May 15, 2018
Cenozoic sedimentary rocks in the southern Texas Gulf Coastal Plains contain abundant continental carbonates that are useful for reconstructing terrestrial paleoclimate and paleoenvironment in a region near sea-level. Our field observations and thin section characterizations of the Oligocene and Miocene continental carbonates in south Texas identified three types of pedogenic carbonates, including rhizoliths, carbonate nodules, and platy horizons, and two types of groundwater carbonates, including carbonate-cemented beds and carbonate concretions, with distinctive macromorphologic and micromorphologic features. Based on preservations of authigenic microfabrics and variations of carbon and oxygen isotopic compositions, we suggest these carbonates experienced minimal diagenesis, and their stable isotopic compositions reflect paleoclimate and paleoenvironment in south Texas. Our Oligocene and Miocene carbonate clumped isotope temperatures (T(Δ47)) are 23–28 °C, slightly less than or comparable to the range of modern mean annual and mean warm season air temperature (21–27 °C) in the study area. These T(Δ47) values do not show any dependency on carbonate-type, or trends through time suggesting that groundwater carbonates were formed at shallow depths. These data could indicate that air temperature in south Texas was relatively stable since the early Oligocene. The reconstructed paleo-surface water δ18O values are similar to modern surface water which could indicate that meteoric water δ18O values also remained stable since the early Oligocene. Mean pedogenic carbonate δ13C values increased ~ − 4.6‰ during the late Miocene, most likely reflecting an expansion of C4 grassland in south Texas. This study provides the first mid- and late Cenozoic continental records of paleoclimate and paleoecology in a low-latitude, near sea-level region.
March 15, 2018
Sediment provenance in the lower reach of rivers is important in reconstructing paleogeography and linking continental tectonic and climate changes with sedimentation in continental margins. This study presents 1300 new detrital zircon U–Pb ages from seven latest Eocene–Miocene sandstone and one modern river sand samples in south Texas and integrates these new data with published detrital zircon data to elucidate the evolution of the Rio Grande paleoriver during the Cenozoic. The new data contain a major population of the Cordillera magmatic province (22–280 Ma) and populations older than 280 Ma that were mostly recycled from sedimentary cover in the southern Rocky Mountains and southern Great Plains. The latest Eocene–Oligocene sandstones contain abundant air-fall zircons, and their detrital zircon maximum depositional ages constrain depositional ages. The changes in detrital zircon age distributions suggest that the paleoriver has changed its drainage extent three times, including a late Eocene drainage reduction by cutting off sediment supply from the west, a late Oligocene drainage reduction to the east of the southern Rocky Mountains and expansion to the East Mexico Arc in northeastern Mexico, and a latest Neogene drainage expansion to the west of the Rio Grande Rift and southern Colorado.
August 15, 2018
Stable isotope-based paleoaltimetry is the most widely used approach for paleoelevation reconstruction. Interpretations of stable isotope data in continental interiors, however, are undermined by surface water isotope compositions that are influenced by multiple factors. Here we present a stable isotope dataset of modern river water samples collected over two summers and one spring from the central Rocky Mountains (Rockies) and the adjacent Great Plains. By examining the spatial and temporal variations of river water O, δD and d-excess values, and their relationships with climatic and geographic parameters, as well as through back trajectory analysis of moisture sources, we elucidate the influences of elevation and climatic parameters on the spatiotemporal variation of river water isotopic values. In the Bighorn River drainage, a typical intermontane drainage in the central Rockies, the isotopic difference between highland and lowland rivers is small, which we attribute to highland precipitation that dominates lowland river discharge. In the North Platte River drainage across the central Rockies and Great Plains, the river water O values show poor correlation with elevation west of 105°W (central Rockies), but increase as elevation decrease east of 105°W (in the western Great Plains). This eastward increase across the western Great Plains leads to an average oxygen isotope lapse rate of −2.3‰/km, which we interpret as being caused primarily by condensation temperature-controlled isotopic fractionation at various elevations, and secondarily by evaporation in the upper reaches of streams that contribute to the North Platte River plus direct contribution of moisture from the Gulf of Mexico in the Great Plains. In this continental interior setting, multiple moisture sources, including recycled continental moisture, contribute to surface water, and evaporation influences river water isotope values to various degrees depending on the relative humidity within an individual river catchment. These results suggest that paleoclimate and atmospheric circulation pattern must be carefully evaluated when applying stable isotope-based paleoaltimetry in continental interiors. Our findings have implications for paleoelevation reconstruction in the study area, including that 1) within the central Rockies, the isotopic difference of river water and unevaporated basinal precipitation can be used to infer paleorelief of the Laramide ranges with respect to the basin floors; 2) along a regional transect crossing the central Rockies and Great Plains, the modern isotope lapse rate of the North Platte River drainage can be used to constrain the paleorelief between the two regions in semi-arid climate.
December 15, 2018
The high-elevation, high-relief landscape of the southern Rocky Mountains (Rockies), USA, reflects interactions between tectonics, paleoclimate, and surface processes. The southern Rockies experienced several episodes of uplift, extension, and major climate changes during the Cenozoic, but the landscape and river drainage evolution remain poorly constrained. Here we apply detrital zircon UPb geochronology to Eocene-Miocene strata in south-central Colorado to constrain the depositional ages and sediment provenance. A total of 1284 concordant UPb ages were determined and are grouped into 75–11 Ma, 1500–1300 Ma, and 1800–1500 Ma populations. Intense late Eocene-Oligocene regional volcanism provided abundant air fall zircons into the latest Eocene-Oligocene sedimentary systems where maximum depositional ages can be used to closely proximate depositional ages and improve the chronostratigraphy. The new data are integrated with interpretation of sedimentary environments, and the detrital zircon signatures of potential source terranes and age-equivalent strata in other nearby basins to interpret landscape and paleodrainage evolution. Specifically, the new provenance data show that (1) after the main phase of the Laramide deformation, the Wet Mountains, but not the Sangre de Cristo Range, was the dominant local topographic feature, and a southward-flowing river connected the Wet Mountain Valley with the Huerfano and Raton Basins to the south; (2) during the Eocene-early Oligocene, aggradation of the Wet Mountain Valley and the Huerfano Basin formed a low-relief surface, and subsequent river erosion changed the drainage pattern eastward and likely formed the modern lower Arkansas River valley; and (3) during the Miocene, dissection of the low-relief surface by the opening of the Rio Grande Rift formed the upper Arkansas River valley, and the upper valley was connected with the lower valley to establish the modern drainage pattern of the Arkansas River in the southern Rockies.
October 04, 2018
Detrital zircon U‐Pb provenance in the northern Great Plains reveals histories of drainage reorganization of the paleo‐Missouri River in response to Cenozoic tectonic and magmatic processes in the central North American Cordillera. During the latest Cretaceous, sediment provenance was confined to the Cordilleran hinterland in central Idaho and southwestern Montana, probably due to the existence of a subtle paleohydraulic divide in northwestern Montana. During the early to middle Paleocene, the paleodrainage was expanded to cover the Belt Supergroup in northwestern Montana in response to the eastward propagation of the Cordilleran thrust belt. During the late Paleocene to early Eocene, the final movement of the thrust belt and the initial extension of the Cordilleran hinterland shifted the drainage divide to the orogenic front and caused focused erosion of the upper Lower to Upper Cretaceous rocks to the east. During the middle Eocene to early Oligocene, a significant increase of Archean grains suggests renewed exhumation of the Laramide province and sediment delivery by the paleo‐Yellowstone River originated in the central Rocky Mountains. The presence of middle Eocene to early Oligocene zircons during this stage also suggests headwater erosion of the paleo‐Upper Missouri River into the Dillion volcanic field in southwestern Montana. These interpretations indicate that a drainage pattern similar to that of the modern Missouri River was established during middle Eocene to early Oligocene time. The detrital zircon maximum depositional ages also improve the chronostratigraphic framework of the Paleogene strata. Our study demonstrates that orogenic processes can be well archived in sedimentary records far away from orogenic systems.
December 27, 2017
One of the most pronounced climate transitions in Earth s history occurred at the Eocene-Oligocene transition, 34.0 - 33.6 m.y. ago. Marine sedimentary records indicate a dramatic decline in pCO2 coeval with global cooling during the transition. However, terrestrial records are relatively sparse, with conflicting interpretations of hydroclimate in continental interiors. Here, we provide quantitative constraints on the response of the continental hydroclimate in the western United States across the Eocene-Oligocene boundary by studying clumped isotope temperatures in eastern Wyoming. Our results show that T(Delta 47) dropped from ~28 C to ~21 C, indicating ~7 C cooling in air temperature, which occurred parallel to the decrease in atmospheric pCO2 during the latest Eocene early Oligocene. We find that aridity and the biome were stable, and ice-volume corrected precipitation decreased ~1.6 percent across the Eocene-Oligocene boundary, attributable to reduced vapor condensation temperatures. These new quantitative data add to the growing body of evidence suggesting a marked terrestrial response in temperature and hydroclimate across the Eocene-Oligocene transition. Our findings indicate a pattern of greenhouse-gas-induced global temperature change in the continental interior of the United States that was roughly 1.5-2 X the magnitude of cooling in the global ocean.
December 20, 2017
We report detrital zircon U-Pb ages in the Fort Worth Basin (southern USA) aimed at understanding sediment dispersal patterns on the southern margin of Laurentia before and during the Laurentia-Gondwana collision. The ages from two Cambrian fluvial-marginal marine sandstone and six Pennsylvanian deltaic-fluvial sandstone samples span from Archean to early Paleozoic time. In the Cambrian sandstones, 80% of zircons are of Mesoproterozoic age (1.451 - 1.325 Ga) and 18% are of Grenvillian age. The high abundance of the Mesoproterozoic population suggests that the grains were dispersed by a local river draining the midcontinent granite-rhyolite province located in the Texas Arch to the northwest of the Fort Worth Basin. In the Pennsylvanian sandstones, 26% of zircons are of Archean - early Mesoproterozoic age, 47% are of Grenvillian age, 15% are of Neoproterozoic earliest Paleozoic age (800 - 500 Ma), and 10% are of early Paleozoic age (500 - 318 Ma), indicating a different dispersal pattern during the Pennsylvanian relative to the Cambrian. Compared to other early Paleozoic detrital zircon records on the southern margin of Laurentia, our Pennsylvanian sandstones have a distinct age peak at ca. 650 - 550 Ma, which we interpreted to be a result of transport by local rivers draining a peri-Gondwana terrane, most likely the Sabine terrane in the Ouachita orogen. The high abundance of Grenvillian zircons reflects either direct transport from the Appalahians by an axial river or recycling from Mississippian - Pennsylvanian sedimentary rocks incorporated in the Ouachita orogenic front. The similarity of detrital zircon age distributions in the Fort Worth Basin, the Arkoma Basin, and the southern Appalachian forelands seems to favor sediment dispersal by a major river with headwaters in the southern Appalachians.
November 13, 2017
Since 2000, the technological advances of horizontal drilling and hydraulic fracturing in the United States have led to a dramatic increase in hydrocarbon (gas and oil) production from shale formations, changing the energy landscape in the US and worldwide. However, total gas recovery is only 12 to 30%, while the tight-oil recovery rate from shale formations is even lower at 5 to 10%. Dr. Hu s research group has been studying the nano-petrophysical aspects, mainly pore connectivity and wettability, which are unique to shale to lead to steep production decline and low overall production. The group uses an integrated approaches of pycnometry ((liquid and gas), pore and bulk volume measurement after vacuum saturation,, porosimetry (mercury injection capillary pressure, low-pressure gas physisorption isotherm, water vapor adsorption/desorption isotherm, nuclear magnetic resonance cyroporometry), imaging (X-ray computed tomography, Wood s metal impregnation, field emission-scanning electron microscopy), scattering (ultra- and small-angle neutron, small angle X-ray), as well and the utility of both hydrophilic and hydrophobic fluids as well as fluid invasion tests (imbibition, diffusion, vacuum saturation) followed by laser ablation-inductively coupled plasma-mass spectrometry imaging of different nm-sized tracers. Among a total of 18 SCI papers published in 2017 from Dr. Hu s group, a recent paper of using SANS (small angle neutron scattering) to quantify accessible pore spaces of various shale is published in Scientific Report, is an online, open access journal from the publishers of Nature, with an impact factor of 4.26. The group has been continuing the work to tease out the relationship between topological connectivity, wettability and effective pore accessibility, using a contrast-matching technique with SANS.
November 1, 2017
The Fort Worth basin in northcentral Texas is a major shale-gas producer, yet its subsidence history and relationship to the Ouachita fold-thrust belt have not been well understood. We studied the depositional patterns of the basin during the late Paleozoic by correlating well logs and constructing structure and isopach maps. We then modeled the one-dimensional (1-D) and two-dimensional subsidence history of the basin and constrained its relationship to the Ouachita orogen. Because the super-Middle Pennsylvanian strata were largely eroded in the region, adding uncertainty to the subsidence reconstruction, we used PetroMod 1-D to conduct thermal-maturation modeling to constrain the postMiddle Pennsylvanian burial and exhumation history by matching the modeled vitrinite reflectance with measured vitrinite reflectance along five depth profiles. Our results of depositional patterns show that the tectonic uplift of the Muenster uplift to the northeast of the basin influenced subsidence as early as the Middle Mississippian, and the Ouachita orogen became the primary tectonic load by the late Middle Pennsylvanian when the depocenter shifted to the east. Our results show that the basin experienced 3.7 -- 5.2 km (12,100 to 17,100 ft) of burial during the Pennsylvanian, and the burial depth deepens toward the east. We attributed the causes of deep Pennsylvanian burial and its spatial variation to flexural subsidence that continued into the Late Pennsylvanian in response to the growth of the Ouachita orogen and southeastward suturing of Laurentia and Gondwana. The modeling results also suggest that the Mississippian Barnett Shale reached the gas maturation window during the Middle Late Pennsylvanian.