consistent with a northward paleoslope as indicated by fluvial systems in underlying and overlying Kootenai members. Facies types, bearing diagnostic tidal structures and trace fossil assemblages, include estuary mudstone, estuary-mouth linear-bar, and estuary-channel facies that typify a central basin area. Tide-dominated shoreface facies lie adjacent to the linear-bar facies, and thin tidal-flat and channel sandstones, locally interbedded with paleosol units, typically mark basin margins near the zero edge of deposition. The stratigraphic succession documents: (1) maximum pre-Sunburst erosion along the central basin (pre-existing valley) axis, (2) estuary flooding and entrapment of non-marine-derived kaolinite mud as well as tidal flat/bar development, (3) a high-energy shoal zone in the estuary mouth consisting of linear sand bars and intervening channels, (4) a flanking tidal shoreface setting, and (5) overall superposition by estuary-channel and tidal-flat facies. Headward encroachment of the linear bar system during transgression is evidenced by a tidal ravinement surface over the estuarine mud-rich facies. The anomalous quartzose composition of Sunburst sandstone compared to otherwise lithic-rich sandstones in the Kootenai resulted from a cratonic (Canadian) source and net headward (southward) transport by tidal currents, consistent with Sunburst paleocurrent data and modern analogues. With regression, the estuary system was abandoned and capped by estuary-channel and tidal-flat facies followed by non-marine coastal plain facies of the overlying Kootenai member.
Handouts
  • GSA'14_Schwartz & Vuke_Kk Estuarine Sedimentation.pdf (6.7 MB)