1. Detrital Zircon Geochronology of the Permian Abo Formation and Permian Laborcita Formation, Sacramento Mountains, New Mexico, USA
MILTON, Jacob1 ,MALONE, David1, HAMPTON, Brian A2 (1) Department of Geology-Geography, Illinois State University (2) Department of Geological Sciences , New Mexico State University
ABSTRACT
The exposures of Paleozoic strata, which are more than 2500 m thick and range in age from Cambrian to Permian, in the Sacramento Mountains  of New Mexico reveal important information for understanding of the paleogeography and timing of the late Paleozoic Ancestral Rocky Mountain orogeny in general, and the Pedernal Uplift in particular.  The lower Permian (Wolcampian) strata consists of two formations: the Laborcita and the Abo Formations, resting above the Ancestral Rocky Mountain Unconformity.  The late synorogenic Laborcita is an ~300 m thick succession of gray and red limestone pebble conglomerate, sandstone, siltstone, shale and limestone deposited in fan delta or deltaic systems.  The flat-lying, post-tectonic Abo Formation ranges from m in thickness and consists of dark-red mudstones, arkoses, and basal quartzite cobble conglomerate.  Otte (1959) recognized that paleocurrent data indicated that Abo sediment was transported south and west along the axis of the Pedernal uplift.  U-Pb geochronology on detrital zircons were conducted on both the Laborcita (n=103) and Abo Formation (n=104) sandstones to determine the sedimentary provenance and sediment dispersal patterns.  Zircons in the Laborcita ranged in age Ma, and have a unimodal peak age of 1220 Ma, and about 80% being Grenville in age.  Zircons in the Abo were exclusively Grenville in age, ranging from Ma with a peak age of 1235 Ma.  The most likely source for these zircons are recycling Grenville synorogenic strata that were exposed to the south during the late Paleozoic.  These include the Lanoria Quartzite, which occurs in the Franklin Mountains, and the Hazel Formation, which occurs near Van Horn, Texas. The protolith sandstone of the Lanoria Quartzites was derived from the central Texas Llano region before being transported west to the Rodinian continental margin ~1110 Ma where it was buried and metamorphosed into quartzite. The similarity of the age spectra for these two units indicates that the same source area supplied sediment during and after the local compressional tectonism.
METHODOLOGY
A sample of sandstone was collected from both the Abo and Laborcita formations in the Sacramento Mountains in New Mexico (Figure 4). The samples were crushed and zircons were extracted and separated through traditional methods of panning, heavy liquids, and multiple magnetic separations with a Franz. The zircons were inspected and placed in sample containers to be shipped to the Arizona LaserChron Center.
U-Pb geochronology of zircons is conducted by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at the Arizona LaserChron Center (Gehrels et al., 2006, 2008; Gehrels and Pecha, 2014). The analyses involve ablation of zircon with a Photon Machines Analyte G2 excimer laser equipped with HelEx ablation cell using a spot diameter of 20 microns. Prior to analysis, grains are imaged to provide a guide for locating analysis pits in optimal locations, and to assist in interpreting results. Images are made with a Hitachi 3400N SEM and a Gatan CL2 detector system. Following analysis, data reduction is performed with an in-house Python decoding routine and an Excel spreadsheet (E2agecalc).
The ages are shown on Pb*/U concordia diagrams and relative age-probability diagrams using the routines in Isoplot (Ludwig, 2008; Figure 7). The age-probability diagrams show each age and its uncertainty (for measurement error only) as a normal distribution, and sum all ages from a sample into a single curve (Figure 8 and 9). Composite age probability plots are made from an in-house Excel program that normalizes each curve according to the number of constituent analyses, such that each curve contains the same area, and then stacks the probability curves (Figure 7).
Figure 8. Frequency plot of detrital zircon age spectrum of Abo Formation sandstone. Peak age is 1238 Ma.
DISCUSSION
The K-S statistical analysis of the detrital zircon age spectra for the Abo sandstone is statistically similar to the Abo quartzite clasts and similar in part to the Laborcita sandstone and Lanoria Quartzite (Figure 8 and 11). The single age peak represented in the Abo sandstone analysis makes it hard to interpret recycled Lanoria Quartzite as a detrital zircon source. It is more likely that the source is a different Neoproterozoic unit of identical detrital zircon age spectrum was exposed and weathered during the Ancestral Rocky Mountain Orogeny and is currently buried.
The sediment for the Abo sandstone was originally eroded prior to 1100 Ma in central Texas from Midcontinent Granite Rhyolite and Grenville basement rocks. The zircons were transported west to the Grenville foreland basin or Neoproterozoic Cordilleran Continental margin (Figure 6). The sediment was buried and lithified before being uplifted and eroded during the Ancestral Rocky Mountain event (Figure 3).
The K-S statistical analysis of the detrital zircon age spectra for the Laboricita sandstone and is statistically similar to the Lanoria Quartzite (Figure 7 and 9). The frequency of Archean age zircons in the Laboricita sandstone however is absent from the Lanoria Quartzite and likely indicates mixing of sediment from multiple sources.
The sediment for the Laboricta sandstone was originally eroded prior to 1050 Ma in central Texas from Grenville, Midcontinent Granite Rhyolite, Yavapai-Mazatzal, Penokian, and Archean age provinces (Figure 6). After transport to the Grenville foreland basin the sediment was buried and lithified before being eroded during the Ancestral Rocky Mountain event (Figure 3).
The Archean age zircons found in the Laborcita sandstone will require a more in depth study of detrital zircon sources for other formations in the area in order understand where they came from and to better understand the local paleogeography and sediment transport patterns before, during, and after the Ancestral Rocky Mountain Orogeny.
Figure 3. Map of the Ancestral Rocky Mountain uplifts (brown) and basins in New Mexico and Texas. The study area, indicated in red, occurs near the margin of the Pedernal Uplift and the adjacent Orogrande basin to the west. Modified from Lindsey et al., 1986.
RESULTS
A total of 207 zircons were analyzed for this project of 104 zircons from the Abo Formation and 103 zircons from the Laborcita Formation (Figure 8 and 9). The sample from the Abo Formation indicates a single peak age of 1238 Ma which are Grenville-age grains ( Ma).
The Laborcita Formation sample indicates five peak ages. The most prominent peak age is 1225 Ma of Grenville-age grains. The lesser percentage of age spectra are 1450, 1665, 1740, and 1870 Ma. These less frequent grains are of Midcontinent Granite Rhyolite Province, Mazatzal and Yavapai Province, and Precambrian age respectively (Figure 6).
INTRODUCTION
The Ancestral Rocky Mountain unconformity is exposed In the Sacramento Mountains in southern New Mexico revealing the Permian Abo and Laborcita Formations overlying folded and faulted Cambrian-Pennsylvanian strata (Figure 1 and 2). The Ancestral Rocky Mountains were a result of the collision of North America with South America and Africa. As a result, the Ouachita-Marathon Orogeny caused foreland deformation to occur in the midcontinent in Early Pennsylvanian time, and cratonic deformation and suturing from the Ouachita to Marathon region in Middle Pennsylvanian Time (Kluth and Coney, 1981). By late Pennsylvanian Time the cratonic deformation spread southward into New Mexico and West Texas resulting in the current thrust faults of the Ouachita-Marathon system (Kluth and Coney, 1981; Ye et al., 1996). The western side of the resulting Pedernal uplift is what is now the Sacramento Mountains.
The Lower Permian (Wolfcampian) Abo formation in the Sacramento Mountains of South Central New Mexico was deposited over top of the Laborcita formation by a silt dominated fluvial system (Mack et al, 2003 Figure 11; Figure 10). The Laborcita is m thick and consists of red and gray shale, sandstone, conglomerate, and a few limestones. It also contains fusulinids of early Wolfcampian age in the upper part (Otte, 1959). The Abo Formation is m thick and consists of coarse-grained sandstone, some shale, and a basal conglomerate. In the Sacramento Mountains the Abo Formation also included dark-red mudstones, arkoses and some limestone (Otte, 1959).
The purpose of this research is to constrain the source area for the sandstone in the Abo and Laborcita Formations using U-Pb Geochronology on detrital zircons. This will be done by determining the maximum depositional age of the protolith of these clasts, and by comparing the zircon age spectrum to other stratigraphic units that occur in the area.
Figure 9. Frequency plot of detrital zircon age spectrum of Laborcita Formation sandstone. Peak ages are 1225, 1450, 1665, 1740, and 1870 Ma.
Figure 4. Geologic Map of the Alamogordo, NM area and the adjacent Sacramento Mountains. The sampling localities are indicated by the red stars. Modified from Anderson et al. (1994).
Figure 11. Correlation diagram of early Permian stratigraphy in southern New Mexico (from Mack et al. 2003).
Figure 6. Sediment dispersal patterns from Neoproterozoic to Permian (modified from Spencer et al., 2014). The zircon grains analyzed here originated largely in the Grenville and Midcontinent Granite-Rhyolite provinces in central Texas and then transported west to the Grenville foreland (blue arrow. These rocks were uplifted, exposed, and eroded during the Ancestral Rocky Mountain Orogeny, and then transported north >100 km to the Sacramento Mountains.
Figure 1. Photograph of the Laborcita Formation along US 82 east of Alamogordo (32°56'44.08"N, 105°52'20.16"W).
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Figure 10. Stratigraphic column of cratonic Paleozoic strata in the Sacramento Mountains.
Figure 7. Stacked frequency of the Abo sandstone and Laborcita sandstone detrital zircon age spectrum as compared to the Abo Formation Quartzite, which occurs in the Franklin Mountains, Hazel Formation, which occurs in the Van Horn area, and the Lanoria Formation, which occurs in the Franklin Mountains. Data for the Texas strata from Spencer et al., G=Grenville, MGR = Midcontinent Granite-Rhyolite, Y-M = Yavapai-Mazatzal, P = Penokean (Trans-Hudson), and Archean age provinces.
Figure 5. Stratigraphic column of Mesozoic and Neoproterozoic strata in the Franklin Mountains and Van Horn area (modified from Spencer et al., 2014). Stars indicate the ages of interbedded volcanic rocks. The Red Bluff Granite intruded the succession at 1120 Ma.
Figure 2. Photograph of conglomeratic basal Abo Formation arkose about 5 km south of High Rolls, NM on the West Side Road (32°55'35.58"N, 105°50'25.64"W).