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Late Tertiary rifting in southern New Mexico took place in two stages: early to late Miocene and Pliocene-Pleistocene. Sedimentary rocks of the earlier stage are represented by the Hayner Ranch and Rincon Valley Formations, which were examined at San Diego Mountain, Dona Ana County, New Mexico. The Hayner Ranch Formation consists of 950 m of red fanglomerate derived from volcanic rocks. The Rincon Valley Formation consists of 325 m of limestone- and volcanic-cobble fanglomerate overlain by 25 m of red playa shale.
Measurements of maximum clast size at 50 m intervals (n = 1,820) and cobble imbrications at 50 m intervals (n = 650) indicate that the initial stage of rifting took place in two distinct phases and involved two separate source areas. The lower 600 m of the Hayner Ranch Formation were derived from volcanic rocks north of San Diego Mountain. Following a period of tectonic quiescence, represented by a 150 m thick fining-upward sequence, uplift occurred to the south of the study area, depositing the upper 350 m of the Hayner Ranch Formation and the lower 300 m of the Rincon Valley Formation. Limestone cobbles recovered from the Rincon Valley Formation are identical to lithologies in the Permian Hueco formation exposed in the Robledo and Dona Ana Mountains, 15 km south and southeast of San Diego Mountain, respectively.
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