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POST-VOLCANIC RIFT CLASTICS

Red ball iconTHE MIDCONTINENT RIFT SYSTEM IN IOWA

 

POST-VOLCANIC RIFT CLASTICS


Early Post-Volcanics Sedimentation

Studies in the Lake Superior Region by several of geologists including Daniels (1982) indicate that as rift volcanism waned, clastic sedimentation became the dominant depositional process along the axis of the rift. Although rift volcanism ceased, probably due to a relaxation of the tensile stresses, subsidence of the rift trend continued, driven in part by isostatic adjustments to the thick, dense pile of rift-filling volcanics. In the Lake Superior area, the Oronto Group and related units were deposited in the central graben. The basal unit of this sedimentary sequence, the Copper Harbor Conglomerate, is dominated by mafic volcanic rock fragments, apparently derived from volcanic rocks in the footwall of the graben-bounding normal faults. This suggests that deposition of Copper Harbor Conglomerate sediments was primarily limited to the area of the central horst. This unit and all other Oronto Group units are known only from this area, although related rocks may lie buried beneath thick Bayfield Group sediments as modelled by Mooney and others (1970) and Chandler and others (1989). In Iowa, Unit B in the M.G. Eischeid #1 well (Witzke, 1990) was interpreted as the distal equivalent of the Copper Harbor Conglomerate. These fluvial sediments were deposited by rivers that eventually flowed into the central graben (Anderson, 1992; Figure 16). Two wells, the Ogden City #2 and Boone City #1 on the Iowa Horst in Boone County encountered clastic rocks, probably Copper Harbor Conglomerate equivalent units.

Standing bodies of water, probably lakes but possibly an arm of the sea, that developed in the MRS grabens hosted the deposition of a sequence of siltstones and shales, the Nonesuch Formation and equivalent units. Although Hieshima and Pratt (1991) used sulfur/carbon elemental ratios to infer a marine environment for Nonesuch sedimentation, it is extremely difficult to imagine an opening through the Grenville Orogenic Belt. This belt was created by a continent-continent collision that produced a structural uplift similar in scale to the Tibet Plateau-Himalayan Mountains (Baer, 1981). It is equally unlikely that the seas entered the structure from the south where the rift apparently narrows to only a zone of dikes.

The water bodies in the MRS were apparently rich in algae and fungae (Moore and others. 1969) which created an organic-rich sediment. The Nonesuch Formation in the Lake Superior area, with a maximum-known thickness of about 215 m (700 ft) (Daniels, 1982), was apparently limited to the area of the central graben. In Iowa, however, the presence of an equivalent rock sequence (Unit C) in the Eischeid well demonstrates that in this area the lacustrine environment extended beyond the limits of the central graben (Anderson, 1990d). The 441 m (1470 ft) of Unit C was deposited about 24 km (15 mi) outside the graben, and about 88 km (55 mi) from the axis of the graben. This suggests that, if interpreted graben margins were correct, in Iowa the lakes were probably very large, possibly over a hundred kilometers wide and may have extended along most of the length of the Iowa Segment of the MRS, and possibly the entire length of the rift. The thickness of the unit near the axis of the graben was probably several times its Unit C thickness.

The rift-axial lakes were eventually filled by prograding alluvial fans that deposited the fluvioclastic rocks of the Freda Formation in the Lake Superior area and equivalent units including Eischeid well Unit D. Deposition of the rift-related clastic rocks of the Oronto Group probably ceased only a few million to a few tens of millions of years at the most after the cessation of volcanism, about 1086 Ma ago (Weiblen, 1993). The end of rift sedimentation was probably indicative of the end of rift subsidence and possibly the initiation of compressive tectonics. This marked the beginning of a period that was apparently dominated by erosion of the recently-deposited clastics.


Late Post-Volcanics Sedimentation

Following a period of regional erosion, the area of the MRS experienced increasing regional shortening. This led to a reversal of movement along the fault system that bordered the central grabens. The central grabens were forced upwards, ultimately forming axial horsts. Oronto Group sediments were eroded from the rising horsts, and deposited in basins that formed along the horst flanks. The remobilized Oronto Group sediments, winnowed during fluvial transport, were also apparently enriched in quartz by mixing with clastics sourced from the granitic terranes in which the MRS was emplaced (Ojakangas and Morey, 1982a). The sandstone-dominated sequences that were deposited by these processes, the Bayfield and equivalent units, reach a maximum of about 2100 m (7000 ft) (ibid.). In the Lake Superior area, the Bayfield Group is composed of three units. Two are feldspathic, fluvial sandstones (the basal Orienta Sandstone and upper Chequamegon Sandstone) and the intervening unit a quartz arenite (the Devils Island Sandstone), interpreted as lacustrine (Ojakangas and Morey, 1982b).

In Iowa, Upper Red Clastics Group units E, F, and G in the Eischeid well were interpreted as equivalent to the Bayfield group by Witzke (1990). Additionally, Eischied Unit H may also be a part of the Upper Red Clastic Sequenc and also equivalent to the Bayfield and related rocks (Anderson, 1990d). All Upper Red Clastic units in the Eischeid well are feldspathic sandstones. Unit G, however, is dominated by very fine- to fine-grained sandstones and has a large component of siltstone, possibly indicative of deposition in a standing body of water. This is similar to the interpreted environment of the Devils Island Sandstone in the Lake Superior area. Units E through G show a progressive increase in the mafic volcanic content of the lithic fragment component of the sandstones, and may record the progressive unroofing of the volcanic rocks of the central horst (Anderson, 1982d).

Seismic data was interpreted by Cannon and others (1989) to indicate the presence of Bayfield Group sediments overlying the Oronto Group beneath central Lake Superior. On land, however, no Bayfield Group sediments have been identified on the central horsts. The exception may be found in Iowa, where the Mineola Basin, centered on the Iowa Horst in southwest Iowa and modelled on Profile 13 (click to view Iowa MRS profiles), is filled with sediments interpreted as low density Upper Red Clastic Sequence rocks.

In Iowa, the Bayfield Group equivalent Upper Red Clastic Sequence rocks in the Eischeid well total 2312 m (7708 ft) in thickness. Along several of the seismically-controlled profiles, the Upper Sequence reaches a thickness of about 3960 m (13,000 ft) in the Wellsburg Basin and on the Ames Block.

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