Mark Pecha

Detrital zircon U-Pb geochronology of Paleozoic strata in the Grand Canyon, Arizona

We determined U-Pb ages for detrital zircons from 26 samples of Paleozoic sandstone from the Grand Canyon. Cambrian strata yield mainly ca. 1.44 and 1.7–1.8 Ga ages that indicate derivation from nearby basement rocks of the Yavapai Province. Devonian strata contain zircons of 1.6–1.8 Ga, 1.34–1.40 Ga, and ca. 520 Ma, suggesting derivation from the Mazatzal and Yavapai Provinces, midcontinent region, and the Amarillo-Wichita uplift, respectively. Mississippian strata record a major change in provenance, with predominantly 415–475 Ma and 1030–1190 Ma grains interpreted to have been shed from the central Appalachian orogen. Pennsylvanian strata contain subequal proportions of 1.4–1.8 Ga grains derived from basement rocks exposed in the Ancestral Rocky Mountains and 409–464 and ca. 1070 Ma grains derived from the Appalachians. Permian strata contain abundant Appalachian zircons, including 270–380 Ma grains, and a lesser proportion of grains derived from the Ancestral Rocky Mountains. Transcontinental transport during Mississippian through Permian time is interpreted to have occurred in large river systems, facilitated by northeasterly trade winds during low sea level and by coastal currents. A compilation of young ages from all Upper Paleozoic strata yields age peaks of 270–365 Ma, 395–475 Ma, and 515–640 Ma, an excellent match for Alleghanian, Acadian, Taconic, and Neoproterozoic (peri-Gondwanan) episodes of magmatism along the Appalachian margin. Lag times of the youngest grains in these Upper Paleozoic strata average ∼25 m.y., suggesting relatively rapid exhumation and erosion of Appalachian source regions.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America

U-Pb geochronologic and Hf isotopic analyses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents significant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early–Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2–1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7–0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with significant production of juvenile crust during late Archean (3.0–2.5 Ga) and Paleoproterozoic (1.78–1.6 Ga) time and phases of mainly crustal reworking at 2.0–1.78, 1.5–1.3, 1.2–1.0, and 0.6–0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3–0.2 Ga.

What happens when n= 1000? Creating large-n geochronological datasets with LA-ICP-MS for geologic investigations

The direct age dating of individual mineral components in sedimentary rocks through the analysis of radiogenic parent and daughter isotopes has been routinely applied to better understand sediment provenance and dispersal patterns for several decades. Time, labor, and financial cost—sadly, not scientific inquiry—are typically the determining factors in the number of analyses run for a sedimentary rock sample during provenance investigations. The number of observations reported for detrital zircon provenance investigations using secondary ion mass spectrometers SIMS and laser-ablation inductively-coupled-plasma mass-spectrometers LA-ICP-MS typically range from n = 60–120. In this range, minor, but commonly geological relevant, age components are commonly not identified from the sample aliquot. In addition, the relative proportions of zircon ages from within an age component are typically unreliable for intersample comparisons because the relative proportions of ages from aliquots of n = 60–120 may poorly reflect the ‘true’ proportions of ages from a sample. This study investigates the practicality and usefulness of generating large-n (n = 300–1000) datasets. A LA-MC-ICP-MS and LA-SC-ICP-MS were used to generate four n ≈ 1000 datasets. We show that precision large-n U–Pb detrital zircon datasets can be created using LA-ICP-MS with total sample-run analysis times that are on par with more traditional studies. At best, most provenance investigations based on n = 60–100 have been statistically limited to identifying principle age components. The statistical robustness on n = 1000 datasets not only significantly increase the probability that exotic or low abundance age components (i.e., f < 0.05) are identified in detrital samples, but it allows for the quantitative comparisons between relatively high abundance age components in samples. This potentially transformative outcome of large-n has the potential to stimulate new avenues of research in sedimentology and tectonics.

Provenance of the Paleogene Colton Formation (Uinta Basin) and Cretaceous-Paleogene provenance evolution in the Utah foreland: Evidence from U-Pb ages of detrital zircons, paleocurrent trends, and sandstone petrofacies

The fluviodeltaic Colton Formation (Late Paleocene–Early Eocene) forms a lobate depositional system that prograded from the south into the Laramide Uinta Basin of northeastern Utah (United States) with a preserved sediment volume of ∼3000 km 3 and a maximum thickness of ∼1000 m. Joint consideration of detrital zircon ages, paleocurrent trends, and sandstone petrofacies permits an assessment of Colton provenance relations in the context of evolving Cretaceous–Paleogene sedimentation in the Utah foreland. Grains with U-Pb ages younger than 285 Ma derived from the Cordilleran magmatic arc form ∼50% of the detrital zircons in arkosic Colton sand, and were transported ∼750 km to the Uinta Basin from the Mojave segment of the arc by the California paleoriver. Colton sedimentation was the Paleogene culmination of a persistent pattern of Cretaceous sediment transport northward, subparallel to the Sevier thrust front, to supplement east-directed sediment delivery to the retroarc foreland from the Sevier thrust belt. The ratio of longitudinally to transversely derived sediment was enhanced in foreland strata after Laramide deformation produced intraforeland uplifts that screened the foreland belt from Sevier sources. The relative abundance of arc-derived detrital zircons that were contributed to strata of the Utah foreland increased in late Campanian time and remained high into Eocene time. Detrital zircon populations in Paleogene forearc strata of southern California are compatible with coeval derivation of arc-derived detritus in the forearc sands and the Colton backarc sand from a common paleodrainage divide crossing the Mojave region to connect hinterland Nevadaplano and Mexicoplano uplands to the north and south.

Importance of groundwater in propagating downward integration of the 6–5 Ma Colorado River system: Geochemistry of springs, travertines, and lacustrine carbonates of the Grand Canyon region over the past 12 Ma

We applied multiple geochemical tracers ( 87 Sr/ 86 Sr, [Sr], δ 13 C, and δ 18 O) to waters and carbonates of the lower Colorado River system to evaluate its paleohydrology over the past 12 Ma. Modern springs in Grand Canyon reflect mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Travertine ( 87 Sr/ 86 Sr and δ 13 C and δ 18 O values that overlap with associated water values, providing justification for use of carbonates as a proxy for the waters from which they were deposited. The Hualapai Limestone (12–6 Ma) and Bouse Formation (5.6–4.8 Ma) record paleohydrology immediately prior to and during integration of the Colorado River. The Hualapai Limestone was deposited from 12 Ma (new ash age) to 6 Ma; carbonates thicken eastward to ∼210 m toward the Grand Wash fault, suggesting that deposition was synchronous with fault slip. A fanning-dip geometry is suggested by correlation of ashes between subbasins using tephrochronology. New detrital-zircon ages are consistent with the “Muddy Creek constraint, ” which posits that Grand Wash Trough was internally drained prior to 6 Ma, with limited or no Colorado Plateau detritus, and that Grand Wash basin was sedimentologically distinct from Gregg and Temple basins until after 6 Ma. New isotopic data from Hualapai Limestone of Grand Wash basin show values and ranges of 87 Sr/ 86 Sr, δ 13 C, and δ 18 O that are similar to Grand Canyon springs and travertines, suggesting a long-lived springfed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87 Sr/ 86 Sr and δ 13 C and increase in δ 18 O in the uppermost 50 m of the Hualapai Limestone indicate an increase in meteoric water relative to endogenic inputs, which we interpret to record progressively increased input of high-elevation Colorado Plateau groundwater from ca. 8 to 6 Ma. Grand Wash, Hualapai, Gregg, and Temple basins, although potentially connected by groundwater, were hydrochemically distinct basins before ca. 6 Ma. The 87 Sr/ 86 Sr, δ 13 C, and δ 18 O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover. The Bouse Limestone (5.6–4.8 Ma) was also deposited in several hydrochemically distinct basins separated by bedrock divides. Northern Bouse basins (Cottonwood, Mojave, Havasu) have carbonate chemistry that is nonmarine. The 87 Sr/ 86 Sr data suggest that water in these basins was derived from mixing of high- 87 Sr/ 86 Sr Lake Hualapai waters with lower- 87 Sr/ 86 Sr, first-arriving “Colorado River” waters. Covariation trends of δ 13 C and δ 18 O suggest that newly integrated Grand Wash, Gregg, and Temple basin waters were integrated downward to the Cottonwood and Mojave basins at ca. 5–6 Ma. Southern, potentially younger Bouse basins are distinct hydrochemically from each other, which suggests incomplete mixing during continued downward integration of internally drained basins. Bouse carbonates display a southward trend toward less radiogenic 87 Sr/ 86 Sr values, higher [Sr], and heavier δ 18 O that we attribute to an increased proportion of Colorado River water through time plus increased evaporation from north to south. The δ 13 C and δ 18 O trends suggest alternating closed and open systems in progressively lower (southern) basins. We interpret existing data to permit the interpretation that the southernmost Blythe basin may have had intermittent mixing with marine water based on δ 13 C and δ 18 O covariation trends, sedimentology, and paleontology. [Sr] versus 87 Sr/ 86 Sr modeling suggests that southern Blythe basin 87 Sr/ 86 Sr values of ∼0.710–0.711 could be produced by 25%–75% seawater mixed with river water (depending on [Sr] assumptions) in a delta– marine estuary system. We suggest several refinements to the “lake fill-and-spill” downward integration model for the Colorado River: (1) Lake Hualapai was fed by western Colorado Plateau groundwater from 12 to 8 Ma; (2) high-elevation Colorado Plateau groundwater was progressively introduced to Lake Hualapai from ca. 8 to 6 Ma; (3) Colorado River water arrived at ca. 5–6 Ma; and (4) the combined inputs led to downward integration by a combination of groundwater sapping and sequential lake spillover that first delivered Colorado Plateau water and detritus to the Salton Trough at ca. 5.3 Ma. We propose that the groundwater sapping mechanism strongly influenced lake evolution of the Hualapai and Bouse Limestones and that groundwater flow from the Colorado Plateau to Grand Wash Trough led to Colorado River integration.

Detrital zircon geochronology from the Bighorn Basin, Wyoming, USA: Implications for tectonostratigraphic evolution and paleogeography

Tectonostratigraphic assemblages record phases of basin history during which the fundamental controls of tectonic setting, subsidence style, and basin geometry are relatively similar. Because these fundamental controls, in combination with climate and eustasy, influence paleogeography and sediment-dispersal patterns, they should also yield similar patterns, or facies, of detrital zircon age spectra. Such age-distribution patterns should be documented on the craton in order to make meaningful comparisons to sedimentary rocks from suspect terranes along continental margins. The Rocky Mountains of western North America provide excellent outcrops of sedimentary rocks that record >500 m.y. of tectonostratigraphic evolution. One such Phanerozoic section is exposed along the margins of the Bighorn Basin in northwest Wyoming, from which we report over 4000 U/Th/Pb detrital zircon ages from 48 samples that span a stratigraphic interval from the Middle Cambrian Flathead Sandstone through the Eocene Willwood Formation. These data provide one of the most complete records of detrital zircon age patterns from this part of cratonic North America. The stratigraphic record of the Bighorn Basin is subdivided into four tectonostratigraphic assemblages (TSA1–TSA4). These assemblages record an initial passive margin, followed by a transition to a convergent margin, followed by a marine-dominated retroarc foreland basin, followed by a retroarc foreland segmented by local basement uplifts. This tectonostratigraphic architecture is expressed as four, first-order patterns within the detrital zircon age distributions. TSA1 represents a Paleozoic–Triassic proximal continental margin assemblage dominated by Proterozoic zircons with abundant grains in the 1600–1950 Ma range, a Grenville population at ca. 1100 Ma, and a Phanerozoic population at ca. 420 Ma. TSA2 is a transitional assemblage associated with the Jurassic–Early Cretaceous organization of a west-facing convergent margin and Cordilleran orogen. The TSA2 detrital zircon age distribution is characterized by the appearance of Mesozoic grains, age peaks at ca. 420 and 600 Ma, and a dominant population of Grenville (1.0–1.1 Ga) grains with a suite of Proterozoic grains diminishing in abundance as age increases to 1.9 Ga. TSA3 sedimentary rocks were deposited in the Cretaceous Interior Seaway in a retroarc foreland basin and are dominated by zircons for which ages are close to the depositional age of the strata, reflecting input from the active Idaho Batholith and Sierran segments of the Cordilleran magmatic arc. The older zircon fractions from TSA3 sedimentary rocks are characterized by a dominant detrital zircon age peak at 1.7–1.8 Ga, which probably reflects reworking of Belt Supergroup metasedimentary rocks from the northwest into the Cretaceous foreland, based on regional paleogeographic patterns. TSA4 reflects the phase of basin fill associated with Paleogene structural segmentation of the retroarc foreland during the Laramide orogeny. Detrital zircon age spectra from this assemblage record erosion and redeposition of all previous sedimentary rocks from surrounding basement uplifts. Patterns of detrital zircon ages reflect fundamental changes in paleogeography and sediment dispersal at the 10–100 m.y. time scale and are clearly related to major tectonic events or phases. Detrital zircon ages also provide evidence for linkages between convergent margin processes such as arc magmatism and sedimentation in the retroarc foreland. During these times of strong arc-retroarc linkage, detrital zircon geochronology provides a potentially useful tool for high-resolution chronostratigraphy.

Basin formation near the end of the 1.60–1.45 Ga tectonic gap in southern Laurentia: Mesoproterozoic Hess Canyon Group of Arizona and implications for ca. 1.5 Ga supercontinent configurations

Detrital zircon data from the upper parts of the Proterozoic Hess Canyon Group of southern Arizona reveal abundant 1600–1488 Ma detrital zircons, which represent ages essentially unknown from southern Laurentia. This basinal succession concordantly overlies a >2-km-thick-section of 1657 ± 3 Ma rhyolite of the Redmond Formation. The rhyolite is intercalated with and hence contemporaneous with the lower parts of the overlying White Ledges Formation, a 300-m-thick orthoquartzite unit at the base of the Hess Canyon Group. These quartzites contain a unimodal detrital zircon age probability distribution with peak ages of 1778, 1767, and 1726 Ma, supporting regional correlation with other ca. 1.65 Ga quartzite exposures in southwestern Laurentia. However, the ∼900-m-thick argillaceous Yankee Joe and minimum 600-m-thick quartzite-rich Blackjack Formations contain younger detrital zircons, with peak ages ranging from 1666 to 1494 Ma and a maximum depositional age of 1488 ± 9 Ma. Prominent age peaks at 1582–1515 Ma and 1499–1488 Ma represent detritus that is exotic and not derived from known southern Laurentian sources. The Blackjack Formation is cut by the 1436 ± 2 Ma Ruin Granite, indicating that deposition, deformation, and intrusion occurred between 1488 and 1436 Ma. This basin likely developed before or in the early stages of the 1.45–1.35 Ga intracontinental tectonism in southwestern Laurentia. Our findings necessitate the presence of an ∼170 m.y. disconformity within the Hess Canyon Group and document a previously unrecognized episode of Mesoproterozoic basin sedimentation (>1.5 km of section) between 1488 and 1436 Ma in southern Laurentia. This new record helps to fill the 1.60–1.45 Ga magmatic gap in southern Laurentia and supports hypotheses for a long-lived Proterozoic tectonic margin along southern Laurentia from 1.8 to 1.0 Ga. The 1.6–1.5 Ga detrital zircon ages offer important new constraints for ca. 1.5 Ga Nuna reconstructions and for the paleogeography of contemporaneous basins such as the Belt Basin in western Laurentia.

U‐Pb and Hf isotope analysis of detrital zircons from Mesozoic strata of the Gravina belt, southeast Alaska

The Gravina belt consists of Upper Jurassic through Lower Cretaceous marine clastic strata and mafic-intermediate volcanic rocks that occur along the western flank of the Coast Mountains in southeast Alaska and coastal British Columbia. This report presents U-Pb ages and Hf isotope determinations of detrital zircons that have been recovered from samples collected from various stratigraphic levels and from along the length of the belt. The results support previous interpretations that strata in the western portion of the Gravina belt accumulated along the inboard margin of the Alexander-Wrangellia terrane and in a back-arc position with respect to the western Coast Mountains batholith. Our results are also consistent with previous suggestions that eastern strata accumulated along the western margin of the inboard Stikine, Yukon-Tanana, and Taku terranes and in a fore-arc position with respect to the eastern Coast Mountains batholith. The history of juxtaposition of western and eastern assemblages is obscured by subsequent plutonism, deformation, and metamorphism within the Coast Mountains orogen, but may have occurred along an Early Cretaceous sinistral transform system. Our results are inconsistent with models in which an east-facing subduction zone existed along the inboard margin of the Alexander-Wrangellia terrane during Late Jurassic-Early Cretaceous time.

U-Pb and Hf isotope analysis of detrital zircons from Paleozoic strata of the southern Alexander terrane (southeast Alaska)

The Alexander terrane is an unusual tectonic fragment in the North American Cordillera in that it contains a long and very complete stratigraphic record, including sedimentary or volcanic rocks representing every period and nearly every epoch between Neoproterozoic and Late Triassic time. The terrane is also unusual in that the southern portion of the terrane experienced arc-type magmatism during Neoproterozoic−early Paleozoic time, whereas the northern portion of the terrane consists mainly of Paleozoic shelf-facies strata. This long and diverse history provides opportunities to reconstruct the evolution and displacement history of the terrane, and specifically test the prevailing interpretation that the terrane formed in the paleo-Arctic realm. This study presents U-Pb geochronologic data and Hf isotopic information for detrital zircons from arc-type rocks in the southern portion of the terrane. Information has been generated from seven samples of Ordovician through Devonian age, complementing the information available from previous studies of Ordovician through Triassic strata. Together, these data sets yield a robust record of the magmatic history of the southern Alexander terrane, with dominant age groups of 640−550 Ma, 490−400 Ma, 380−340 Ma, and 310−275 Ma (dominant ages of 579, 441, 361, and 293 Ma). There are few pre−640 Ma grains in any of the samples. Hf isotope compositions of the detrital zircons are exceptionally juvenile, with most epsilon Hf (t) values between +15 and +5. Collectively, the available geologic, U-Pb geochronologic, and Hf isotopic evidence suggests that the southern Alexander terrane formed within a juvenile Neoproterozoic−early Paleozoic arc system, with little continental influence, whereas the northern portion of the terrane formed in proximity to a continental landmass that experienced similar Neoproterozoic−early Paleozoic ages of continental-affinity magmatism. Our data are consistent with previous suggestions that the Alexander terrane resided in the paleo-Arctic realm during early Paleozoic time, with the northern portion of the terrane adjacent to Baltica and the Caledonides, and the southern portion of the terrane forming further offshore as a juvenile north-facing oceanic arc.

Detrital zircon U‑Pb geochronology and Hf isotope geochemistry of the Yukon-Tanana terrane, Coast Mountains, southeast Alaska

Rocks of the SE Alaska subterrane of the Yukon-Tanana terrane (YTTs) consist of regionally metamorphosed marine clastic strata and mafic to felsic volcanic-plutonic rocks that have been divided into the pre-Devonian Tracy Arm assemblage, Silurian–Devonian Endicott Arm assemblage, and Mississippian–Pennsylvanian Port Houghton assemblage. U-Pb geochronologic and Hf isotopic analyses were conducted on zircons separated from 23 igneous and detrital samples in an effort to reconstruct the geologic and tectonic evolution of this portion of YTT. Tracy Arm assemblage samples are dominated by Proterozoic (ca. 2.0–1.6, 1.2–0.9 Ga) and Archean (2.7–2.5 Ga) zircons that yield typical cratonal eHf( t ) values. Endicott Arm assemblage samples yield U-Pb ages that range from Late Ordovician to Early Devonian and eHf( t ) values that range from highly juvenile to moderately evolved. Port Houghton assemblage samples yield similar Ordovician–Devonian ages and eHf( t ) values, and also include early Mississippian zircons with highly evolved eHf( t ) signatures. Comparison of these age-Hf patterns with data from nearby assemblages suggests the following: (1) Results from YTTs are similar to (or compatible with) available data from rocks of the Yukon-Tanana terrane in eastern Alaska, Yukon, and northern British Columbia (YTTn) and pericratonic strata in east-central Alaska (NAa). (2) YTTs contains abundant Late Ordovician–Early Devonian magmatism that is not recorded in YTTn and NAa. (3) The eHf( t ) values from YTTs display two excursions from juvenile to evolved eHf( t ) values, which are interpreted to record cycles of crustal thinning and then thickening within a convergent margin system. (4) Available data from both YTTs and YTTn support Neoproterozoic(?)–early Paleozoic positions along the northern Cordilleran margin. (5) The Late Ordovician–Early Devonian magmatic record of the southern Alexander terrane is very similar to that of YTTs, which raises the possibility that these assemblages evolved in the same convergent margin system along the northern (Alexander) and northwestern (YTT) margins of Laurentia. These results support a tectonic model in which: (1) YTTs formed outboard of (or northward along strike of) YTTn and NAa along the northern Cordilleran margin during Neoproterozoic(?)–early Paleozoic time; (2) initial subduction-related magmatism during Late Ordovician to Early Devonian time records a progression from crustal thinning to crustal thickening, and is preserved only in YTTs; (3) a second phase of magmatism records Middle–Late Devonian crustal thinning followed by early Mississippian crustal thickening; (4) YTTs and YTTn evolved as an intra-oceanic arc outboard of the Slide Mountain ocean basin during Carboniferous–Permian time and were accreted to the continental margin during Triassic time; and (5) YTTs is interpreted to have been displaced ∼1000 km southward, from an original position outboard of YTTn/NAa to its present position outboard of the Stikine terrane, along a sinistral fault system of Late Jurassic–Early Cretaceous age.