Chronology of terraces in the Rio Grande rift, Socorro basin, New Mexico: Implications for terrace formation

The Rio Grande rift hosts a remarkable record of Quaternary river incision preserved in an alluvial terrace sequence that has been studied for more than a century. However, our understanding of Rio Grande incision history in central New Mexico since the end of basin filling ca. 0.78 Ma remains hampered by poor age control. Robust correlations among Rio Grande terrace sequences in central and southern New Mexico are lacking, making it difficult to address important process-related questions about terrace formation in continental-scale river systems. We present new age controls using a combination of 40Ar/39Ar, 36Cl surface-exposure, and 14C dating techniques from alluvial deposits in the central New Mexico Socorro area to document the late Quaternary incision history of the Rio Grande. These new age controls (1) provide constraints to establish a firm foundation for Socorro basin terrace stratigraphy, (2) allow terrace correlations within the rift basin, and (3) enable testing of alternative models of terrace formation. We identified and mapped a high geomorphic surface interpreted to represent the end of basin filling in the Socorro area and five distinct, post–Santa Fe Group (ca. 0.78 Ma) alloformations and associated geomorphic surfaces using photogrammetric methods, soil characterization, and stratigraphic descriptions. Terrace deposits exhibit tread heights up to 70 m above the valley floor and are 5 to >30 m thick. Their fills generally have pebble-to-cobble bases overlain by fine-to-pebbly sand and local thin silt and clay tops. Alluvial-fan terraces and associated geomorphic surfaces grade to former valley levels defined by axial terrace treads. Carbon-14 ages from detrital charcoal above and below a buried tributary terrace tread show that the most recent aggradation event persisted until ca. 3 ka during the transition from glacial to modern climate conditions. Drill-log data show widespread valley fill ∼30 m thick that began aggrading after glacial retreat in northern New Mexico and southern Colorado (ca. 14 ka). Aggradation during this transition was likely due to hillslope destabilization, increased sediment yield, decreased runoff, and reduced stream competence. Chlorine-36 ages imply similar controls on earlier terraces that have surface ages of ca. 27–29, 64–70, and 135 ka, and suggest net incision during glacial expansions when increased runoff favored down-cutting and bedload mobilization. Our terrace chronology supports existing climate-response models of arid environments and links tributary responses to the axial Rio Grande system throughout the central Rio Grande rift. The terrace chronology also reflects a transition from modest (60 m/m.y.) to rapid (300 m/m.y.) incision between 610 and 135 ka, similar to patterns observed throughout the Rio Grande rift and the western United States in general.

The stratigraphic evolution of a submarine channel: linking seafloor dynamics to depositional products

ABSTRACT We investigate the relationship between the cross-sectional geomorphic expression of a submarine channel as observed on the seafloor and the stratigraphic product of long-lived erosion, bypass, and sediment deposition. Specifically, by reconstructing the time–space evolution of an individual channel fill (i.e., channel element) exposed in outcrop, we establish a genetic link between thick-bedded channel-element-axis sandstone to thinly interbedded channel-element-margin deposits. Although the bounding surface between axis sandstone and margin thin beds is sharply defined, it is composed of a series of geomorphic surface segments of various ages; as such, the composite stratigraphic surface (∼ 17 m relief) was formed from numerous incision events that repeatedly sculpted the conduit. By demonstrating the origin of the stratigraphic surface, we conclude that geomorphic surfaces with 2–7 m of erosional relief were largely responsible for the observed intra-channel-element architecture (and ultimately, the composite 17-m-thick element). The widely documented channel element axis-to-margin architecture is a product of submarine-channel thalweg dynamics, primarily recording interactions between the seafloor and the basal high-concentration layers of channelized turbidity currents.

Holocene earthquake history and slip rate of the southern Teton fault, Wyoming, USA

The 72-km-long Teton normal fault bounds the eastern base of the Teton Range in northwestern Wyoming, USA. Although geomorphic surfaces along the fault record latest Pleistocene to Holocene fault movement, the postglacial earthquake history of the fault has remained enigmatic. We excavated a paleoseismic trench at the Buffalo Bowl site along the southernmost part of the fault to determine its Holocene rupture history and slip rate. At the site, ∼6.3 m of displacement postdates an early Holocene (ca. 10.5 ka) alluvial-fan surface. We document evidence of three surface-faulting earthquakes based on packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian modeling of radiocarbon and luminescence ages yields earthquake times of ca. 9.9 ka, ca. 7.1 ka, and ca. 4.6 ka, forming the longest, most complete paleoseismic record of the Teton fault. We integrate these data with a displaced deglacial surface 4 km NE at Granite Canyon to calculate a postglacial to mid-Holocene (14.4–4.6 ka) slip rate of ∼1.1 mm/yr. Our analysis also suggests that the postglacial to early Holocene (14.4–9.9 ka) slip rate exceeds the Holocene (9.9–4.6 ka) rate by a factor of ∼2 (maximum of 3); however, a uniform rate for the fault is possible considering the 95% slip-rate errors. The ∼5 k.y. elapsed time since the last rupture of the southernmost Teton fault implies a current slip deficit of ∼4–5 m, which is possibly explained by spatially/temporally incomplete paleoseismic data, irregular earthquake recurrence, and/or variable per-event displacement. Our study emphasizes the importance of minimizing slip-rate uncertainties by integrating paleoseismic and geomorphic data sets and capturing multiple earthquake cycles.

Soil-landscape relationship in sandstone-gneiss topolithosequence in Amazonas, Brazil

Soil position in the landscape reveals its history of formation and genesis. Therefore, the landscape is the combination of features of the surface of the earth with subsurface components (parent material), while the soil is a three-dimensional, dynamic natural body inserted in the landscape. This research aimed to study the soil-landscape relationship in a sandstone-gneiss topolithosequence in Amazonas, Brazil. The study was carried out along a 9.253-meter transect from the top downwards the softer slope. Soil profiles were selected in five landscape compartments (top, upper third, lower third, transport foothill, and deposition foothill). Morphological, mineralogical, physical, chemical, and ray diffraction characterizations were performed. Soils had different morphological, physical, chemical, and mineralogical attributes due to the variations of the geological substrate and landscape position. The mineralogy of the clay fraction is composed of kaolinite, goethite, hematite, and gibbsite, with goethite being the predominant iron oxide. A sand fraction dominance was observed in relation to the other fractions in all the profiles, being related to the alluvial nature of the parent material, with the highest values occurring in the lower third. The separation of the landscape into geomorphic surfaces and identification of the parent material were effective for understanding the variation of soil attributes along the landscape.

Inferring the timing of abandonment of aggraded alluvial surfaces dated with cosmogenic nuclides

Information about past climate, tectonics, and landscape evolution is often obtained by dating geomorphic surfaces comprising deposited or aggraded material, e.g. fluvial fill terraces, alluvial fans, volcanic flows, or glacial till. Although surface ages can provide valuable information about these landforms, they can only constrain the period of active deposition of surface material, which may span a significant period of time in the case of alluvial landforms. In contrast, surface abandonment often occurs abruptly and coincides with important events like drainage reorganization, climate change, or landscape uplift. However, abandonment cannot be directly dated because it represents a cessation in the deposition of dateable material. In this study, we present a new approach to inferring when a surface was likely abandoned using exposure ages derived from in situ-produced cosmogenic nuclides. We use artificial data to measure the discrepancy between the youngest age randomly obtained from a surface and the true timing of surface abandonment. Our analyses simulate surface dating scenarios with variable durations of surface formation and variable numbers of exposure ages from sampled boulders. From our artificial data, we derive a set of probabilistic equations and a MATLAB tool that can be applied to a set of real sampled surface ages to estimate the probable period of time within which abandonment is likely to have occurred. Our new approach to constraining surface abandonment has applications for geomorphological studies that relate surface ages to tectonic deformation, past climate, or the rates of surface processes.

Holocene slip rate of the frontal thrust in the western Qilian Shan, NE Tibetan Plateau

SUMMARY The activities of frontal thrusts in the northern Qilian Shan are critical for understanding the deformation of the Qilian Shan and the northeastern Tibetan Plateau. In this study, we estimate the slip rate of the active Fodongmiao–Hongyazi thrust along the northern margin of the Qilian Shan. High-resolution satellite imagery interpretations and detailed field investigations suggest that the fault displaced late Pleistocene terraces and formed fresh prominent north-facing fault scarps. To quantify the slip rate of the fault, we measured the displacements along the fault scarps using an unmanned aerial vehicle system and dated the displaced geomorphic surfaces using optically stimulated luminescence (OSL) and 14C methods. The vertical slip rate of the fault is estimated at 1.0 ± 0.3 mm yr−1 for the western segment. The slip rates for two branches in the eastern segment are 0.3 ± 0.1 and 0.6 ± 0.1 mm yr−1. Using a fault dip of 40 ± 10°, we constrain the corresponding shortening rates to 1.4 ± 0.5 and 1.2 ± 0.4 mm yr−1, respectively. The rates are consistent with values over different timescales, which suggests steady rock uplift and northeastward growth of the western Qilian Shan. Crustal shortening occurs mainly on the range-bounding frontal thrust.