Tammy M. Rittenour

Fluvial evolution of the lower Mississippi River valley during the last 100 k.y. glacial cycle: Response to glaciation and sea-level change

The lower Mississippi valley contains multiple large braid belts for which age control has been limited. Application of the optically stimulated luminescence technique has produced a new chronology of lower Mississippi valley channel-belt formation and insight into the valley9s evolution during the last glacial cycle. Fluvial deposits range from last interglacial meander belts (85 ± 7 to 83 ± 7 ka) to multiple braid belts (64 ± 5 to 11 ± 1 ka) and record large-amplitude responses of the Mississippi River to glacially induced changes in discharge and sediment supply during the last glacial cycle. Slackwater deposits in buried tributary valleys from the middle Mississippi valley and northern lower Mississippi valley suggest that the river was flowing at a position 8–21 m below the present flood plain during the last interglacial, then rapidly aggraded and switched to a braided regime to form the highest and oldest braid belt by 64 ± 5 to 50 ± 4 ka, coincident with initial glaciation of the upper drainage basin. The Mississippi River remained braided until final meltwater withdrawal from its headwaters in the earliest Holocene. Braid-belt formation and incision was controlled by fluctuations in meltwater and sediment discharge, while glacio-eustatic sea level controlled the elevation to which the river was graded, causing late glacial braid belts to dip below the Holocene flood plain in the southern lower Mississippi valley. Moreover, avulsions in the middle Mississippi valley and northern lower Mississippi valley during the last glaciation have pinned the river over regions of shallow bedrock, preventing the modern river from incising to its last interglacial profile. The new chronology and longitudinal profiles presented here provide insight into the response of this continental-scale river system to climatic (glacial) and base-level forcing during the last 100 k.y. glacial cycle.

Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall

Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced base-level fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development. The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys.

Unraveling tectonic and climatic controls on synorogenic growth strata (Northern Apennines, Italy)

We develop a new high-resolution stratigraphic age model to unravel the contributions of tectonic and climatic processes on early to late Pleistocene synorogenic growth strata. We capitalize on excellent, continuous exposures along the flank of the Po foreland in northern Italy to elucidate hydrologic, geomorphic, and sedimentologic processes that are regularly attributed to, but rarely proven to be caused by, glacial-interglacial climatic changes and unsteady rock uplift. We perform our analysis on the Enza section, a succession of marine and terrestrial strata exposed along the Enza River, between Parma and Reggio Emilia, northern Italy. Bedding in the Enza section displays synorogenic growth strata geometry, with bedding dips that range from 2° to 55°, that becomes progressively shallower upsection. We develop an age model that incorporates biostratigraphy, magnetostratigraphy, rock-magnetic cyclostratigraphy, cosmogenic radionuclide burial dating, and optically stimulated luminescence dating and shows that the Enza section spans the interval between 0.04 and 1.65 Ma. Furthermore, the age model pins the time of deposition for several lithostratigraphic units of regional significance and shows that sediment accumulation was unsteady, ranging from 14–31 cm/k.y. in the marine part of the section to 5–362 cm/k.y. in the overlying littoral and terrestrial part of the section. Unsteady deposition is most pronounced in the terrestrial deposits where thick fluvial gravel packages accumulated in short (∼10–15 k.y.) time periods that coincide with Quaternary glacial intervals. There is direct evidence for a dominant tectonic control in the older, marine part of the section. Here, sediment accumulation rates on the limb of the fold growing along this portion of the Northern Apennine mountain front show that between 1.07 and 1.65 Ma, repetitive progradation of neritic sand units directly followed pulses of rapid, punctuated uplift. In contrast, the cyclic terrestrial facies variations in the Enza section reveal that once the section became emergent at ca. 1 Ma and uplift slowed, climate was the dominant control on sediment production and deposition.

An optical age chronology of Late Pleistocene fluvial deposits in the northern lower Mississippi valley

The lower Mississippi valley (LMV) contains many large braided channel belts that are preserved west of the Holocene floodplain. Previous efforts to establish geochronologic control on channel-belt construction have been hindered by the lack of organic material for radiocarbon dating. Luminescence techniques provide a burial date for the sediment itself and may prove useful in this context. Samples from three channel belts in the northern LMV were analyzed using the single aliquot-regenerative technique on 90–125mm quartz. Optical ages (19.7–17.8, 16.1–15.0 and 12.5–12.1 ka) are consistent with geomorphic relationships and indicate that channel belts were formed in the late Pleistocene under glacial conditions. These optical ages provide the first detailed chronology of LMV channel-belt formation and are the first step towards developing a chronology for the entire LMV. r 2003 Elsevier Science Ltd. All rights reserved.

Forecasting the response of Earth's surface to future climatic and land use changes: A review of methods and research needs

In the future, Earth will be warmer, precipitation events will be more extreme, global mean sea level will rise, and many arid and semiarid regions will be drier. Human modifications of landscapes will also occur at an accelerated rate as developed areas increase in size and population density. We now have gridded global forecasts, being continually improved, of the climatic and land use changes (C&LUC) that are likely to occur in the coming decades. However, besides a few exceptions, consensus forecasts do not exist for how these C&LUC will likely impact Earth-surface processes and hazards. In some cases, we have the tools to forecast the geomorphic responses to likely future C&LUC. Fully exploiting these models and utilizing these tools will require close collaboration among Earth-surface scientists and Earth-system modelers. This paper assesses the state-of-the-art tools and data that are being used or could be used to forecast changes in the state of Earths surface as a result of likely future C&LUC. We also propose strategies for filling key knowledge gaps, emphasizing where additional basic research and/or collaboration across disciplines are necessary. The main body of the paper addresses cross-cutting issues, including the importance of nonlinear/threshold-dominated interactions among topography, vegetation, and sediment transport, as well as the importance of alternate stable states and extreme, rare events for understanding and forecasting Earth-surface response to C&LUC. Five supplements delve into different scales or process zones (global-scale assessments and fluvial, aeolian, glacial/periglacial, and coastal process zones) in detail.

Colorado River chronostratigraphy at Lee’s Ferry, Arizona, and the Colorado Plateau bull’s-eye of incision

ABSTRACTLee’s Ferry (Arizona, United States) lies at an important geo-logic transition between the Grand Canyon margin and the Canyon-lands center of the Colorado Plateau. It marks a knickpoint along the Colorado River at the top of the steep Grand Canyon, and it is central to debate about the patterns of erosion and sources of uplift in this famous landscape. New chronostratigraphic data from the suite of fi ll terraces here indicate a strong fl uvial response to climate driv-ers superimposed upon an integrated mid-to-late Pleistocene incision rate of ~350 m/m.y. A regional compilation of well-constrained results over the same timescale reveals that this is intermediate between slower rates downstream in Grand Canyon and even faster rates in the central Colorado Plateau, which taper off again farther upstream near the plateau’s eastern edge. This bull’s-eye pattern of rapid inci-sion in the central Colorado Plateau does not match proposed sources of uplift from mantle dynamics at the south and west fl ank of the pla-teau, nor patterns of river steepness and energy. Instead we suggest that this incision pattern is primarily driven by transient response to drainage integration and isostatic feedback from the deep exhuma-tion of weak rocks in the central plateau.INTRODUCTION

Shortening rate and Holocene surface rupture on the Riasi fault system in the Kashmir Himalaya: Active thrusting within the Northwest Himalayan orogenic wedge

New mapping demonstrates that active emergent thrust faulting is occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The >60-km-long Riasi fault system is the southeasternmost segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India. Two fault strands, the Main Riasi and Frontal Riasi thrusts, dominate the fault system in the study area. The Main Riasi thrust places Precambrian Sirban Formation over folded unconsolidated Quaternary sediments and fluvial terraces. New age data and crosscutting relationships between the Main Riasi thrust and the Quaternary units demonstrate that the Main Riasi thrust accommodated shortening between 100 and 40 ka at rates of 6−7 mm/yr. Deformation shifted to the southern Frontal Riasi thrust splay after ca. 39 ka. Differential uplift of a 14−7 ka terrace yields a range of shortening rates between 3 and 6 mm/yr. Together, shortening across the two strands indicates that a 6−7 mm/yr shortening rate has characterized the Riasi fault system since 100 ka. Geodetic data indicate that an 11−12 mm/yr arc-normal shortening rate characterizes the interseismic strain accumulation across the plate boundary due to India-Tibet convergence. These data combined with rates of other active faults in the Kashmir Himalaya indicate that the Suruin-Mastgarh anticline at the thrust front accounts for the remainder 40%−50% of the convergence not taken up by the Riasi fault system. Active deformation, and therefore earthquake sources, include both internal faults such the Riasi fault system, as well as rupture of the basal decollement (the Main Himalayan thrust) to the thrust front. Limited paleoseismic data from the Riasi fault system, the historical earthquake record of the past 1000 yr, the high strain rates, and partitioning of slip between the Riasi fault system and the thrust front demonstrate that a substantial slip deficit characterizes both structures and highlights the presence of a regionally important seismic gap in the Kashmir Himalaya. Slip deficit, scaling relationships, and a scenario of rupture and slip on the basal decollement (the Main Himalayan thrust) parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw > 8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake on the Balakot-Bagh fault in Pakistan Azad Kashmir.

User guide for luminescence sampling in archaeological and geological contexts

Abstract Luminescence dating provides a direct age estimate of the time of last exposure of quartz or feldspar minerals to light or heat and has been successfully applied to deposits, rock surfaces, and fired materials in a number of archaeological and geological settings. Sampling strategies are diverse and can be customized depending on local circumstances, although all sediment samples need to include a light-safe sample and material for dose-rate determination. The accuracy and precision of luminescence dating results are directly related to the type and quality of the material sampled and sample collection methods in the field. Selection of target material for dating should include considerations of adequacy of resetting of the luminescence signal (optical and thermal bleaching), the ability to characterize the radioactive environment surrounding the sample (dose rate), and the lack of evidence for post-depositional mixing (bioturbation in soils and sediment). Sample strategies for collection of samples from sedimentary settings and fired materials are discussed. This paper should be used as a guide for luminescence sampling and is meant to provide essential background information on how to properly collect samples and on the types of materials suitable for luminescence dating.

Rapid river incision across an inactive fault: implications for patterns of erosion and deformation in the central Colorado Plateau

The Colorado Plateau presents a contrast between deep and seemingly recent erosion and apparently only mild late Cenozoic tectonic activity. Researchers have recently proposed multiple sources of epeirogenic uplift and intriguing patterns of differential incision, yet little or no quantitative constraints exist in the heart of the plateau to test these ideas. Here, we use both optically stimulated luminescence (OSL) and uranium-series dating to delimit the record of fluvial strath terraces at Crystal Geyser in southeastern Utah, where the Little Grand Wash fault crosses the Green River in the broad Mancos Shale badlands of the central plateau. Results indicate there has been no deformation of terraces or surface rupture of the fault in the past 100 k.y. The Green River, on the other hand, has incised at a relatively rapid pace of 45 cm/k.y. (450 m/m.y.) over that same time, following a regional pattern of focused incision in the “bull’s-eye” of the central plateau. The Little Grand Wash fault may have initiated during Early Tertiary Laramide tectonism, but it contrasts with related structures of the ancestral Paradox Basin that are presently active due to salt dissolution and focused differential erosion. We also hypothesize there may be a Pliocene component of fault slip in the region linked to broad-wavelength erosional unloading, domal rebound, and extension. An apparent rapid decrease in incision rates just upstream through Desolation Canyon suggests the Green River here may have recently experienced an upstream-migrating wave of incision.

Exploring relations between arroyo cycles and canyon paleoflood records in Buckskin Wash, Utah: Reconciling scientific paradigms

A new chronostratigraphy for a suite of Holocene alluvial deposits along Buckskin Wash, Utah, shows that dynamic upstream alluvial systems have an important and as-yet-unrecognized infl uence on the origin and preservation of downstream paleofl ood records. An ephemeral stream featuring an upstream alluvial reach that fl ows into a narrow slot canyon, Buckskin Wash has previously been the subject of important arroyo cut-and-fi ll and paleofl ood studies. We expand upon and reconcile these earlier efforts through detailed sedimentology and a diverse geochronology (radiocarbon, optically stimulated luminescence, treering, and 137 Cs). The alluvial-reach deposits preserve at least four arroyo cut-and-fi ll cycles since ca. 3 ka. In contrast, the majority of the constricted-reach deposits date to ca. A.D. 1850‐1950, coincident with the most recent arroyo-cutting event upstream. These new data show that transfer of sediment from allu vial valleys during historic arroyo cutting temporarily enhanced preservation of alluvial deposits in the bedrock canyon downstream. Thus, we argue that slackwater deposition in canyons like Buckskin Gulch can be as much a function of upstream geomorphic changes as of fl ood frequency and magnitude. This suggests that some paleofl ood records may be seriously incomplete, and that paleofl ood chronologies could be as much related to the phase of the arroyo cycle as to temporal variability in storm frequency. This fi nding has important implications for studies that utilize paleofl ood records and for our broader under standing of the ways in which dryland fl uvial systems respond to climate change.