STRATIGRAPHY OF THE SUCCOR CREEK AREA: UPDATING THE EXPLOSIVE AND EFFUSIVE CHRONOLOGY AT THE MAHOGANY MOUNTAIN – THREE FINGERS RHYOLITE FIELD

The Mahogany Mountain and Three Fingers calderas with their associated tuffs, the tuff of Leslie Gulch and tuff of Spring Creek, respectively, were the centerpiece of a larger rhyolite center that developed in response to Columbia River Basalt volcanism as numerous other mid Miocene rhyolite centers in a corridor from Baker City in the north to northern Nevada. Recently, the younger tuff of Spring Creek was found to be actually part of the tuff of Leslie Gulch thus demanding only one caldera.

This study refines the eruptive stratigraphy along the eastern margin of this rhyolite field with important implications for the entire field. Several distinct rhyolitic units are identifiable, these are (from oldest to youngest) the tuff of Leslie Gulch, the “Old” McIntyre rhyolite, the newly named tuff of Succor Creek, the “Young” McIntyre rhyolite, and a sequence of thin, nonwelded ignimbrites. In addition, intermediate to mafic lavas under- and overlie rhyolites.

The stratigraphy of the tuff of Leslie Gulch is variable but generally consists of a lower (15.95±0.05 Ma) and upper pyroclastic flow section separated by a fine grained and possibly reworked section. All is nonwelded, except for the densely welded, glassy top ignimbrite that underlies “Old” McIntyre rhyolite in the north. The western section includes lithic-rich lapilli tuff. Surge deposits found in the middle of the area are up to 100 ft thick. Our overall thickest section is 200 ft.

The most towering outcrops are made up of the older McIntyre rhyolite in the north and the younger one in the south and distinguishing features between both are age (16.02±0.02, 15.98±0.03, 15.94±0.16 vs. 15.76±0.02 Ma), bulk rock composition, phenocryst content, and mineral compositions. Both rhyolites have a lower glassy section.

The tuff of Succor Creek (named tuff of Spring Creek before) are thin welded ignimbrites along the eastern margin of the area but rhyolite with matching composition is also found as lava flows and dikes cutting tuff of Leslie Gulch in the central portion. The age is 15.74±0.09 Ma, (Marcy, 2013 PSU MS thesis) and its least silicic unit is chemically and mineralogically distinct. High precision yet overlapping ages highlight a 250k lasting, prolific silicic rhyolite field where stratigraphic field relationships are paramount in devising the correct eruptive stratigraphy.