Detrital zircon geochronology of the Grenville/Llano foreland and basal Sauk Sequence in west Texas, USA Christopher J. Spencer1,2,†, Anthony R. Prave1, Peter A. Cawood1, and Nick M.W. Roberts2 1Department of Earth and Environmental Sciences, University of St. Andrews, North Street, St. Andrews KY16 9AL, UK 2NERC Isotopes Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK ABSTRACT INTRODUCTION This is recorded by the progressive onlap and blanketing of North America as preserved in the U-Pb detrital zircon ages from Meso- The Grenville orogeny is a record of the Sauk Sequence (Sloss, 1963). protero zoic and Cambrian siliciclastic assembly of the supercontinent Rodinia. In Lau- The stratigraphic location of the base of the units in west Texas (USA) constrain the rentia, syn-collisional detritus shed off the evolv- Sauk Sequence has been one of prolonged debate depositional setting, provenance, and tec- ing Grenville deformation front is preserved in a and has generally been ascribed on the basis of tonic history of the region within a late variety of settings, ranging from widely dispersed sedimentology and biostratigraphy (e.g., Hogan Mesoproterozoic Grenville foreland basin extensional basins (e.g., Midcontinent Rift, Fort et al., 2011; Peters and Gaines, 2012). In west and the early Paleozoic Sauk transgressive Wayne Rift) to broad, fl uvial aprons hundreds to Texas and New Mexico, its position is placed at sequence. Two key units, the Hazel and La- even thousands of kilometers in width (Cawood the base of the Cambrian–Ordovician Bliss For- noria Formations, have detrital zircon age et al., 2007; Hadlari et al., 2012; Rainbird et al., mation (Hayes, 1972; Amato and Mack, 2012). spectra dominated by detritus derived from 2012, and references therein). In contrast, the In New Mexico, this surface is a nonconformity the Grenville orogen (the Llano uplift and preserved lateral extent of foreland basin depo- with Mesoproterozoic igneous and metamor- eroded equivalents), the ca. 1.4 Ga Granite- sition (as defi ned by Allen and Allen, 2005) is phic rocks or the Cambrian Florida Mountains Rhyolite, and the ca. 1.7–1.6 Ga Yavapai/ thought to be limited to a small number of locali- pluton (Clemons, 1988). In west Texas, how- Mazatzal provinces. These data, combined ties proximal to the Grenville thrust front (Santos ever, the Bliss Formation overlies the braided with sedimentological data, permit inter- et al., 2002; Cawood et al., 2007; Baranoski fl uvial sandstones of the Van Horn Formation, preting those formations as the proximal et al., 2009; Rainbird et al., 2012). In west the age of which has been ambiguous but typi- and distal deposits, respectively, of a mo- Texas (USA), the late Mesoproterozoic Lanoria cally considered to be latest Precambrian (Deni- lasse shed into the Grenvillian foreland and Hazel Formations have been interpreted as son, 1980; Davidson, 1980) because of the lack basin. having formed in extensional basins (Bickford of trace- and/or macrofossils. We use detrital Detrital zircons as young as ca. 520 Ma et al., 2000) or as proximal molasse (Soegaard zircon U-Pb ages in the Van Horn Formation to show that the Van Horn Formation, previ- and Callahan, 1994). Consequently, those units assess its maximum depositional age and rela- ously considered to be Precambrian in age, should contain a provenance signal fi ngerprint- tion of the Van Horn Formation to the location is no older than middle Cambrian. Further, ing their connection to source terranes exhumed of the basal Sauk surface in west Texas. the overall detrital zircon age spectrum by the Grenville orogeny. We use laser ablation of the Van Horn Formation is similar to inductively coupled plasma–mass spectrometry GEOLOGICAL SETTING that of the overlying Cambro-Ordovician (ICP-MS) detrital zircon U-Pb geochronology Bliss Formation: both indicate derivation to assess this prediction and as a potential test of Pre–1.3 Ga Rocks from sources that included the Colorado- correlations of the formations in west Texas. Oklahoma aulacogen, Grenville, Granite- Nearly 300 m.y. after the assembly of Rodinia, Pre–1.3 Ga basement rocks in the southwestern Rhyolite, and Yavapai/Mazatzal provinces. the supercontinent fragmented. The initial epi- United States belong to the 1.8–1.6 Ga Yavapai/ The similarities between the depositional sode of rifting along Laurentia’s margins began Mazatzal and 1.5–1.3 Ga Granite-Rhyolite prov- history of the Van Horn and Bliss Forma- in the early Neoproterozoic (ca. 780–740 Ma), inces (Fig. 1). These units incorporate arc and tions lead us to conclude that the base of the and extensional tectonism continued for nearly arc-related supracrustal rocks as well as A-type Sauk Sequence in west Texas occurs at the 250 m.y. (Macdonald et al., 2013, and refer- granites emplaced behind active continental arcs base of the Van Horn Formation. Base-level ences therein). Thermal subsidence analyses during intra- and back-arc extension (Karlstrom rise associated with the Sauk transgression have revealed that, despite this protracted period and Bowring, 1993; Slagstad et al., 2009). affected drainage patterns and sediment of continental rifting, the rift-to-drift transition deposition along southwestern Lauren- occurred near the time of the Precambrian-Cam- Grenville Orogeny and Sedimentation tia some 20 m.y. earlier than previously brian boundary along both the eastern and west- assumed. ern margins of Laurentia (ca. 540 Ma) (Armin Along the southeast Laurentian margin, the and Mayer, 1983; Bond and Kominz, 1984; Wil- Grenville orogeny (Rivers, 1997; Carr et al., †E-mail: [email protected] liams and Hiscott, 1987; Cawood et al., 2001). 2000; Chiarenzelli et al., 2010) is attributed to GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–12; doi: 10.1130/B30884.1; 10 fi gures; 2 tables; Data Repository item 2014179. For permission to copy, contact [email protected] 1 © 2014 Geological Society of America Spencer et al. Figure 1. Map showing tec- Yavapaii Wichita Mtns (McMillan and McLemore, 2004; Keller and tonic provinces and distribu- Province 539±2, 535±8, Stephenson, 2007). Extensive exposures of tion of surface exposure of 533±4, 533±2 Ma Arbucklele Mtns Cambrian-age bimodal igneous rocks within Oklahoma 539±5, 536±56 Ma Precambrian and Cambrian Mazatzal Provincev the Oklahoma-Colorado aulacogen indicate that rocks (after Whitmeyer and crustal extension (Larson et al., 1985; McMil- Aul.uulul. Karlstrom, 2007; Stoeser et al., Franklin Mountainsn ll. lan and McLemore, 2004; Gilbert and Hogan, (fig. 2) 2007; position of Pre cordi llera te 2010) propagated broadly cratonward with ages N 33° after Thomas, 2006). U-Pb 1111–1120 MaM ranging from 539 Ma to 528 Ma, based on laser Grani Precordillera?r crystallization ages of Meso- Rhyolite ablation (LA)–ICP–MS and ID-TIMS U-Pb zir- proterozoic and Cambrian ig- Grenville Thrust FrontProvince Llano Uplift 1288–1232, con geochronology (Larson et al., 1985; Lam- neous rocks are also indicated 1147–1128, bert et al., 1988; Hames et al., 1995; Hogan and 1256– 1119–1070 Ma (see text for references). Politi- 1243 Ma Grenville Province Gilbert, 1998; McConnell and Gilbert, 1990; cal boundary base map is from McMillan and McLemore, 2004). In southwest- Vanann HornHo (fig. (fig 2) Wikimedia Commons (http:// ern New Mexico, the Florida Mountains pluton N 29° commons .wikimedia .org /wiki has also been dated by zircon U-Pb geochronol- /File: Blank _US _Map .svg) and inset ogy as crystallizing at ca. 510 Ma (Amato and is used under the Creative above Cambrianm Mack, 2012). 0 200 km Commons Attribution-Share- Precambrianca Alike license. Oklahoma Aul.— N W 100° W 95° Sauk Transgression Oklahoma aulacogen. The breakup of Rodinia and subsequent ther- mal subsidence of the Laurentian margins facili- collision with the Kalahari and/or Amazon cra- broadly equivalent to the Castner Marble as well tated deposition of a thick sequence of largely tons (Dalziel et al., 2000; Tohver et al., 2006; as with the Bass Limestone in the Grand Can- marine sediments (e.g., Bond and Kominz, Jacobs et al., 2008; Hynes and Rivers, 2010). yon. Overlying the Allamoore is the Tumble- 1984; Levy and Christie-Blick, 1991). These This orogenic episode is archived in rocks form- down Formation, a unit of volcanic and carbon- defi ne the Sauk Sequence (Sloss, 1963), which ing the broad Grenville province in southeastern ate breccia in which a felsic tuff near the top of spans the late Neoproterozoic to mid-Ordovi- Canada, inliers in the Appalachian Mountains, the formation has yielded an ID-TIMS U-Pb zir- cian and blankets approximately half of North and the Llano uplift of central Texas (Fig. 1) con age of 1243 ± 10 Ma (Bickford et al., 2000). America (outcrop and borehole data; Peters and where three main phases of magmatic activ- The Tumbledown Formation is overlain by the Gaines, 2012). ity are known: arc volcanism and accretion Hazel Formation, which is a ~3000-m-thick unit The Sauk Sequence overlies progressively between 1288 and 1232 Ma, collision-related of conglomerate and interbedded fi ne-grained older units cratonward, from rift/post-rift– magmatism between 1150 and 1120 Ma, and sandstone whose thickness decreases signifi - related Neoproterozoic sequences along the post-collisional magmatism between 1120 and cantly to the north (Bickford et al., 2000). margins of Laurentia to Archean–Proterozoic 1070 Ma (Mosher, 1998). The initial Neoproterozoic rifting of the west- basement in the central regions of the craton In west Texas, exposures of Precambrian ern margin of Laurentia is recorded by a ca. (Rankin, 1993). This transgressive episode was rocks are restricted to the Franklin Mountains 780 Ma magmatic event stretching from Utah to long lived, with the base of the Sauk Sequence north of El Paso, and the region around Van the Yukon (Jefferson and Parrish, 1989; Harlan being ~30–40 m.y.