THE AGE OF CRATER ELEGANTE, A MAAR IN THE PINACATE VOLCANIC FIELD, SONORA, MEXICO

Crater Elegante is a dry maar in the Sonoran Desert that is 1.6 km in mean diameter from rim crest to rim crest, 244 m deep, and surrounded by a blanket of tuff breccia. A thick section of basalt flows crops out in its steep walls. The crater-forming eruptive sequence began with a basalt flow followed by construction of a small cinder cone in the south-central part of what is now the crater. Another basalt flow that is nearly identical both petrographically and geochemically crops out on the north side of the crater. The tuff breccia that overlies all these units is cut and baked by a dike that emanates from this north rim flow; the tuff breccia is also tilted and faulted over squeeze-ups in this flow. The age of Crater Elegante is that of the north rim flow. We obtained a ⁴⁰Ar/³⁹Ar combined isochron age for this flow of 32 ± 6 ka.

This age is not significantly different from a ⁴⁰Ar/³⁹Ar combined isochron age of 42 ± 13 ka that we obtained on the youngest flow at the south rim of the crater but it is far younger than the 149 ± 19 ka K-Ar age reported for Crater Elegante by Lynch (1981). The 32± 6 ka age helps account for the voluminous, pebbly lake deposits that are as much as 60 m above the floor of the crater and form a discontinuous bench at that elevation around the crater's inner walls. These deposits were derived chiefly by erosion of tuff breccia and have ¹⁴C ages up to at least 20 ka (17.2 ka uncorrected ¹⁴C age determined by Damon, Long and Sigalove, 1963). Had tens of thousands of years elapsed between crater formation and lakeshore sedimentation, the inner slopes of soft tuff deposits on the rim would have attained a relatively stable angle; and any tuff on the crater walls or at their base would either have been washed to the crater floor or buried by the talus that now covers the benches near the crater walls. No large supply of sediment would then have been available to build the elevated, near-shore lacustrine deposits. Indeed, the lake may have first appeared when the crater did, and a relatively high water table may have promoted formation of both.