Late Caldera-fill Sequence
The Late Caldera-Fill sequence includes the Mount Pleasant Porphyry, the Big Scott Mountain formation, and the Kleef formation. The ages of the rocks are not well-established, but they are most likely Late Devonian and Mississippian. In addition the Intracaldera sequence is cut into multiple intrusions of varying origins. Many of the intrusions are thought to have formed without eruption, when mineral deposits "leaked."
The Mount Pleasant Porphyry formation occurs as dykes and small rock areas that have been associated with local breccia. The dyke structures seem to indicate multiple periods of intrusion. Two types of breccias have been identified, and older and more common felsic series, and a younger neutral phase. The porphyries making up the rocks were emplaced at the pre-existing volcanic margin.
The Big Scott Mountain formation consists of rhyolite of variable composition, lapilli-tuff, and crystal tuff. Most of the rhyolites are characterized by large amounts of pyroxene. One rhyolite unit appears to overlie the McDougall Brook Granite formation. The tuff is littered by clasts from the Seelys and McDougall Brook Granite formations. The tuff also appears to be layered. It is colored dark orange on the map, and the exposed part partially rings the north of the volcano.
The final formation, the Kleef formation, includes redbed, basalt and pumice-ridden tuff. Pebble to cobble sized conglomerate is also seen complimenting the volcanic rock, and parts seem to come from older formations. The basalt is characterized by large crystals (up to 2 cm (1 in)). The tuff are characterized by their reddish-brown color and abundant fossil-pumice. The formation is very rare exposed and of interest to economic geologists, as it contains many of the possibly gold-bearing intrusive areas. It is colored ligh blue on the map.
Read more about this topic: Mount Pleasant Caldera, Geology, Eruptive History
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