227 Sedimentary Geology: Clastic Sediments from the Cambrian to the Quaternary

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 211CD

NEW INSIGHTS INTO THE BASIN-SCALE ALLUVIAL ARCHITECTURE OF THE OGALLALA GROUP (NEOGENE, CENTRAL NEBRASKA) USING STATISTICAL ANALYSES OF GRAIN SIZE TRENDS

Paper number: 227-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

One hundred and sixty-six borehole resistivity logs were used to interpret grain size trends in fluvial sediments of the Ogallala Group. Thicknesses were determined for 227 fining upward (FU) units, 81 coarsening upward (CU) units, and 76 sandbody (SB) units showing straight sided, “cylindrical” log signatures. Non-parametric univariate statistics were used to compare the thicknesses of units in the western part of the study area to those in the eastern part. Using the Mann-Whitney test, the null hypothesis of equal medians is rejected for the CU units (p = 0.009) and the SB units (p = 0.030), but it is not rejected for the FU units (p = 0.909). Using the Kolmogrov-Smirnov test, the null hypothesis of equal distributions is rejected for the CU units (p = 0.017) and the SB units (p = 0.060), but it is not rejected for the FU units (p = 0.671). Cluster analysis and prinicipal component analysis (PCA) were used to investigate spatial trends in the overall resistivity log patterns from the lower 90 m of the Ogallala Group in the central part of the study area. Four distinct clusters of boreholes were identified using Ward’s method. These groups generally align to paleovalleys and paleoridges on the sub-Ogallala erosion surface. PCA reveals that grain size trends in the lower 20 – 30 m of the Ogallala Group also distinguish groups of boreholes that align to these sub-Ogallala features.

Results show that CU units thin downgradient (eastward), a trend compatible with: (1) interchannel deposition on a downstream-thinning megafan, or (2) the existence of more accommodation to the west. SB units, interpreted as amalgamated channel fills or as paleovalley fills, thicken eastward. This downgradient trend suggests either (1) more superimposed channels, (2) deeper paleovalleys, or (3) less accommodation to the east. Results also reveal that paleovalleys are filled with 20 – 30 m of FU or SB units, whereas sub-Ogallala paleoridges are overlain by 20 – 30 m of CU units. Therefore, we interpret a marked shift toward the lateral migration of channels and an increase in overbank deposition after the filling of basal Ogallala paleovalleys during the Late Miocene.