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Quantitative Petrographic Analysis of Cretaceous Sandstones from Southwest Montana

T. S. Dyman, K. B. Krystinik, K. I. Takahashi

The Albian Blackleaf Formation and the Cenomanian lower Frontier Formation in southwest Montana lie within or east of the fold and thrust belt in the Cretaceous foreland basin complex. Petrography of these strata record a complex interaction between source-area tectonism, basin subsidence, and sedimentation patterns associated with a cyclic sequence of transgressions and regressions. Because the petrographic data set was large (127 thin sections) and difficult to interpret subjectively, statistical techniques were used to establish sample and variable relationships.

^THgr-mode cluster and correspondence analysis were used to determine the contributing effect (total variance) of key framework grains. Monocrystalline quartz, plagioclase, potassium feldspar, and sandstone-, limestone-, and volcanic-lithic grain content contribute most to the variation in the framework-grain population. ^THgr-mode cluster and correspondence analysis were used to identify six petrofacies. Lower Blackleaf petrofacies (I-III) contain abundant monocrystalline quartz (55-90%) and sedimentary lithic grains (10-50%), which are distributed throughout the study area. Petrofacies I-III are differentiated by variable monocrystalline quartz and sedimentary lithic grain content. Upper Blackleaf and lower Frontier petrofacies (IV-VI) exhibit highly variable, sedimentary and volcan c lithic ratios, and contain less monocrystalline quartz (20-50%) than lower Blackleaf petrofacies.

Information from quantitative analyses combined with available paleocurrent data indicates that Blackleaf and lower Frontier detritus was derived from variable source areas through time. Lower Blackleaf detritus was derived from Precambrian through Paleozoic sedimentary terranes to the west, north, and east; whereas, upper Blackleaf and lower Frontier detritus was derived from both sedimentary and volcanic terranes to the south.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.