New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping of the Socorro area identifies four major piedmont units of Quaternary age (Qlc, Qvo1, Qvo2, Qvy) modified after McGrath and Hawley, 1987. Recent detailed mapping and correlation of surfaces using soil characteristics has indicated that these piedmont units can be further subdivided. Two major types of landforms are recognized in the piedmont west of Socorro, 1) those that grade to past levels of the Rio Grande and 2) those landforms that have not been influenced by past levels of the Rio Grande. The first set of landforms are moderate gradient fans and terraces associated with the larger streams flowing primarily from the Magdalena Mountains, whereas the second set of landforms are those found along the foot of Socorro Peak, and consist of steep, coarse, debris flow fans.

Landforms related to levels of the Rio Grande consist of two fans (at Socorro Canyon and Nogal Canyon) and a series of inset terraces. The oldest fan surface, forming the southern boundary of Socorro Canyon, has been correlated to the Las Canas (Qlc) surface, which marks the top of the Santa Fe Group and the top of the Sierra Ladrones Formation. It has a stage III-stage IV carbonate horizon and is probably several hundred thousand years old. The next oldest fan surface (Qvo1) is located north of Nogal Canyon; it has a Stage II carbonate horizon, but this has been stripped and it probably was more strongly developed than this. At a minimum, this surface is late Pleistocene in age

Both these fan surfaces have been incised by tributary streams of the Rio Grande, resulting in nested terrace sequences. Both terrace sequences consist of three traces. In some places there is an apparent tectonic control on the development (age) of fan surfaces where Quaternary faults are down-to-the-west (against slope) causing small segments of the upthrown block to bypassed by streams. In the Socorro Canyon sequence, terraces soils have stage III, and stage II, and the third a very weakly developed stage I calcic horizons, respectively. In the Nogal Canyon sequence, the highest terrace has a stage II calcic horizon and the youngest terrace has no calcic horizon. Only a few remnants exist of an intermediate surface between the two terraces in this sequence.

The debris fans along the Socorro piedmont (Qpu, piedmont-slope deposits undifferentiated) have a range of soils with different degrees of development. Of the six soils described on this area, three had stage II calcic horizons, and one of these had been stripped. The other three surfaces all had soils of different degrees of development on them. One of the soils was developed on a sand sheet and had stage I calcic horizon.

The two major types of surfaces respond to different geomorphic conditions, the first set of surfaces are formed as the tributary streams respond to drops in the level of the Rio Grande, whereas the second set of surfaces probably form in response to climatically wetter periods in the late Pleistocene and Holocene. It is possible that there has been almost continuous deposition of the debris fans since the middle to late Pleistocene, which has implications for recognizing the top of the Sanat Fe Group in these sediments.