Richard M. Chamberlin

SummaryThe San Lorenzo Spring quadrangle includes the west flank of the Lemitar Mountains and the eastern margin of the La Jencia Basin about 19 km northwest of Socorro, New Mexico. San Lorenzo Arroyo (Canyon) flows around the north flank of the Lemitar Mountains and eastward to the Rio Grande near the north margin of the quadrangle.  San Lorenzo Spring occurs where sandy arroyo-fill deposits wedge out across a bedrock sill of Oligocene dacitic lava. The dense lava forces easterly groundwater flow in the shallow alluvium to the surface; several other springs along San Lorenzo Arroyo occur at similar bedrock exposures. In contrast, “Query” spring, about 1.4 km east of San Lorenzo Spring, issues from sandstones of the Popotosa Formation adjacent to the upthrown corner of a major fault block. San Lorenzo Canyon is a popular area for picnics and day hiking.            

The San Lorenzo Spring quadrangle lies within the central Rio Grande rift, where WSW-directed lithospheric extension over the last 29-32 million years has broken an Oligocene volcanic plateau into a north-trending array of tilted fault-block ranges and alluvial basins. Contemporaneous dip slip and westward rotation of early rift domino blocks is recorded in the Lemitar Mountains and NE of San Lorenzo Spring by wedge-shaped prisms of basaltic-andesite lavas (Tl1-3) found between progressively less tilted ignimbrite sheets of late Oligocene age (see cross section A-A’). Two or three episodes of domino-style extension have rotated originally high-angle, east-dipping normal faults to near horizontal attitudes (e.g. large elliptical block centered 3 km south of Red Mountain). Minor displacement high-angle normal faults offset large displacement low-angle fault traces at several localities.  Previously unmapped andesite porphyry lavas (Tar) and dacitic lavas (Tds) cap the La Jara Peak Basaltic Andesite near Red Mountain and San Lorenzo Spring. Mafic agglomerates, dacitic debris flows and felsic tuffs mark a small vent complex (Tvs) southeast of San Lorenzo Spring. Andesite lavas are preferentially preserved on downthrown early rift fault blocks. Boulder-rich andesitic breccias of the Popotosa Formation (Tpx3), which are interpreted as scarp-derived colluvial deposits, locally cap the downthrown andesitic lavas. Notably the same porphyritic lavas are absent on the adjacent uplifted footwall blocks (e.g. Red Mountain) that were eroded and then buried by stratigraphically higher units in the Popotosa Formation. The earliest rift faulting is locally demonstrated by two down-to-the east normal fault scarps that were buried by the 28.7 Ma La Jencia Tuff in the area NE of Red Mountain.A complex structural/tectonic evolution is apparent in the San Lorenzo Spring quadrangle. Strongly tilted domino blocks of the early Rio Grande rift are superimposed on a Laramide highland. Absence of Mesozoic strata and limestone-cobble conglomerates of the Baca Formation (Tb) record uplift and erosion of the Lemitar Mountains in early Tertiary time. Granite-derived boulder conglomerates within the Baca Formation in the footwall of the Puerto fault suggest that this normal fault was a Laramide reverse fault, upthrown on the north, prior to regional extension. The Polvadera thrust fault, near Canoncito del Lemitar, represents N-S crustal shortening associated with the Ancestral Rocky Mountains orogeny in middle Pennsylvanian (Atokan) time. The large ignimbrite hogback about 1.5 km NE of San Lorenzo Spring is underlain by 30m of thin bedded limestones and arkosic sandstones of the lower Permian Bursum Formation, which unconformably overlies a two-mica granite of Proterozoic age.  The absence of Pennsylvanian strata at this locality implies that it was an ancestral Rocky Mountain highland similar to the Joyita Hills, located 17 km to the east. Proterozoic granite forms the structurally high crystalline core of the west-central Lemitar Mountains.      

Moderate to strongly tilted volcanic-rich conglomerates, sandstones and mudstones of the Miocene Popotosa Formation accumulated in evolving tilt-block depressions of variable orientation (dominantly north-south), lateral extent and age of activity. Map patterns and intraformational unconformities indicate that the floor of the Popotosa basin (top of Oligocene volcanic pile) was complexly faulted prior to and during Popotosa deposition. The Popotosa Formation west of San Lorenzo Spring is as much as 2.1 km thick and locally fills a transverse sag or graben between structurally high blocks just south of Red Mountain and northeast of San Lorenzo Spring (ignimbrite hogback). Popotosa beds in the transverse graben contain a distal flow unit of the 15.4 Ma basaltic andesite of Silver Creek and several water-laid pumiceous sandstones that project northward along strike into the Silver Creek quadrangle, where they have been dated at 15.6 to 14.5 Ma (Cather and Read, 2003). Older, presently undated ash beds, locally occur between basal debris flow units (Tpd2/Tpd1) exposed in the walls of San Lorenzo Canyon. Traverses across uniformly dipping stratigraphic sequences show that the conglomerates locally exhibit abrupt up section changes in the dominant lithology of entrained volcanic pebbles and cobbles. Thus conglomeratic subunits have locally been mapped and defined on the basis of distinctive clast suites. These compositional subunits, in conjunction with textural units defined by dominant grain-size and bedding character, provide some sense of stratigraphic sequence and position, which is not apparent by mapping only textural units (cf. Cather and Read, 2003). Map patterns imply that the Popotosa Formation generally thickens and becomes older northward along the west flank of the Lemitar Mountains. A 15-20° angular unconformity observed in the southwestern Lemitar Mountains is not apparent at San Lorenzo Spring, where the basal debris flows essentially parallel the underlying lavas at the erosional unconformity. Anomalous southeasterly to northeasterly dips of Popotosa beds at San Lorenzo Canyon and Canoncito de las Cabras represent a complex ramp structure associated with westward and eastward splaying of the large displacement Silver Creek fault (> 2km), which bounds the east flank of the ignimbrite hogback north of San Lorenzo Canyon. Additional dating of ash beds in the Popotosa formation is underway; new age data should help test preliminary correlations of Popotosa units as presented here. Pliocene and Pleistocene sedimentary deposits of the Sierra Ladrones Formation record the transition from early-rift closed basins to late-rift, valley-fill deposits of the ancestral Rio Grande and its tributary drainages (e.g  La Jencia Creek). Moderately west tilted basal conglomerates  of the Sierra Ladrones piedmont facies (QTse), which were derived from the Silver Creek uplift to the east (“Rock” ridge), lie in sharp angular unconformity on steeply tilted upper Popotosa conglomerates (Tpcu) near the SW corner of the Sevilleta National Wildlife Refuge. The Sierra Ladrones Formation may be as much as 1100 m thick under La Jencia Creek. Northeast transported piedmont gravels (QTsp) cap the rim of La Jencia Creek and locally define the original paleovalley walls of La Jencia Creek where the gravels are inset against nearly horizontal mudstones and sandstones beds of the underlying basin floor facies (QTsb).            

Groundwater in La Jencia Basin is largely untapped, except for a few stock wells and one irrigation well SE of the La Jencia playa. Poorly cemented and moderately sorted piedmont gravels of the Sierra Ladrones Formation are the most likely aquifer below the La Jencia playa. Much of the spring flow along San Lorenzo Canyon probably represents overflow from the totally saturated La Jencia Basin and perennial recharge of Holocene arroyo sands (Qa/Qvy) from sandstone aquifers in the Popotosa Formation. Coarse conglomeratic units in the Popotosa Formation are typically well cemented with carbonate or silica and probably represent poor aquifers, except for local fracture permeability. Sandstones in the upper Popotosa Formation (Tps) are commonly friable or poorly cemented and probably represent the best aquifer zones in the Popotosa Formation.            

Quaternary faulting is not evident in the San Lorenzo Spring quadrangle. However, graded piedmont slopes underlain by the upper Sierra Ladrones Formation (Qsp) appear to reflect slight westerly tilting (~1°) of  the La Jencia Basin in middle to late  Pleistocene time. Graded surfaces on the east flank of the basin are anomalously steep (~2°) and those on the west flank are anomalously shallow (~1/2°).  Westward tilting in Pleistocene time maybe associated with inflation of the mid-crustal Socorro magma body and/or tectonic tilting associated with active faults that bound the Lemitar uplift and the western margin of the La Jencia Basin.