THE COLDWATER AND CEDAR CREEK BRECCIAS AND ASSOCIATED UNITS ALONG HIGHWAY 67, ST. FRANCOIS MOUNTAINS, SOUTHEASTERN MISSOURI: ERUPTIVE PHASES OF THE MUDLICK DELLENITE? A GEOLOGIC AND GEOCHEMICAL STUDY

In 2009 a new road-cut was created one mile north of Coldwater, Missouri on Highway 67. In this roadcut, seven major geologic units are exposed based on fieldwork completed during summer 2014 (unit numbers are included): (1) Cedar Creek Breccia, (2) Small Dike, (3) Red Magmatic Area, (4) Coldwater Breccia, (5) Cedar Creek Breccia 2, (6) well-bedded avalanche deposits, and (7), a capping pyroclastic flow. Images found on Google Maps^TM of the Coldwater road-cut in 2009 show that the currently identified pyroclastic units may contain more units than can be seen today. The units are hypothesized here to be related to the hypabyssal Mudlick Dellenite, located several miles to the southwest in Sam Baker State Park. The Dellenite is a potential source of the rhyodacitic volcanics in the region (Malkames and Hood, 1976). Units 1, 4 and 5 are believed to be pyroclastic in origin while units 2 and 3 are intrusive into the pyroclastic units. Unit 6 is unconformably overlain by a well-lithified pyroclastic flow (Unit 7), the upper 15 feet of which, is significantly hydrothermally altered. Units 1-5 have been intruded by several small (<10 cm wide by 2 m long) subvertical dikes of unknown composition.

The vertical orientation of the pyroclastic units may indicate that this block was once part of a caldera wall that collapsed into a caldera moat. This block was then was intruded by the small dikes (Unit 2 and others) and a red rhyolite (Unit 3), and later, covered by avalanche and pyroclastic flow deposits (Units 6, 7). At this time it is not clear whether these units are part of the Greene or Piedmont calderas as defined by Kisvarsanyi (1981).

The whole-rock geochemistry of Units 1-5 indicates that the pyroclastic units are rhyolitic and potentially co-magmatic based on the uniform liquid line of descent shown on Harker variation diagrams. Chondrite-normalized REE and spider diagrams also indicate that the units, except for the Cedar Creek Breccia, are all similar in composition. The geochemical relations displayed by chondrite-normalized spider diagrams provide strong evidence that the source for these volcanics is the Mudlick Dellenite, which in turn, is geochemically correlative with the Lodi Granite.