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Most information on Midcontinent Rift volcanic rocks was derived from study of exposures in the Lake Superior region. In that area the volcanic rocks (mafic dominated) flowed from fissures along the margins of the central graben and formed a series of discrete basalt plateaus (Green, 1972). In Iowa extremely limited drilling has sampled only late stage volcanic rocks.

Early Rift Volcanism

The deposition of the basin-fill clastics was short-lived, as evidenced by a maximum-known thickness of about 120 m (400 ft) (Morey and Ojakangas, 1982). Their deposition was interrupted when a series of mafic dikes breached the crust and issued floods of mafic dominated lavas which filled the deepening basins (Green, 1972). The dikes include the Gunflint Dikes (Minnesota) and related dikes (Weiblen, 1982; Fahrig, 1987). The lavas are given a variety of names in various areas of the Lake Superior region (click to view MRS stratigraphic nomenclature), but Anderson (1992) grouped them under the name Mamainse Point Group. Soft sediment deformational features in the underlying Nopeming and related rocks indicate that they were not yet lithified when overridden by the magmas. Also, basal volcanic rocks display pillow structures in some areas, indicating that they were erupted into bodies of water, probably the lakes in which the clastic rocks were being deposited.

The Mamainse Point Group rocks, which reach thicknesses in excess of 6.3 km (3.9 mi), erupted as a series of individual flows ranging in thickness from less than a meter to more than 400 meters (Green, 1982). A series of coalescing basalt plateaus that thicken towards the axis of the rift were eventually constructed. Typical of rift environments, these lavas were dominated by mafic (tholeiitic) compositions, but also included minor felsic magmas. In periods of quiescence, the axis of the basin continued to subside. Rivers flowing over the lava plains eroded the distal portions of the lava plateaus and drained towards the rift axis, depositing interflow clastic units (Merk and Jirsa, 1982). In the Lake Superior area the rift axis subsided slowly. These volcanic rocks, and the overlying North Shore Group, lack depositional structures indicative of the presence of a central graben at the time of active volcanism (Green, 1972). In Iowa, however, seismic evidence, supported by gravity modelling in this study, failed to identify basalts on the footwalls of horst-bounding faults along any of the six studied profiles. This suggests that the central graben in Iowa was active during magmatism. If any of the Iowa basalt flows extended beyond the limits of the graben they were probably not extensive and were mostly soon removed by erosion.

The Mamainse Point Group was deposited between 1109 - 1098 Ma ago (Van Schmus and Hinze, in press). This period includes two magnetic field reversals: (1) Keweenawan lower reversed to lower normal polarity and (2) lower normal to upper reversed polarity. Both reversals are associated with periods of volcanic quiescence in known exposures, and neither reversal has yet been observed in the volcanic rocks. The top of the Mamainse Point Group is marked by a third reversal, also associated with a period of volcanic inactivity. This was apparently a period during which a regional unconformity developed.

In Iowa the lower volcanic unit rocks occupy the same stratigraphic position and are probably contemporaneous with the Mamainse Point Group of the Lake Superior region. This lower volcanic unit reaches thicknesses of about 8 km (5 mi) on several of the gravity profiles (10, 11, and 12) (click to view Iowa MRS profiles), assuming the arbitrary base of the volcanics at the base of the upper crust, 13 km (8 mi). There is little to distinguish the upper and lower volcanic units on the seismic profiles. In general, the upper unit displays more continuous parallel reflectors than the lower unit, the upper unit is generally more nearly horizontal, and an angular unconformity can usually be identified between the units. The difficulty in identifying the units with seismic reflection data and their similarity in the gravity models, suggests that large inaccuracies in their geometries may be possible, perhaps as much as 50%.

Late Rift Volcanism

A second package of volcanic rocks, with an upper normal polarity remanence, has been identified in the Lake Superior region. These rocks, like the lower package, have been assigned different names in various areas, but in his study Anderson (1992) referred to all related late rift volcanic units as the North Shore Group. In the Lake Superior area these rocks are similar in lithology and chemistry to the Mamainse Point Group and reach a maximum known thickness of 7.1 km (4.4 mi) (Green, 1982). The U-Pb ages of the North Shore Group rocks range from 1096 to 1086 Ma (Van Schmus and Hinze, in press), with age determinations bracketing the contact between the North Shore Group and Mamainse Point Group at 1097+1 Ma (Davis and Paces, 1990).

In Iowa, the upper volcanic package is considered equivalent to the North Shore Group. This unit, identified on all seismic and gravity profiles, is characterized on the seismic profiles by long, continuous sets of reflectors, generally horizontal to subhorizontal. The unit displays a maximum thickness of about 6.4 km (4.0 mi) on most profiles, but reaches 9.6 km (6.0 mi) along Profile 9.

Rocks correlated by Anderson (1992) with the upper volcanic package were encountered in 44 drill holes in Iowa, 36 wells with cutting samples totaling 116 m (388 ft) of basalt and 8 cores totalling 51 m (170 ft). Most of these samples are currently reposited at the Iowa Department of Natural Resources Geological Survey's Oakdale Research Facility. The most complete and informative basalt samples are from the Northern Natural Gas #1 Sharp core, drilled in Webster County near the town of Thor, which encountered upper package volcanics at a depth of 647 m (2156 ft) below the surface, and penetrated 17 m (57 ft) of the unit. All or portions of three individual basalt flows can be observed in the core, separated by weathered, vesicular flow tops. The rocks in this core are the "type" for the informal name "Thor volcanic Group" that is applied to upper volcanic package rocks in Iowa (click to view depositional model for Thor volcanics). A second deep core, the Finnegan #1 drilled in 1973 in Guthrie County, penetrated 72 feet (21.6 m) of gabbro and was studied by Fischer (1982). This gabbro may represent an unroofed late-stage pluton or possibly the crystallized interior of a thick upper package flow.

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