Calcium Carbonate Cement (Caliche) in Keweenawan Sedimentary Rocks (∼ 1.1 Ga), Upper Peninsula of Michigan
Department of Geological and Mining Engineering and Sciences
A complex caliche profile, and numerous widespread zones of carbonate cement occur in a thick sequence of fluvial siliciclastic sandstones and conglomerates that were deposited in the Lake Superior basin during and after continental basaltic volcanism, as interflow conglomerates of the Portage Lake Volcanics and as the Oronto Group. In conglomerates the carbonate cement forms discontinuous zones, up to 60 cm thick and several meters in length, consisting of up to 25 vol.% carbonate.
Whereas the caliche horizon at the Centennial Mine shows typical upward-grading features characteristic of pedogenic caliche, the widespread sparry carbonate cement is less diagnostic. It is interpreted also as caliche for the following reasons: (1) it predates regional epidote metamorphism; (2) a caliche profile has been described from the same strata; (3) textures characteristic of caliche such as carbonate-rimmed pebbles, pendulose calcite on pebbles, sand grains floating in a carbonate matrix, and exfoliated chlorite; (4) possible vadoids; (5) the carbonate-rich zones are widespread stratigraphically and geographically in basinward-inclined alluvial fans, and (6) the distribution of these zones, and the total quantity of contained carbonate requires other than a point source for the origin of the carbonate.
Several lines of evidence suggest that the prevailing climate may have been arid, with a fluctuating, perhaps seasonal rainfall pattern required for caliche: (1) a paleolatitude about 30° north; (2) carbonate-filled shrinkage cracks denoting high evaporation; (3) intermittent, flashy, or ephemeral streams recorded locally in fluvial beds by numerous superimposed horizons of mud-chip conglomerate, enormous, previously cemented mudstone rip-ups; and desiccation-cracked mud drapes.
Interpretation of the cement as vadose caliche is compatible with a sedimentation model for the region as a semi-arid intermontane basin, flanked by alluvial fans and with a central saline lake.
Calcium Carbonate Cement (Caliche) in Keweenawan Sedimentary Rocks (∼ 1.1 Ga), Upper Peninsula of Michigan.
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