Age constraint for the Moreno Hill Formation (Zuni Basin) by CA-TIMS and LA-ICP-MS detrital zircon geochronology

The “mid-Cretaceous” (~125–80 Ma) was punctuated by major plate-tectonic upheavals resulting in widespread volcanism, mountain-building, eustatic sea-level changes, and climatic shifts that together had a profound impact on terrestrial biotic assemblages. Paleontological evidence suggests terrestrial ecosystems underwent a major restructuring during this interval, yet the pace and pattern are poorly constrained. Current impediments to piecing together the geologic and biological history of the “mid-Cretaceous” include a relative paucity of terrestrial outcrop stemming from this time interval, coupled with a historical understudy of fragmentary strata. In the Western Interior of North America, sedimentary strata of the Turonian–Santonian stages are emerging as key sources of data for refining the timing of ecosystem transformation during the transition from the late-Early to early-Late Cretaceous. In particular, the Moreno Hill Formation (Zuni Basin, New Mexico) is especially important for detailing the timing of the rise of iconic Late Cretaceous vertebrate faunas. This study presents the first systematic geochronological framework for key strata within the Moreno Hill Formation. Based on the double-dating of (U-Pb) detrital zircons, via CA-TIMS and LA-ICP-MS, we interpret two distinct depositional phases of the Moreno Hill Formation (initial deposition after 90.9 Ma (middle Turonian) and subsequent deposition after 88.6 Ma (early Coniacian)), younger than previously postulated based on correlations with marine biostratigraphy. Sediment and the co-occurring youthful subset of zircons are sourced from the southwestern Cordilleran Arc and Mogollon Highlands, which fed into the landward portion of the Gallup Delta (the Moreno Hill Formation) via northeasterly flowing channel complexes. This work greatly strengthens linkages to other early Late Cretaceous strata across the Western Interior.


METHODS
All Moreno Hill samples were collected from the eastern slopes of Santa Rita Mesa at Cox Ranch, located north of Quemado (Salt Lake Coal Field), west-central New Mexico (BLM Permit NM17-02S). Three key stratigraphic zones were selected based on stratigraphic position or proximity to known fossil horizons (Fig. 2). Specifically, bulk samples were collected from the contact with the underlying Atarque (basal Moreno Hill (Cox 2), along with the middle (Cox 3) and uppermost (Cox 4) bedded sandstone strata of the Moreno Hill Formation. A fourth sample was collected near Zuni Salt Lake from the underlying Dakota Sandstone and is also presented herein (whilst Carpenter (2014) notes that "Naturita Sandstone" is a more apt name for this unit on the Colorado Plateau we prefer "Dakota Sandstone" to facilitate reference to previous research). All samples collected were fresh and unweathered; at least 0.5-1.0 m of the exterior surface of the rock was removed from the outcrop face before collecting due to surficial weathering. Two gallon-sized bags of sandstone were collected at each site (Dakota Sandstone, Cox 2, 3, and 4) and processed accordingly to the techniques set forth by the Central Analytical Facility at Stellenbosch University including crushing, milling, panning, Frantz magnetic separation, and density liquid separation. Thereafter, analysis of the recovered zircon grains was undertaken via standard methods of Boise State University's Isotope Geology Laboratory (Boise, Idaho, USA) as noted in several other studies (Bold, 2016;MacNaughton et al., 2016;Normore et al., 2018;Tucker et al., 2020).

LA-ICP-MS methods
Zircon grains were annealed at 900 C for 60 h in a muffle furnace, and randomly selected grains were mounted in epoxy and polished until their centers were exposed (Fig. 3). Cathodoluminescence (CL) images were obtained with a JEOL JSM-300 scanning electron  microscope and Gatan MiniCL. Zircons were analyzed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) using a ThermoElectron X-Series II quadrupole ICPMS and New Wave Research UP-213 Nd:YAG UV (213 nm) laser ablation system. In-house analytical protocols, standard materials, and data reduction software were used for acquisition and calibration of U-Pb dates and a suite of high field strength elements (HFSE) and rare earth elements (REE). Zircons were ablated with a laser spot of 25 µm wide using fluence and pulse rates of 5 J/cm 2 and 5 Hz, respectively, during a 45-s analysis (15-s gas blank, 30-s ablation) that excavated a pit~15 µm deep. Ablated material was carried by a 1.2 L/min He gas stream to the nebulizer flow of the plasma. Quadrupole dwell times were 5 ms for Si and Zr, 200 ms for 49 Ti and 207 Pb, 80 ms for 206 Pb, 40 ms for 202 Hg, 204 Pb, 208 Pb, 232 Th, and 238 U and 10 ms for all other HFSE and REE; total sweep duration is 950 ms. Background count rates for each analyte were obtained prior to each spot analysis and subtracted from the raw count rate for each analyte. For concentration calculations, background-subtracted count rates for each analyte were internally normalized to 29 Si and calibrated with respect to NIST SRM-610 and -612 glasses as the primary standards. Ablations pits that appear to have intersected glass or mineral inclusions were identified based on Ti and P signal excursions, and associated sweeps were discarded. U-Pb dates from these analyses are considered valid if the U-Pb ratios appear to have been unaffected by the inclusions. Signals at mass 204 were normally indistinguishable from zero following subtraction of mercury backgrounds measured during the gas blank (<1,000 cps 202 Hg), and thus dates are reported without common Pb correction. Rare analyses that appear contaminated by common Pb were rejected based on mass 204 greater than baseline. Temperature was calculated from the Ti-in-zircon thermometer (Watson, Wark & Thomas, 2006). Because there are no constraints on the activity of TiO 2 , an average value in crustal rocks of 0.8 was used. Data were collected in three experiments in July 2020. For U-Pb and 207 Pb/ 206 Pb dates, instrumental fractionation of the background-subtracted ratios was corrected and dates were calibrated with respect to interspersed measurements of zircon standards and reference materials. The primary standard Plešovice zircon (Sláma et al., 2008) was used to monitor time-dependent instrumental fractionation based on two analyses for every 10 analyses of unknown zircons.
Radiogenic isotope ratio and age error propagation for all analyses include uncertainty contributions from counting statistics and background subtraction. The standard calibration uncertainty for U/Pb is the local standard deviation of the polynomial fit to the fractionation factor of Plešovice vs time and for 207 Pb/ 206 Pb is the standard error of the mean of the fractionation factor of Plešovice. These uncertainties are 0.6-1.0% (

U-Pb dates were obtained by the Chemical Abrasion Isotope Dilution Thermal Ionization
Mass Spectrometry (CA-TIMS) method from analyses composed of single zircon grains (Table 1), modified after Mattinson (2005). Zircon was removed from the epoxy mounts for dating based on CL images and LA-ICP-MS dates. In two samples, grains that yielded the five youngest LA-ICP-MS dates from an initial round were analyzed in a second round. Dates from both rounds agreed in most cases. Grains were selected for CA-TIMS from this population. For a third sample, the grains were too small to permit a second round of analysis.
Zircon was put into 3 ml Teflon PFA beakers and loaded into 300 ml Teflon PFA microcapsules. Fifteen microcapsules were placed in a large-capacity Parr vessel and the zircon partially dissolved in 120 ml of 29 M HF for 12 h at 190 C. Zircon was returned to 3 ml Teflon PFA beakers, HF was removed, and zircon was immersed in 3.5 M HNO 3 , ultrasonically cleaned for an hour, and fluxed on a hotplate at 80 C for an hour. The HNO 3 was removed and zircon was rinsed twice in ultrapure H 2 O before being reloaded into the 300 ml Teflon PFA microcapsules (rinsed and fluxed in 6 M HCl during sonication and washing of the zircon) and spiked with the EARTHTIME mixed 233 U-235 U-202 Pb-205 Pb tracer solution (ET2535). Zircon was dissolved in Parr vessels in 120 ml of 29 M HF with a trace of 3.5 M HNO 3 at 220 C for 48 h, dried to fluorides, and re-dissolved in 6 M HCl at 180 C overnight. U and Pb were separated from the zircon matrix using an HCl-based anion-exchange chromatographic procedure (Krogh, 1973), eluted together and dried with 2 µl of 0.05 N H 3 PO 4 .
Pb and U were loaded on a single outgassed Re filament in 5 µl of a silica-gel/phosphoric acid mixture (Gerstenberger & Haase, 1997), and U and Pb isotopic measurements made on a GV Isoprobe-T multicollector thermal ionization mass spectrometer equipped with an ion-counting Daly detector. Pb isotopes were measured by peak-jumping all isotopes on the Daly detector for 160 cycles and corrected for mass fractionation using the known 202 Pb/ 205 Pb ratio of the ET2535 tracer solution. Transitory isobaric interferences due to high-molecular-weight organics, particularly on 204 Pb and 207 Pb, disappeared within approximately 30 cycles, while ionization efficiency averaged 10 4 cps/pg of each Pb isotope. Linearity (to ≥ 1.4 × 10 6 cps) and the associated deadtime correction of the Daly detector were determined by analysis of NBS982. Uranium was analyzed as UO þ 2 ions in static Faraday mode on 10 12 ohm resistors for 300 cycles, and corrected for isobaric interference of 233 Jaffey et al. (1971), and 238 U/ 235 U of 137.818 (Hiess et al., 2012). The 206 Pb/ 238 U ratios and dates were corrected for initial 230 Th disequilibrium using D Th/U = 0.2 ± 0.1 (2 sigma) and the algorithms of Crowley, Schoene & Bowring (2007),  Notes: (a) z1, z2, etc. are labels for analyses composed of single zircon grains that were annealed and chemically abraded (Mattinson, 2005). z1a and z1b are fragments from the same grain.
(c) Pb * and Pbc are radiogenic and common Pb, respectively. mol % 206 Pb * is with respect to radiogenic and blank Pb.
(d) Measured ratio corrected for spike and fractionation only. Fractionation correction for single-collector Daly analyses is based on measurement of 202Pb/205Pb in the EARTHTIME ET2535 tracer solution.
resulting in an increase in the 206 Pb/ 238 U dates of~0.09 Ma. All common Pb in analyses was attributed to laboratory blank and subtracted based on the measured laboratory Pb isotopic composition and associated uncertainty. U blanks are estimated at 0.013 pg. A weighted mean 206 Pb/ 238 U date is calculated from equivalent dates (probability of fit > 0.05) using Isoplot 3.0 (Ludwig, 2003). The error is given as ±x/y/z, where x is the internal error based on analytical uncertainties only, including counting statistics, subtraction of tracer solution, and blank and initial common Pb subtraction, y includes the tracer calibration uncertainty propagated in quadrature, and z includes the 238 U decay constant uncertainty propagated in quadrature. Internal error should be considered when comparing our date with 206 Pb/ 238 U dates from other laboratories that used the same tracer solution or a tracer solution that was cross-calibrated using EARTHTIME gravimetric standards. Error including the uncertainty in the tracer calibration should be considered when comparing our date with those derived from other geochronological methods using the U-Pb decay scheme (e.g., laser ablation ICPMS). Error including uncertainties in the tracer calibration and 238 U decay constant (Jaffey et al., 1971) should be considered when comparing our date with those derived from other decay schemes (e.g., 40 Ar/ 39 Ar, 187 Re-187 Os). Errors are at 2 sigma.

CA-TIMS
Results from CA-TIMS dating are used to establish MDAs for the Moreno Hill Formation (Table 1). Two grains from the lower Moreno Hill Formation (Cox 2) yield CA-TIMS dates of 91.275 ± 0.048 and 90.855 ± 0.040 Ma, indicating that deposition occurred after 90.9 Ma. Two grains from the middle Moreno Hill Formation (Cox 3) yield dates of 99.095 ± 0.051 and 91.454 ± 0.043 Ma. These dates are older than the MDA from the underlying sample, and thus the grains are not close in age to sedimentation. For the upper Moreno Hill Formation (Cox 4) two grains were analyzed by CA-TIMS. One yields a date of 94.338 ± 0.142 Ma that is older than the MDA from the underlying lower Moreno Hill Formation, and thus the grain is not close in age to sedimentation. The other was broken into two fragments that were analyzed separately and yield equivalent dates of 88.648 ± 0.074 and 88.401 ± 0.286 Ma, with a weighted mean of 88.632 ± 0.072/0.084/ 0.127 Ma (MSWD = 2.8, probability of fit = 0.09) ( Table 1). This is taken as the MDA, indicating deposition was after 88.6 Ma. Based on the assumption that the MDAs from the lower and upper Moreno Hill Formation are close in age to deposition, the formation is interpreted as being deposited between the Late Turonian and earliest Coniacian.

LA-ICP-MS
The four samples (Dakota Sandstone and lower, middle, and upper Moreno Hill Formation) contained a broad spectrum of detrital zircon. Individual grains range from well-faceted euhedral, to somewhat rounded or minorly cracked, to fractured, rounded, or fragmented. All of the grains (save for a minor few) have oscillatory and sector zoning indicative of igneous growth (Fig. 3). Many grains have distinct cores. Zircons in the Dakota Sandstone are more commonly round, indicative of transport, compared with the well-faceted zircon that is common in the lower to middle Moreno Hill. Based on the recent work of Coutts, Matthews & Hubbard (2019), Herriott et al. (2019), and Beveridge, Roberts & Titus (2020), our study also ran an additional filter based on Tucker et al. (2013Tucker et al. ( , 2016Tucker et al. ( , 2020, which omits analyses with a greater than 5% (at 2σ analytical uncertainty) discordance based on the 207 Pb/ 235 U and 206 Pb/ 238 U ratios. The 5% filter is more rigorous than Tucker et al. (2013Tucker et al. ( , 2016, which utilized 10-15%, based on the recent results by Beveridge, Roberts & Titus (2020). For LA-ICPMS-based MDAs, we utilized the following five analyses for sample sets ranging from 30 to 100 grains: YDZ (n = 6) (Youngest Detrital Zircon); YC2σ (n = 6) (Youngest Cluster of grains with overlapping 2σ uncertainty); Weighted Average (n = 6); and TuffZirc (n = 6); with the new addition of YSP (n > 6) (Youngest Statistical Population) (Ludwig, 2003;Coutts, Matthews & Hubbard, 2019;Herriott et al., 2019). Results are described herein and depicted in Fig. 4.

Dakota sandstone
In agreement with Pike (1947), Wolfe (1989) and more recently Carpenter (2014), and based on regional characterization of the trough-bedded alluvial sandstone, this sample was recovered from the laterally extensive lower "Dakota alluvial unit" or "main body of the Dakota Sandstone" (McLellan et al., 1983b). We do not, however, recognize the "Cliff Dwellers Sandstone" nomenclature for this unit (Wolfe, 1989). The Dakota Sandstone has the widest spectrum of dates and grain morphologies. Of the 97 grains ablated (86 included Table 2). The mean of the youngest date signatures is 96.1 Ma (Table 2; Table S1). Based on this date we suspect that this lower "Dakota sandstone" is equivalent to the basal Naturita Sandstone in Southern Utah (Barclay et al., 2015;Laurin et al., 2019;Tucker et al., 2020).

Lower Moreno Hill Formation
Of the 101 grains ablated (88 included Table 2). The mean of the youngest date signatures is 89.6 Ma ( Table 2; Table S1).

Detrital populations: precambrian & phanerozoic-paleozoic
This study builds upon well-documented tectonic reconstructions and source terranes (Fig. 5) (Willis, 1999;Dickinson & Gehrels, 2003, 2008DeCelles, 2004;Lawton & Bradford, 2011;Laskowski, DeCelles & Gehrels, 2013), which allow for reliable linkages. The Dakota Sandstone sample displays a diverse assemblage of grain ages and populations. In contrast, zircons from the Moreno Hill Formation are either Precambrian n > 1.0 Ga or Mesozoic (n < 251 Ma). All samples contain individual grains or minor populations that are >2.0 Ga and are likely reworked continental fragments (Gehrels et al., 1995;Linde et al., 2016;Tucker et al., 2020). Large populations of grains between 1.9 and  (Gehrels et al., 1995;Van Schmus, Bickford & Turek, 1996;Whitmeyer & Karlstrom, 2007). Due to the distance of the Sevier (west) and Maria (south-southwest) Fold and Thrust Belts, grains between 1.7 and 1.2 Ga are likely from uplifted basement (Yavapai/Mazatzal). Therefore this study is in agreement with regional studies that the most likely source terrane for the above-mentioned grains is the Mogollon Highlands (Barth et al., 2004;Dickinson & Gehrels, 2008;Salem, 2009;Lawton & Bradford, 2011;Laskowski, DeCelles & Gehrels, 2013;Szwarc et al., 2015). Our study also recognized the possibility, though less likely, that source terranes include but are not limited to (1) Antarctica; (2) Australia; (3) Africa; or even yet to be identified proto-Rodinian terranes (Gehrels & Stewart, 1998;Laskowski, DeCelles & Gehrels, 2013;Linde et al., 2016). Of the four samples included in this study, only the Dakota Sandstone contains a population of Paleozoic grains (n = 4). This may reflect a bias in sampling or more likely that sources such as the Amarillo-Wichita to Appalachian Orogeny as noted by Laskowski, DeCelles & Gehrels (2013) did not contribute much into this area. In any event, dates from this period in North America's tectonic history are distinctly absent within the Moreno Hill Formation (Fig. 5). All of these grain populations can also be linked to heavily-reworked multi-generational recycling of sedimentary blankets lying east of the Sevier Highlands and north of the Mogollon Highlands, potentially including proximal units underlying the sub-Dakota Sandstone angular unconformity (Wolfe, 1989;Molenaar et al., 2002;Dickinson & Gehrels, 2003Lawton, Pollock & Robinson, 2003;DeCelles, 2004;Laskowski, DeCelles & Gehrels, 2013;Gehrels & Pecha, 2014). Yonkee & Weil, 2015). Cretaceous grain populations could yet be linked to the Mogollon Highlands where Pike (1947) noted sedimentary rocks (with interbedded volcanics) indicated by fossil content to be "Benton-aged" near Deer Creek, Arizona.

K-S analysis
The Kolmogorov-Smirnov test (K-S test) was applied to determine the likelihood that the age profiles of sampled zircons obtained from the Dakota Sandstone and from the lower, middle and upper members of the Moreno Hill are statistically similar (pass) or dissimilar (fail). In this study, we utilized a Cumulative Distribution Function (CDF) via date and its corresponding uncertainty with a 95 % confidence interval, in that p-values > 0.05 pass and < 0.05 fail the test (Fig. 7) (Dickinson & Gehrels, 2008;Barbeau et al., 2009a;Tucker et al., 2016Tucker et al., , 2020. When comparing all four samples based on a CDF across all reported dates spanning Precambrian-Proterozoic-Mesozoic-Cretaceous we note no statistical similarity between the underlying Dakota Sandstone and the overlying Moreno Hill (p-value = 0.00). Furthermore, with the same parameters, the lower and middle Moreno Hill samples have genetic similarity (p-value = 0.222), yet neither showed similarity with the upper Moreno Hill sample (p-values = 0.003 and 0.001). The lack of genetic similarity between Moreno Hill samples reflects (1) the limited number of recovered zircon grains from the upper Moreno Hill; (2) statistical effects of the 5% cutoff, and; (3) the very youthful multi-grain population between 89 and 88 Ma that is not present in the lower to middle Moreno Hill. This pattern is somewhat different when the K-S test is applied only to Mesozoic populations. For instance, the Dakota and the middle Moreno Hill have genetic similarity (p-value = 0.790), yet the Dakota fails to be significantly different from all other comparisons. When we compare only Moreno Hill samples, the lower and middle samples present genetic similarity (p-value = 0.220), and no similarity to the upper Moreno Hill (p-value = 0.003/0.001) (Fig. 7). Such subtle variances in confidence are potentially linked to variably geographically influenced drainage systems and/or to temporally dissimilar volcanic inliers within the westerly lying arc rather than to a single, enduring source (Fitz-Diaz, Hudleston & Tolson, 2011 and references therein).
The above described multi-faceted source terrane narrative is fairly complex; therefore, we sought to confirm these observations by utilizing the methods described by Cawood, Hawkesworth & Dhuime (2012), which links cumulative proportion curves with tectonic sources (Fig. 7). When results for dates between 0 and 3.5 Ga are plotted on cumulative proportion curves, all four samples variably plot between zones A (convergent), B (collisional), and C (extensional) confirming the complex detrital source terrane history (Cawood, Hawkesworth & Dhuime, 2012). By and large, all youthful populations (n < 100 Ma) from all four samples indicate a convergent margin (westerly lying arc); yet, weak genetic similarity between youthful samples indicates that these are likely derived from different volcanic inliers (Dakota and the middle Moreno Hill p-value = 0.790 and the lower and upper Moreno Hill p-value = 0.211, with all other comparisons failing the 0.05 significance threshold). It should be noted that with the 5% filter, only 16 grains were approved for the upper Moreno Hill, and should be treated as a proxy only. However, when all the samples are presented on a detrital zircon age probability plot, irrespective of the final

DISCUSSION
This study sought to (1) refine the depositional age and duration of sedimentation of the Moreno Hill Formation; (2) provide newly calibrated linkages based on the revised temporal framework; and (3) determine likely source terranes. Our results demonstrate that emplacement of sediment into the Moreno Hill depo-center was diachronous, occurring in two distinct phases. The first phase of sedimentation occurred after 90.9 Ma and terminated before 88.6 Ma (lower to middle Moreno Hill), spanning the late Turonian to very early Coniacian (Fig. 8). The second phase of sedimentation occurred shortly thereafter 88.6 Ma (upper Moreno Hill), very early Coniacian (Fig. 8) (Cohen et al., 2013). This two-phased deposition is reflected in the results of the K-S test, with only the lower and middle Moreno Hill having genetic similarity. Therefore, a comparison of stratigraphic position and the resulting MDAs would strongly indicate that there was no synchronicity between sedimentation and the emplacement of detrital zircons in the middle Moreno Hill, rather that only the lower and upper Moreno Hill could potentially be nearer to syndepositional (Rossignol et al., 2019;Tucker et al., 2020). In light of the seemingly strong temporal relationship between the lower and middle members of the Moreno Hill, the current informal subdivision of the formation into its three members will be reassessed in a forthcoming manuscript. This study notes that based on the depositional nature of detrital zircon (Gehrels, 2014), sedimentation into the Moreno Hill could have been entirely within the very early Coniacian; however, key pieces of evidence seemingly corroborate staggered pulses of detrital-rich sedimentation into the Moreno Hill depo-center. The first phase of sedimentation is well-documented to have co-occurred with anoxic paleosols and distinct coal horizons within a distal alluvial plain ( Fig. 9) (McLellan et al., 1983a;Landis et al., 1985;Mack, 1992;Hoffman, 1996;Sweeney et al., 2009). On the other hand, the upper Moreno Hill is distinctly different, preserving paleosols generally indicative of subaerial conditions and aridification with co-occurring fluvial channel belts (Roybal, 1982;Campbell, 1984;Hoffman, 1996). While the complete lithological review of the Moreno Hill is the focus of a forthcoming manuscript, we wish to highlight the above-mentioned differences between the lower and upper Moreno Hill are supported by our own observations. The lower Moreno Hill is characterized by thickly interbedded dark fissile to blocky plant hash-rich mudrocks with associated coal seams, finely laminated siltstones, fine to medium-grained laterally discontinuous sandstones, and fine to coarsely-grained sublitharenitic to subarkosic laterally continuous multi-story planar and distinctly trough-cross-bedded ledge-forming sandstones more prevalent towards the upper parts of the member; whereas, the upper Moreno Hill is characterized by lighter-colored more fissile mudrocks, siltstones and lesser-occurring very finely to medium-grained subarkosic lenticular trough-cross-bedded sandstones (Campbell, 1984;Cook & Arkell, 1987) (details to be updated in forthcoming manuscript). Beyond local sedimentological differences, regional linkages can be utilized to corroborate this study's interpretation of diachronous sedimentation. Based on regional biostratigraphic linkages of C. woollgari woollgari and either M. labiatus (Wolfe & Kirkland (1998, p. 304) or M. mytiloides (Kirkland, Smith & Wolfe (2005, p. 89)), the underlying diachronous Atarque Sandstone was emplaced during the latest early-Turonian to early-middle Turonian (~93.4 and~92.5 Ma) Cobban et al., 2006). Biostratigraphically controlled radiometric

Date in Ma
First Pulse of Deposition Second Pulse of Deposition Figure 8 Sedimentation and zircon input. Plot displays temporal relationship between crystallization age and the delayed depositional input. Due to the similarity of the lower and middle Moreno Hill, this study finds evidence for a singular, longer-lived pulse of sediment input. Thereafter, a second phase with more youthful zircons, and a varied zircon history thus indicates a potential new subdivision of the Moreno Hill Formation into lower and upper members only. K-S analysis after Barbeau et al. (2009b).  from other diachronous units including the Ferron Sandstone, Utah, and from the Juana Lopez Member of the Mancos Shale in San Juan County, New Mexico (Obradovich, 1993;Cobban et al., 2006). Both dates have been re-calibrated to current standards, dating the bentonite beds within the P. hyatti and P. macombi Zones to 91.1 ± 0.5 Ma and 90.8 Ma ± 0.7, respectively (Fowler, 2017). A bentonite from within the S. preventricosus Ammonite Zone (Lower Coniacian), Marias River Shale in Montana, was dated and recalibrated to 88.9 ± 0.6 Ma, thus constraining the age of Flemingostrea elegans within the Mulatto Tongue of the Mancos Shale (which overlies the upper member-correlative Dilco Coal Member of the Crevasse Canyon Formation) to early Coniacian (Obradovich, 1993;Molenaar et al., 2002;Hook, 2010;Fowler, 2017).
In light of the above interpretations and if it assumed the most recent revision of the Gallup Delta by Lin & Bhattacharya (2019) and Lin, Bhattacharya & Stockford (2019) is accurate, the Moreno Hill depo-center was likely a sedimentary conduit for the Gallup Delta. Seminal work by Lin, Bhattacharya & Stockford (2019) indicates that initial sedimentation in the Gallup System occurred near ±89.6 Ma for the Lower Gallup and terminated by ±88.4 Ma for the Upper Gallup (Lin, Bhattacharya & Stockford, 2019, Fig. 20), and geographically is placed at or just north to northeast of the Moreno Hill depo-center (Lin, Bhattacharya & Stockford, 2019, Figs. 2A and 2B). Therefore, based on this study's initial findings, we interpret the lower Moreno Hill would have formed the proximal portions of the delta plain with co-occurring channel complexes, vast back swamps, and accumulations of water-saturated floodplain fines (Fig. 9). By approximately 88.6 Ma (but no older) the delta prograded further north by northeast Lin, Bhattacharya & Stockford, 2019, Fig. 20, p. 571, Sequence 5, 4, 3), which is reflected in the Moreno Hill sedimentation. The once delta plain shifted to a distal floodplain with the continued development of the fluvial complex and adjacent floodplain fines preserved in slightly more arid climatic conditions (Roybal, 1982;Campbell, 1984;Hoffman, 1996; this study). In a broader context, with the newly interpreted MDAs for the Moreno Hill, emplacement of sediment into the depo-center would have initiated during the latest phase of the Greenhorn continuing through the Frontier-Ferron regression (Mancos Seaway) (Kauffman, 1984, Blakey, 2014Lowery et al., 2018, Fig. 4, p. 14;Miall & Catuneanu, 2019; and references therein). The revision of lithostratigraphic, sequence stratigraphic, and biostratigraphic ties are the focus of a forthcoming manuscript and is beyond the scope of this particular study. However, these findings thus far are in agreement with the regional framework(s) (Fig. 9) by Roberts & Kirschbaum (1995, Fig. 10, p. 23 and Fig. 13, p. 29) Pecha et al. (2018) and Lin, Bhattacharya & Stockford (2019).
The oldest detrital zircons in the Moreno Hill Formation are Precambrian and are interpreted as being from the uplifted and eroding Yavapai and Mazatzal blocks in the adjacent Mogollon Highlands. All Moreno Hill Formation samples are void of Paleozoic multi-grain populations. Triassic to middle Jurassic recycled grains and populations are interpreted as being from the Appalachian and Amarillo-Wichita uplifts (300-200 Ma) with younger grains being from westerly lying terranes within the early phases of the Cordilleran Arc and heavily-reworked multi-generational recycling of sedimentary blankets (aeolianites) lying east of the Sevier Highlands and north of the Mogollon Highlands (Dickinson & Gehrels, 2003Lawton, Pollock & Robinson, 2003;DeCelles, 2004;Laskowski, DeCelles & Gehrels, 2013;Gehrels & Pecha, 2014). Late Jurassic-"mid-Cretaceous" zircon populations can be linked to igneous terranes in the southwestern portion of the Cordilleran Arc, specifically the western zones of the Sierra Nevada and Peninsular Ranges Batholiths (Laskowski, DeCelles & Gehrels, 2013;Tucker et al., 2020). Based on regional paleo-drainage reconstructions from southwest to northeast, we conclude that the most youthful (near-syndepositional) populations are most likely derived from the southeastern zone of the Sierra Nevada Batholith and the northeastern zone of the Peninsular Ranges Batholith (Roberts & Kirschbaum, 1995;Hildebrand & Whalen, 2014;Yonkee & Weil, 2015;Pecha et al., 2018;Lin, Bhattacharya & Stockford, 2019). If the Moreno Hill is compared to the Dakota Sandstone, our study confidently identifies a long-term shift in probable source terranes and is linked to an evolving catchment system. Specifically, the impacts of the eastward migration of the forebulge to the north (Currie, 1997;DeCelles, 2004;White, Furlong & Arthur, 2002;DeCelles & Coogan, 2006;Yonkee & Weil, 2015;Miall & Catuneanu, 2019) may have extended further south to southwest than previously recognized, thus creating a topographic high, which diverted drainage into the north (foredeep) or northeast (backbulge) (Figs. 9B and 9C). We interpret that as the forebulge migrated eastward, it slowly cut off westerly lying sources, including the Sevier Highlands and northern Sierra Nevada Batholith (Fig. 9). Volcaniclastic to volcanilithic-rich sediment that blanketed the Mogollon Highlands during eruption phases would have been eroded and mixed with other Mogollon Highland sediments and transported northeast to the Moreno Hill depo-center and the Gallup Delta. Temporally, the tectonic driver for sediment and resulting influence on drainage (southwest to northeast) can be confidently linked to the 90-86 Ma development of the Maria Fold and Thrust Belt (Spencer & Reynolds, 1990;Knapp & Heizler, 1990, Barth et al., 2004Salem, 2009;Szwarc et al., 2015).
Specifically, the Moreno Hill Assemblage (sensu Nesbitt et al., 2019), which currently derives solely from the lower Moreno Hill Formation (Wolfe & Kirkland, 1998), has been used to pinpoint first and last appearance dates for a variety of key taxa including therizinosauroids, hadrosauroids, and ceratopsians (Wolfe & Kirkland, 1998;Kirkland & Wolfe, 2001;McDonald, Wolfe & Kirkland, 2010;Gates et al., 2011), and fills in biodiversity data otherwise only supplemented temporally by the more poorly categorized Straight Cliffs Formation regionally (Titus, Roberts & Albright, 2013;Albright & Titus, 2016). Although Wolfe & Kirkland (1998) suggest a middle-upper Turonian age for the lower Moreno Hill Formation based on ammonite biostratigraphy (Molenaar et al., 2002), recent paleontological studies have used an early middle Turonian age (~92 Ma) in taxon descriptions (Nesbitt et al., 2019). Our MDA of 90.9 Ma from the Moreno Hill Assemblage compares well with the temporal framework of Wolfe & Kirkland (1998) and suggests that taxon ages should be refined to be approximately 1 million years younger than previously recognized, whereas, the limited fossils recovered from the upper Moreno Hill are Coniacian.

CONCLUSION
This study presents a newly calibrated chronostratigraphic framework for the Moreno Hill Formation exposed within the Zuni Basin, New Mexico. By coupling CA-TIMS and LA-ICP-MS data, we identify that emplacement of the most reliable youthful zircon populations preserved within the Moreno Hill depo-center occurred in two distinct phases. The first pulse of deposition occurred after 90.9 Ma (lower Moreno Hill), and the second pulse of sediment emplacement occurred after 88.6 Ma. Based on the principle of detrital zircon, the emplacement of the Moreno Hill is diachronous, Turonian/Coniacian. Based on LA-ICP-MS data this study was able to detangle a complex history of detrital input and confidently identify likely volcanic source terranes. Youthful populations are interpreted to derive from the westerly Cordilleran Arc (Phase C), and more likely Peninsular Ranges Batholith and the southernmost to central Sierra Nevada Batholith (Fig. 9). Reworked volcanic detritus and co-occurring detrital sediment from the Maria Fold and Thrust Belt and the Mogollon Highlands were enriched with Cordilleran Arc detritus and transported via fluvial complexes to the developing Gallup Delta (Fig. 9). The Moreno Hill is interpreted to be the proximal delta plain and distal fluvial system to the Gallup Delta (Hutsky & Fielding, 2016;Lin, Bhattacharya & Stockford, 2019). Our future investigations into the Moreno Hill Formation will seek to couple this newly calibrated temporal framework with that of historically significant lithostratigraphic and biostratigraphic records, which are now somewhat juxtaposed with our current results. Future work will also seek to provide a comprehensive review of the Moreno Hill sedimentary system including a stratigraphic revision of its current subdivision. Finally, these efforts present a novel temporal framework for the Moreno Hill