STRATIGRAPHY OF THE MIDCONTINENT RIFT SYSTEM IN IOWA
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UNIT D |
The upper most unit in the Lower Red Clastic Group, Unit D (depth 3153 to 4494 m; 10,510 to 14,980 ft), was described by Witzke (1990) as a thick, generally fining upward sequence that is dominated by sandstones in the lower half and siltstone to shale in the upper half. He subdivided the unit into four intervals, from the top D 4, D 3, D 2, and D 1. The sandstones in Unit D are quartz dominated (with a mean framework grain composition of Q=76 F=21 L=3) and the lithic component includes the highest percentage of sedimentary and metamorphic rock fragments (average 74% of lithic grains) observed in any unit (click here for QFL information) in the Eischeid well (Ludvigson and others, 1990). Interval D 4 (depth 3153 to 3588 m; 10,510 to 11,960 ft) is dominated by red to brown (with minor light green to gray) shales in its upper half and red brown siltstones in the lower half (Witzke, 1990). Core #2 (depth 3414.3 to 3418.5 m; 11,381 to 11,395 ft) was recovered from this interval and was described by Ludvigson and others (1990) as displaying two facies, one composed of red, very fine to fine grained, cross stratified sandstones, the second a very fine grained, horizontally stratified sandstone with minor mudstone. These sediments are interpreted as representative of a fluvial setting with shallow channel fills, subaerial exposure, inundation by over bank deposits, and then a return to shallow channel deposition (Ludvigson and others, 1990). The cored interval also displays folds, slickensides, and other fault-related deformational features illustrated and discussed by Ludvigson and Spry (1990).
The underlying interval, D 3 (depth 3588 to 3780 m; 11,960 to 12,600 ft ), is siltstone dominated with minor interbedded sandstone and shale. This interval also contains gray to dark gray siltstones and shales and the first occurrences of black siltstones and black carbonaceous specks. The lower portion of this interval is dominated by light gray, fine to medium grained sandstones with minor shales and siltstones (Witzke, 1990). The upper portion of interval D 2 (depth 3780 to 4335 m; 12,600 to 14,450 ft), is dominated by varicolored siltstone with some dark gray to black shaley laminations (some pyritic). The majority of the interval is sandstone dominated with minor red-brown to gray-green siltstone and shale interbeds. Coarse sand grains are abundant in the basal portions of interval D 2. The basal interval in Unit D, D 1 (depth 4335 to 4349 m; 14,450 to 14,498 feet), was described by Witzke (1990) as a sandstone dominated sequence with an upper varicolored shale and siltstone package. The sandstone is primarily light gray to red-brown and fine to medium grained with minor coarse grains. Shales are red-brown to gray (some black) and contain siltstone interbeds. Down-hole logs indicated traces of methane and ethane in this interval. Witzke (1990) reported intergranular black residues, possibly hydrocarbon residues indicative of past petroleum movement through this unit.
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