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UNIT C

Red ball iconSTRATIGRAPHY OF THE
MIDCONTINENT RIFT SYSTEM IN IOWA

 

UNIT C


Unit C (depth 4349 to 4935 m; 14,980 to 16,450 ft) is the most distinctive Keweenawan Supergroup clastic unit encountered in the M.G. Eischeid well. It is unique in its abundance of gray to black siltstones and shales, calcite cements, calcite vein fills, and structural deformation. The unit is also the most thoroughly cored, with two cores totaling 39 feet (11.7 m). The unit was subdivided into two intervals, an upper interval (C 2) and a lower interval (C 1). Interval C 2 (depth 4349 to 4710 m; 14,980 to 15,700 ft) is an interbedded sequence of sandstones, siltstones, and shales, with gray to black siltstones and shales more common in the upper portion of the interval and red-brown to green-gray colors more common in the basal portions (Witzke, 1990). Methane and ethane were detected throughout the interval and black intergranular residues, possibly relict hydrocarbons, were reported. Also reported on the mud log in this interval, at 4632 m (15,440 ft), were traces of chalcopyrite or native copper. A careful examination of samples from this interval, however, failed to confirm the presence of copper or chalcopyrite.

Core #3 (depth 4528.8 -to4536.0 m;= 15,096 to 15,120 ft), from the upper part of the interval, consists of horizontally laminated, millimeter- to decimeter-thick interlayered medium to dark gray shales and lighter gray siltstones to fine-grained sandstones (Ludvigson and others, 1990). These strata were interpreted to have been deposited in a lake or other body of standing water with fluctuating water depth and intermittent influxes of coarse detritus. Petrologic study of two samples of coarse detritus from interval C 2 yielded a mean of Q=72 F=24 L=3 (Ludvigson and others, 1990). The 24% feldspar observed in this interval is the greatest concentration observed in any interval in the Eischeid well (click here for QFL information). Interval C 1 (depth 4710 to 4935 m; 15,700 to 16,450 ft) is dominated by light to dark gray siltstones and shales with dark brown to black shaley interbeds (Witzke, 1990). Calcite cements, spar, and vein fills are common in this interval, and a carbonate-rich region near the bottom of the interval was identified as limestone on the mud log. Core #4 (depth 4812.9 to 4817.4 m; 16,043 to 16,058 ft) showed a steep tectonically derived dip ranging from 65 degrees to vertical and slightly overturned. The cored interval is composed of well laminated, interbedded light gray, micaceous siltstone and medium gray to black shale (Ludvigson and others,1990). Calcite veinlets and slickensided fault surfaces with calcitic rough facets are common in the core and indicate reverse faulting, requiring lateral shortening (Ludvigson and Spry, 1990).

click for depositional model for Unit C

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