Análisis de Infraestructura para servicios culturales en Chiclayo

Neoproterozoic rocks of southeastern Idaho and northern Utah formed within extensional basins as part of the multi-phased Rodinian supercontinent breakup that culminated with the development of a passive continental margin extending from eastern Alaska to California by ~540 Ma (e.g., Bond, Kominz, and Devlin, 1983; Ross, 1991, Levy and Christie-Blick, 1991; Lund et al., 2003; Dickinson, W.R., 2006; Colpron,

Logan, and Mortensen, 2002; Macdonald 2013b; Fedo and Cooper, 2001; Yonkee et al., 2014) (Figure 1). Regional exposures today are generally located along the eastern edge of the late Jurassic to early Cenozoic Cordilleran fold-and-thrust belt of the eastern Basin-and-Range province. Commonly correlated glacial deposits in the region include the Pocatello Formation in Idaho, the Mineral Fork Formation, Perry Canyon Formation, Horse Canyon Formation, Sheeprock Group, and Trout Creek sequence in Utah (e.g., Crittenden, Christie-Blick, and Link, 1983; Link et al., 1993; Balgord et al., 2013;

Yonkee et al., 2014). Thrust sheets associated with these exposures belong to the Willard – Paris – Putnam system of northern Utah and southeastern Idaho, and the Tintic – Sheeprock – Canyon Range system of west-central Utah (Link et al., 2011 and references therein).

The greenschist facies Pocatello Formation is a roughly 1.5 km thick package of dominantly siliciclastic, lesser volcanic rocks, and rare carbonate rocks exposed in the Pocatello and Bannock Ranges of southeastern Idaho and northern Utah (Link et al., 1993; Link and Christie-Blick, 2011; Dehler, Anderson, and Nagy, 2011). The formation was transported eastward ~100 km in the Mesozoic by the Paris-Willard thrust and was subsequently exposed via regional normal faulting during Basin and Range extension (Link, 1983; Yonkee et al., 2014). The type section of the Pocatello Formation, located at the Portneuf Narrows east of Pocatello, ID, is divided into three members: the lower Bannock Volcanic Member, the middle Scout Mountain Member and an informal upper member (Link, 1983) (Figure 1).

Exposures of the Bannock Volcanic member comprise mafic metavolcanics and volcaniclastic rocks (Link, 1983). The volcanic rocks are tholeiitic-alkaline to alkaline in

composition, and are products of intra-plate rift volcanism (Harper and Link, 1986; Keeley and Link, 2011). Felsic volcanic clasts within the overlying Scout Mountain Member represent a cryptic felsic component of bimodal volcanism within the Bannock Volcanic Member (Fanning and Link, 2004). Additionally, basalts are locally interbedded within the Scout Mountain Member, providing evidence that the transition between the two members may be conformable (Keeley and Link, 2011; Keeley et al., 2013). Exposures of the unit vary in thickness (from 200-450 m) and grade upward into the overlying Scout Mountain Member.

A variety of lithotypes comprise the ~800m thick Scout Mountain Member (Link, 1982) (Figure 1). The base is composed primarily of a matrix-supported diamictite that contains predominantly mafic and rare felsic volcanic clasts, some of which are glacially striated. This “lower diamictite” is overlain by siltstone, sandstone, cobble conglomerate and an “upper diamictite”. The upper matrix-supported diamictite is clast-rich, containing granitic, gneissic, quartzitic, and felsic volcanic clasts, including rare glacially striated, yet abundant faceted, clasts (Link, 1982). Overlying the upper diamictite is a pink to buff-colored dolomite with interbedded sandstone that culminates with aragonite crystal fans (Lorentz, Corsetti, and Link, 2004; Dehler et al., 2011). The lateral extent (>100 km), correlation with glacial deposits to the south (northern Utah’s Oxford Mountain and Perry Canyon, Yonkee et al., 2014), and occasional striated/faceted clasts indicate a glacial origin for the diamictites (e.g., Crittenden et al, 1983; Link, Miller, and Christi- Blick, 1994). As a whole, this unit has been interpreted to record the relatively rapid deposition of immature subaqueous sediments containing glacial till (Crittenden et al., 1983; Link et al., 1994) representing two phases of the ~717-660 Ma Sturtian glaciation

(Fanning and Link, 2004; 2008; Balgord et al., 2013; Yonkee et al., 2014). However, based upon the distinctive lithology of the upper cap carbonate, others have correlated the upper diamictite unit with the ~645-635 Ma Marinoan glaciation (e.g. Dehler et al., 2011; Petterson et al., 2011; Macdonald et al., 2013a). The Scout Mountain Member grades into the informal upper member, which comprises primarily (>600 m) laminated sandstone and a lesser carbonate unit (Link, 1982). The upper carbonate and sandstone units

represent shoreface and deeper water conditions interpreted to be the result of post-glacial eustatic sea level rise (Link, 1983; Link et al., 1994; Dehler et al., 2011).

Radioisotopic age determination for the Pocatello Formation have proved to be problematic. Fanning and Link (2004) interpreted the diamictite portion of the Pocatello Formation to be younger than their U-Pb zircon SHRIMP date of 717 ± 4 Ma from a rhyolitic clast within the upper diamictite at Portneuf Narrows, which they later revised to 701 ± 4 Ma (Fanning and Link, 2008). They went on to contend that a U-Pb zircon

SHRIMP date of 709 ± 5 Ma, from an epiclastic tuff breccia some 50m below diamictite, further constrains the maximum age for the formation (Fanning and Link, 2004). A second SHRIMP analysis of the original sample by Fanning and Link (2008) yielded a revised date of 686 ± 4 Ma, and those authors also acknowledged the stratigraphic

position was not well constrained due to extensive Sevier age thrusting within the section. Condon and Bowring (2011) re-evaluated the original epiclastic tuff breccia sample of Fanning and Link (2004), along with a recollected equivalent sample (with Fanning and Link) from within the diamictite, using high-precision CA-IDTIMS (precision ~0.1%) and found a multimodal detrital signal. Condon and Bowring (2011) concluded that the relatively low precision of SHRIMP dates (precision >1%) lacked the

resolution to determine mixed date populations, and assigned a maximum depositional age of 687 ± 1.3 Ma on the basis of the youngest single CA-IDTIMS date. This is not to say that in situ techniques are inaccurate themselves, but are more subjective in

interpretation as they lack sufficient resolution to determine mixed date populations within a sample at less than a few percent difference, or identify those grains that have experienced small amounts of Pb-loss thereby recording spuriously younger dates. The lack of resolution by in situ techniques alone indicates that regional correlation based on weighted means of the youngest grains in detrital units should not be made as they may or may not be accurate or equivalent.

This point was further highlighted in a subsequent investigation by Keeley et al. (2013). The authors describe a suite of facies within the Oxford Mountain tuffite

(including Fanning and Link’s (2004, 2008) and Condon and Bowring’s (2011) epiclastic tuff breccia), interbedded and with gradational contacts within the lower diamictite of the Scout Mountain member. CA-IDTIMS analysis of five different tuffite samples identified a range of crystal ages from ~685 Ma up to ~709 Ma, which must represent epiclastic detritus from protracted regional volcanism. A youngest weighted mean date of 685.5 ± 0.4 Ma for ten grains from one sample of volcaniclastic diamictite with cobble-sized volcanic clasts relatively high in the lower diamictite stratigraphy provided a robust maximum depositional age for any overlying units. Keeley et al. (2013) favorably correlated this 685.5 ± 0.4 Ma detrital maximum age with relatively imprecise weighted mean SHRIMP dates of 685.6 ± 7 and 684.6 ± 4 Ma reported for volcanics from central Idaho’s Edwardsburg Formation (Lund et al., 2003). However, the SHRIMP data from Lund et al.’s (2003) investigation are complex (ranging from ~660 Ma to 717 Ma with a

few analyses at ~600, 740 and 1180 Ma), may have experienced from variable degrees of Pb-loss (thus skewing the apparent dates), and likely only subjectively correlates with the more precise CA-IDTIMS analysis.

The disparity in maximum ages for the Pocatello Formation raise uncertainty about its published minimum age as well. In accord with Condon and Bowring’s (2011) work, Petterson et al. (2011) suggest caution accepting Fanning and Link’s (2004) minimum SHRIMP date for glaciation of 667 ± 5 Ma from their reworked fallout tuff situated above the upper diamictite at Portneuf Narrows as robust minimum age

constraint of glaciation. Data from this study demonstrate that the “reworked fallout tuff” is in fact an epiclastic siltstone and should be regarded as a maximum depositional age only, with the likeliness that the enclosing strata are much younger.