Cordell Formation

The Cordell formation is the uppermost member of the Manistique group. The Cordell member is a Silurian aged dolomite deposited within the The Manistique The Manistique group is approximately 60 meters of varying dolostones. The lower group, the Schoolcraft member, can be distinguished from the Cordell by its massive and crystalline texture. Additionally, the Schoolcraft unit was deposited following a lull and drop in regional sea level as indicated by the massive erosional structure found at the base of the member (Ehlers, 1973). In his paper, Sloss observes three major unconformities within the basin that coincide with the eustatic transgressions. The large unconformity located the bottom of the Schoolcroft correlates temporally with the

The Cordell varies in lithology throughout the 47 meters of unit thickness. The Cordell has chert nodules throughout the strata unlike the Schoolcroft which only has chert nodules at the top of the member. Additionally, the Cordell is more uniformly and thinly bedded than the Schoolhouse. The very upper Cordell unit is nicely exposed at Scott’s quarry in the HNF. The quarry exposes approximately 15 meters of Cordell dolostone. The fossils which can be observed include brachiopods and rugose corals. Specific species of corals have been identified in Bellapenna’s report since they are the most widespread fossil within the Cordell dolostone. The interbedded chert can be observed in the field. The gradation of gray to blue to brown chert nodules can also be observed. Bellapenna also suggests there are quartz and garnet grains within the Dolostone. The minerals were not obvious in hand sample so further mineralogical analysis would likely be needed to recognize the accessory minerals. The Cordell unit is difficult to distinguish in the field from the lower Engadine unit, the Rockview, due to the high degree of dolomitization and recrystallization. However, unlike the Rockview, the Cordell member is more thinly bedded and fossiliferous and lacks algal mats. The bottom of the Rockview dolostones appear to weather to a lighter color than that of the exposed Cordell and is considered

Along the southern boundary were three separate arches which were rising due to the Taconic orogeny: Kankakee, Findlay, and Cincinnati. The upward movement of these arches were integral in the depressive basins forming adjacent to them: Michigan, Illinois, and Appalachian. However, the original weakening of the continental lithosphere is suggested to have been caused by the introduction of a rising diaper (Haxby, Turcotte, Bird 1976). Erosional and weathered material from the nearby forming mountains were transported and deposited into the many basins west of the mountain belt. The sediment would continue to accumulate and accentuate the subsidence which was occurring and further deepen the basin. The Manistique member runs along the southern margin of the Upper peninsula of Michigan. The region was relatively stable and underwent little deformation. The subsequent subsidence would be caused directly and indirectly by the rise of the Appalachians mountains. Subsidence, however, would continuous cease (Howell, van der Pluijm 1990). They investigated the region and used drill core, sequence stratigraphy, and backstripping to indicate the subsidence of the area could not be explained by simple magmatic origins. They suggest there are correlations between significant Appalachian building events and cessations of the basin subsidence. Their interpretation withdraws credit away from the popular thermal fit theory