PRECAMBRIAN TECTONIC PROVINCES 



33 



It is called the Duluth gabbro. Three divisions of the Keweenawan are 

 recognized, namely, a lower clastic sequence 1400 feet thick, then a 

 thick unit of basic amydaloidal lava flows interbeddcd with sandstones 

 and conglomerates, and at the top a continental clastic sequence possibly 

 reaching a thickness of 25,000 feet in the center of the basin of ac- 

 cumulation. The widespread extent of the flows and the paucity of ash 

 suggest that the flows issued from a system of fissures rather than central 

 vents. Associated with the flows and intruded into them are numerous 

 dikes and sills, dominantly basic. The most prominent sill is the Duluth 

 gabbro. 



The thick upper Keweenawan elastics consist of red feldspathic shaly 

 sandstones at the base and these grade upward into arkosic and quartzose 

 sandstones. They accumulated as the basin foundered in response, pre- 

 sumably, to the extrusion of the large volume of volcanics. Highlands 

 existed on both sides of the basin (Hamblin and Horner, 1961). 



Several large faults break the Keweenawan series. The Douglas and 

 Keweenawan are found on opposite sides of the synclinal or basin axis 

 with thrusting away from the axis. See map, Fig. 4.3 and cross sections 

 of Fig. 4.7. Vertical displacements up to 4 miles are indicated by the cross 

 sections. The North Shore fault, postulated from physiographic data 

 solely (principally from the straight shorelines) was not detected by 

 gravity surveys, but the surveys do not rule out its existence. If it is a 

 reality, it may be a normal fault and of later age than the reverse faults. 

 The orogeny of post-Keweenawan time consisting of volcanism and fault- 

 ing has been called the Killarnean and is dated at about 950 m.y. ( Fair- 

 bairn et al, 1960). 



The sills and volcanic rocks are strongly reflected by positive gravity 

 anomalies, and the deep basins of clastic rocks by negative anomalies. 

 Thiel (1956) has recognized this fact and traced the Keweenawan series 

 under the Paleozoic sedimentary rock cover by means of these strong 

 anomalies southwestward to the Salina basin of Kansas. The positive 

 feature has an average width of 30 miles and an amplitude of 100 miligals 

 above the regional gravity value. For the greater part of its length it is 

 flanked on both sides by gravity lows. The igneous rock masses are 

 responsible for the gravity highs and the clastic-filled basins, the lows. 



CLASTICS 



RED-ROCK 

 PHASE 



DULUTH GABBRO 



-I 



20 MILES 

 Fig. 4.7. Keweenawan orogenic belt. Sections in the Duluth area after Thiel, 1956. 



The Keweenawan belt projects toward the volcanic and gabbroic 

 terranes of Oklahoma and Texas, and perhaps these are part of the same 

 tectono-igneous belt. No strong gravity anomalies are known between 

 Kansas and Texas, but the grain of gravity contours (Lyons, 1950) is 

 southwesterly, and thus the belt may be marked by sedimentary rocks 

 and an absence of volcanic in this region. 



The Precambrian rocks of the Wichita Mountains of Oklahoma rep- 

 resent the upper granitic part of a large gabbroic lopolith which 



