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. Furthermore, recent studies have sought to address the variable relationship of volcaniclastic primary-deposit and volcanilithic secondary-deposit rock used to date sedimentary sequences Rosol et al.
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. are described herein and depicted in Fig. Also displayed is the K—S test to show the variable weak genetic relationship across the samples across the stratigraphic section. Temporal placement of the Atarque Sandstone is linked to the Collignoniceras woollgari biozone coupled with the occurrence of the middle Turonian ammonite Collignoniceras woollgari woollgari and a Mytiloides bivalve reported as either M.
In spite of this biostratigraphic framework, correlation between terrestrial and marine sequences is challenging. 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.
Most research has focused on regional correlation and economic potential of three key coal seams, the lower Antelope, the medial Cerro Prieto, and the upper Rabbit, all within the lower Moreno Hill Anderson, ab; McLellan et al. These uncertainties are 0. All samples collected were fresh and unweathered; at least 0. Whilst ly identified ash beds tonstein or paratonstein Admakin, occur within its coal seams Hoffman,these are stratigraphically limited to the lower portions of the formation.
Due to the distance of the Sevier west and Maria south-southwest Fold and Thrust Belts, grains between 1. Large populations of grains between 1. This study presents the first systematic geochronological framework for key strata within the Moreno Hill Formation. Based on current mapping, the three unnamed members lower, middle, and upper of the Moreno Hill constitute a clastic wedge Anderson, ; Roybal, ; McLellan et al. All common Pb in analyses was attributed to laboratory blank and subtracted based on the measured laboratory Pb isotopic composition and associated uncertainty.
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.
For concentration calculations, background-subtracted count rates for each analyte were internally normalized to 29 Si and calibrated with respect to NIST SRM and glasses as the primary standards. 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.
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Individual grains range from well-faceted euhedral, to somewhat rounded or minorly cracked, to fractured, rounded, or fragmented. These youngest date atures are YDZ at The mean of the youngest date atures is Comparison of five 5 different metrics utilized within this study to interpret the Maximum Depositional Age MDAs for each detrital sample modified from Tucker et al. Of the 97 grains ablated 86 included31 are Precambrian, four are Paleozoic, and 51 grains are Mesozoic. This refined temporal context will provide crucial context for paleobiogeographic and macroevolutionary studies involving terrestrial vertebrate assemblages during the mid-Cretaceous.
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. Ablated material was carried by a 1. The youngest date atures are YDZ at Of the 99 grains ablated 94 included57 are Precambrian; Paleozoic grains are absent, and 37 grains are Mesozoic. 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.
Rare analyses that appear contaminated by common Pb were rejected based on mass greater than baseline.
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U blanks are estimated at 0. Confident age constraints for the Moreno Hill Formation are key to detailing patterns of vertebrate evolution and biostratigraphy in the early-Late Cretaceous given its well-preserved floral angiosperm, gymnosperm and faunal osteichthyan fish, squamate, trionychid, crocodilian, dinosaurian and mammalian assemblages Wolfe et al. The first formal lithostratigraphic identification of the Moreno Hill Formation was by McLellan et al. 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.
K—S analysis after Barbeau et al.
This study builds upon well-documented tectonic reconstructions and source terranes Fig. The Dakota Sandstone sample displays a diverse assemblage of grain ages and populations. Error including uncertainties in the tracer calibration and U decay constant Jaffey et al.
With these MDAs we are be able 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 and provide a reliable reconstruction for its emplacement. Specifically, the western margin of the Western Interior Seaway underwent ificant shifts in coastal morphology and coupled with on-going tectonism resulted in numerous unconformities, lateral discontinuities, and cryptic or tenuous stratigraphic linkages Molenaar, a ; Molenaar et al.
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This is increasingly important, particularly when detangling the complexities of Maximum Depositional Ages MDAs and ecosystem evolution within terrestrial depo-centers that are marine-adjacent, including possible linkages to the Gallup Delta Jinnah et al. Background count rates for each analyte were obtained prior to each spot analysis and subtracted from the raw count rate for each analyte.
The youngest grain atures are YDZ at Of the 24 grains ablated 16 includedthree grains are Precambrian, one is Paleozoic, and 12 are Mesozoic. To provide broader context, we focused on utilizing U-Pb detrital geochronology on suspected near-syndepositional igneous well-faceted zircon grains from channel sandstone deposits in the key members spanning the Moreno Hill Formation.
Learn More. The four samples Dakota Sandstone and lower, middle, and upper Moreno Hill Formation contained a broad spectrum of detrital zircon.
Based on established linkages to marine biostratigraphic records tied to radiometric dates derived from co-occurring bentonites, ly reported ages for the terrestrial Moreno Hill Formation place its deposition entirely within the Turonian McLellan et al. One yields a date of The other was broken into two fragments that were analyzed separately and yield equivalent dates of This is taken as the MDA, indicating deposition was after 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.
More recently, Chin et al. Try out PMC Labs and tell us what you think. As a result of multi-phased sedimentary recycling, these detrital zircons can be meaningfully associated with 1 the Trans-Hudson and Wopmay orogenies 2. Paleontological evidence suggests terrestrial ecosystems underwent a major restructuring during this interval, yet the pace and pattern are poorly constrained.
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Whilst this stratigraphic column is based on our own measurements and observations the original type section and principle reference section stratigraphic columns for the Moreno Hill Formation were deated by McLellan et al. Current biostratigraphic age control for the Atarque Sandstone is based on the local occurrence of the range zones of Collignoniceras woollgari and Mammities nodosoides. Three key stratigraphic zones were selected based on stratigraphic position or proximity to known fossil horizons Fig.
Specifically, bulk samples were collected from the contact with the underlying Atarque basal Moreno Hill Cox 2along with the middle Cox 3 and uppermost Cox 4 bedded sandstone strata of the Moreno Hill Formation. Dates from both rounds agreed in most cases.
Two grains from the middle Moreno Hill Formation Cox 3 yield dates of These dates are older than the MDA from the underlying sample, and thus the grains are not close in age to sedimentation. The Dakota Sandstone has the widest spectrum of dates and grain morphologies. All of the grains save for a minor few have oscillatory and sector zoning indicative of igneous growth Fig. Many grains have distinct cores.
Errors on the dates are given at 2 sigma.
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Data were collected in three experiments in July Radiogenic isotope ratio and age error propagation for all analyses include uncertainty contributions from counting statistics and background subtraction. Errors are at 2 sigma.
Because there are no constraints on the activity of TiO 2an average value in crustal rocks of 0. Zircon was dissolved in Parr vessels in ml of 29 M HF with a trace of 3. U-Pb dates from these analyses are considered valid if the U-Pb ratios appear to have been unaffected by the inclusions. The following information was supplied regarding data availability:.
Whilst the Western Interior Seaway has produced one of the most robust marine biostratigraphic frameworks in the world Obradovich, ; Cobban et al. For a third sample, the grains were too small to permit a second round of analysis. This work greatly strengthens linkages to other early Late Cretaceous strata across the Western Interior.
Ablations pits that appear to have intersected glass or mineral inclusions were identified based on Ti and P al excursions, and associated sweeps were discarded. Of the grains ablated 88 included60 are Precambrian, Paleozoic grains are absent, and 28 are Mesozoic.