INTERPRETATION OF ALLUVIAL ARCHITECTURE IN THE OGALLALA GROUP OF NEBRASKA FROM TEST-HOLE LOGS

The stratigraphic architecture of the Miocene Ogallala Group (OG) remains far from fully understood over the large areas of Nebraska in which exposures are minimal or altogether lacking. Resistivity and lithologic logs produced by the Nebraska Conservation and Survey Division test-hole drilling program, however, can be applied in these areas. The analysis of these data is of sedimentologic interest, but it also has practical implications in the exploitation of groundwater.

In east-central Nebraska, the OG is dominated by sands and silts and large outcrops are very rare. Resistivity logs, cross-checked against lithologic logs of rotary cuttings, indicate four readily-discernible types of stratigraphic trends: (1) smaller-scale fining-upward (SFU), (2) larger-scale fining-upward (LFU), trending from sand into sandy silt or silt and, rarely, diatomite or “marl”; (3) coarsening-upward (CU); and (4) thick sand/sandstone bodies dominated by fine to medium sand (TSB). SFU are 1.82 to 12.19 m thick and show a distribution skewed toward the low end of their range (N = 168, μ = 5.41, σ = 2.22, median = 5.18, mode = 4.27). The three examples of LFU identified so far are 18.90 to 24.38 m thick. CU are 0.91 to 17.37 m thick and also show a distribution skewed to the low end of their range (N = 54, μ= 4.06, σ = 2.81, median = 3.51, mode = 3.66). TSB range from 6.10 to 38.10 m thick (N = 36, μ = 16.101, σ = 8.26, median = 13.72, mode = 14.02). LSB signatures on resistivity logs range from nearly “straight-sided” (rare) to “ragged” (common), to “peaked” (consisting of a lower, coarsening-upward trend and an upper, fining-upward trend). “Ragged” resistivity signatures indicate variations in sediment grain size within sand bodies, which can be verified in the corresponding lithologic logs of rotary cuttings. We interpret SFU as individual fluvial fining-upward trends associated with rarer coarsening-upward trends of similar scales. TSB are almost certainly multistorey fluvial sands. Both TSB and LFU may, in many cases, be discrete valley fills. Similar stratigraphic trends have been identified in the OG in test-hole data from the western Sand Hills. In both study areas, OG resistivity-log patterns contrast markedly with the resistivity signatures of the overlying, gravelly fluvial sediments of the Broadwater Formation (Pliocene).