Groundwater Sapping Research Articles

Hydrologic evolution of two Martian deltas in the Ismenius Cavus system

Ismenius Cavus (33.76° N, 17.05° W) is a large Martian basin that has been interpreted as an ancient paleolake. It is situated in the middle of the path Mamers Valles traces from the Cerulli Crater to the Borealis Basin. This ancient lake served as a key deposition area (or depocenter) for multiple lake chains originating in the southern highlands. The collected water was then transported downstream to the Borealis Basin. The ancient hydrologic activity of this zone is evidenced in the widespread appearance of valley networks and fretted channels, but especially in the presence of deltaic deposits converging in Ismenius Cavus. We made a hydrological and geomorphological analysis of two of these deltas, which compose a three-crater lake chain system. It was interpreted that both deltas, although being close to each other, were created by different processes and at different times. The Aracataca Delta was deposited during the Noachian by a valley network system. The Ariguani Delta, in turn, was the result of the discharge of a fretted channel carved by groundwater sapping. The transformations of the hydrologic systems in the Ismenius Cavus region were established by analyzing the change in the deltaic pulses over time. It was determined that the first hydrologic stage was dominated by widespread valley networks that probably represent subglacial hydrologic systems, a second stage started when these systems were replaced by groundwater activity; and finally, when the water sources were exhausted, a final stage of glacial processes prevailed until the present.

Spring erosional processes and small sapping valleys in southwestern lower Michigan, USA

AbstractTributary valleys along the Pigeon River in southwestern Michigan USA possess many previously defined criteria for sapping valleys. Direct observations of seasonal variations in groundwater levels, hydraulic gradients and water temperatures confirm that perennial springs and their streams are present in these small valleys. Estimated spring‐fed stream discharges are low and variable, but continuous. Direct observations, grain‐size distributions and bedload transport estimates indicate that significant transport of fine to medium sand occurs in all seasons.We present a process‐form model for small‐scale sapping processes. Stream terraces and paleo‐meander scarps are being modified by sapping, and elongated valleys are likely being lengthened headward by groundwater sapping. Sapping processes have been occurring for at least the past ~4,500 years after Pigeon River adjusted to a fall in base level from the Nipissing high stand to the modern level of Lake Michigan. Sapping processes are currently producing a variety of small‐scale landforms dependent on four main factors: 1) hydraulic gradients and spring discharges, 2) available grain size of material being eroded, 3) transport rate of sand bedload by spring‐fed streams and finally 4) time available for groundwater sapping process to occur.

Drainage evolution in the Piraí depression (southern Brazil): evidence for headward erosion in large south american river systems

The long-term evolution of rivers draining westward from Brazilian Great escarpment remain unclear and one of the main questions is how western draining rivers organized and defined their modern courses. In this study, we evaluated how the Piraí Depression (PD) evolved and how rivers draining the PD interior defined their valleys and became an important contributing area for the Paraná River catchment. We used three sets of data: (1) topographical metrics (ks(n), knickpoints, local relief, χ-index values, Gilbert metrics, Swath Profiles and Seppômen Maps); (2) relief and river morphology (elbows and low-divides) from remote sensing derived data; and (3) sedimentologic analysis from a drainage capture previously identified with vibro-corer drilling. Ks(n), local relief and χ indices, and morphological evidence, point to an ongoing process of drainage rearrangement all over PD as it has been continuously eroded by Tibagi, Cinzas, Itararé and, mainly the Atlantic draining rivers from the Ribeira do Iguape River System. Morphometric, morphological and sedimentologic data imply long-lasting recurrence of captures and divide migration. The oldest capture evidence in the study area corresponds to the incision of the Iapó River which created the Guartelá Canyon. At least three generations of captures followed, and the younger generation is exemplified by the Maracanã-Jotuba capture as shown by the fieldwork data and described sedimentary facies. The field-documented underground capture illustrates the progressive abandonment and valley filling of the Maracanã River and capture by Jotuba river. Headward erosion and groundwater sapping are crucial for drainage (re)arrangements. In the PD drainage transition from endorheic to exorheic system was a direct result of captures. Atlantic rivers have greater erosion power and also migrate westward capturing areas from Paraná-La Plata which does not prevent drainage rearrangements to take place between westward-draining rivers.

An insight to the cryospheric level in Mars: Case study from the Thaumasia Minor

The presence of a subsurface Martian cryosphere has been widely acknowledged. Rampart craters and outflow channels are thought to be the prominent geomorphic signatures in support of the presence of this cryospheric layer. Valley networks, on the other hand, are thought to be the signatures of precipitation, seasonal meltwater flow due to glacial melting and/or groundwater sapping. A rampart crater essentially has an asymmetric lobate-shaped ejecta blanket around it. In the present study area within Thaumasia Minor, situated near the equatorial region of Mars, rampart craters are present while outflow channels are absent. Therefore, the rampart craters are the only windows to locate the existence and extent of the cryospheric level in the area. It has been observed that these rampart craters remained unaffected by surface fluvial activities. Rampart Craters, both Single Layered Ejecta craters (SLE) and Multiple Layered Ejecta craters (MLE) present in Thaumasia Minor were identified, mapped and dated. Excavation depths were calculated (with respect to the areoid) for all craters of varied ages. The excavation depth data sets were used to draw statistical surfaces for MLE craters and SLE craters. It was concluded from the comparative analysis of the excavation depths (between the MLE and SLE craters situated in the relatively higher altitudes within TM) at different time intervals, the base of the Martian cryosphere was at greater depths prior to ~2Ga than it was around 0.5Ga in the study area. This suggests that within the study area the Martian cryosphere did evolve with time and our study of the excavation depths of the craters suggests that the cryosphere must have thinned out as time progressed.

Vegetation and cloud cover shape semi‐arid carbonate landform development

AbstractWe study the relationships among precipitation, vegetation, and morphological characteristics of watersheds draining either side of the Dhofar Mountains in southeastern Oman to understand the geomorphic signature of water availability in a semi‐arid carbonate landscape. Water availability is expressed in terms of vegetation and cloud cover. The integral and the statistical moments of the hypsometric curve were used to determine whether hyper‐arid, inland‐draining watersheds are significantly different from seasonally wet watersheds on the coast side of the mountain range. We demonstrate that the vegetation and cloud cover are correlated, with locations with longer cloud periods also having a longer period with a vegetation canopy. The analysis shows that the hypsometric curve and its statistical moments capture the morphological difference between wet watersheds shaped by groundwater sapping and dry watersheds with fluvial morphology. Specifically, the curves exhibit two shapes: watersheds with more vegetation and cloud cover are characterized by higher convexity, and those with less vegetation and cloud cover are characterized by higher concavity. A variance analysis of cloud cover, vegetation, and hypsometric integral shows that they are significantly different between the wet and dry watersheds. The link between hydrology and morphology is not strong at the scale of a single watershed, but it is significant when the watersheds are aggregated in zones. The statistical moments of the hypsometric curve in the range of values of the integral and skewness show good separation between watersheds dominated by sapping and fluvial erosion processes. We can separate the watersheds draining the mountain range in two distinct groups on the basis of their bimodal hydrological and morphological characteristics. Our findings support other studies that hypothesize a trade‐off from chemical‐ to mechanical‐dominated denudation in carbonate terranes as precipitation decreases.

Valley formation on early Mars by subglacial and fluvial erosion

The southern highlands of Mars are dissected by hundreds of valley networks, which are evidence that water once sculpted the surface. Characterizing the mechanisms of valley incision may constrain early Mars climate and the search for ancient life. Previous interpretations of the geological record require precipitation and surface water runoff to form the valley networks, in contradiction with climate simulations that predict a cold, icy ancient Mars. Here we present a global comparative study of valley network morphometry, using a principal-component-based analysis with physical models of fluvial, groundwater sapping and glacial and subglacial erosion. We found that valley formation involved all these processes, but that subglacial and fluvial erosion are the predominant mechanisms. This is supported by predictions from models of steady-state erosion and geomorphological comparisons to terrestrial analogues. The inference of subglacial channels among the valley networks supports the presence of ice sheets that covered the southern highlands during the time of valley network emplacement. Some valleys in the southern highlands of Mars may have formed by subglacial erosion, consistent with a cold and icy early Mars, according to a statistical analysis of valley morphometry.

The case against vast glaciation in Valles Marineris, Mars

The Valles Marineris of Mars form the largest system of interconnected canyons in the Solar System, where morphological, mineralogical, and structural evidence of widespread glaciation has been recently reported. However, neither precipitation models nor global water budgets can account for such a colossal fill, and the hypothesis has thus far remained unchallenged by additional scrutiny. Here, we present the first thorough case against a Valles Marineris glaciation by describing new evidence that precludes the existence of a glacier. Most crucially, we review High Resolution Imaging Science Experiment (HiRISE) images of chaos terrain in Candor Chasma — previously interpreted as remnant glacial ice — and identify layered, boulder-rich scarps bearing no similarity to scarps of massive ice in Promethei Terra or the North Polar Layered Deposits. We also find no significant differences in the structure, morphology, and composition of chaos terrain in Candor and the Chryse Planitia region, which suggests Candor's Chaos, like Chryse's, is fractured highland terrain. We also review several other key supports for the glacial hypothesis, including the coincidence of an apparent glacial trimline with the water-bearing mineral jarosite, and the apparent presence of glacial features including kettle holes and sandur plains. These too are found to have more plausible explanations with non-glacial origins including tectonic reorganisation, groundwater sapping, and chaotic fracturing. We conclude that the Valles Marineris glacial hypothesis is inconsistent with both observed morphology and our understanding of the ancient Martian climate, and we find no evidence to support its existence.

Evidences of early aqueous Mars: Implications on the origin of branched valleys in the Ius Chasma, Mars

This study shows results of morphological and spectroscopic analyses of Ius Chasma and its southern branched valleys using Orbiter datasets such as Mars Reconnaissance Orbiter (MRO)-Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), High Resolution Imaging Science Experiment (MRO-HiRISE) and digital terrain model (HRSC-DTM). Result of the spectral analysis reveals presence of hydrated minerals such as opal, nontronite and vermiculite in the floor and wall rock areas Ius Chasma indicating alteration of parent rock in an water rich environment of early Mars. Topographic gradient and morphological evidences such as V-shaped valleys with theatre shaped stubby channels, dendritic drainage and river piracy indicate that these valleys were initially developed by surface runoff due to episodic floods and further expanded due to groundwater sapping controlled by faults and fractures. Minerals formed by aqueous alteration during valley formation and their intricate association with different morphological domains suggest that surface runoff played a key role in the development of branched valleys south of Ius Chasma on Mars.

Canyon incision chronology based on ignimbrite stratigraphy and cut-and-fill sediment sequences in SW Peru documents intermittent uplift of the western Central Andes

Based on an 40Ar/39Ar- and U/Pb-based chronostratigraphy of ignimbrite sheets and the geomorphological features of watersheds, river profiles and slope deposits in the Ocoña–Cotahuasi–Marán (OCM) and Colca valleys of southwest Peru, we reconstruct the valley incision history of the western Central Andes over the last c. 25Myr. We further document the Pleistocene and Holocene evolution of deep valleys on the basis of 14 10Be surface-exposure ages obtained on debris-avalanche deposits and river straths.The data suggest that uplift was gradual over the past 25Myr, but accelerated after c. 9Ma. Valley incision started around 11–9Ma and accelerated between 5 and 4Ma. Incision was followed by several pulses of valley cut-and-fill after 2.3Ma. Evidence presented suggest that the post-5Ma sequence of accelerated canyon incision probably resulted from a combination of drainage piracy from the Cordilleran drainage divide towards the Altiplano, accentuated flexural tilting of the Western Cordillera towards the SE, and increased rainfall on the Altiplano after late Miocene uplift of the Eastern Cordillera. The valley deepening and slope steepening driven by tectonic uplift gave rise to large occurrences of rockslope failure. The collapsed rock masses periodically obstructed the canyons, thus causing abrupt changes in local base levels and interfering with the steadiness of fluvial incision. As a result, channel aggradation has prevailed in the lower-gradient, U-shaped Pacific-rim canyons, whereas re-incision through landslide deposits has occurred more rapidly across the steeper V-shaped, upper valleys. Existing canyon knickpoints are currently arrested at the boundary between the plutonic bedrock and widespread outcrops of middle Miocene ignimbritic caprock, where groundwater sapping favouring rock collapse may be the dominant process driving headward erosion.

The geometry and complexity of spatial patterns of terrestrial channel networks: Distinctive fingerprints of erosional regimes

The morphology of channel networks related to long‐term erosion reflects the mechanisms involved in their formation. This study aims to identify quantitative metrics, drawn from topographic data and satellite imagery, that are diagnostic of the distinctive styles of erosion by rivers, glaciers, subglacial meltwater, and groundwater sapping. From digital elevation models, we identify three geometric metrics: the minimum channel width, channel aspect ratio (longest length to channel width at the outlet), and tributary junction angle. We also characterize channel network complexity in terms of its stream order and fractal dimension. To validate our approach, we perform a principal component analysis (PCA) on measurements of these five metrics on 70 channel networks. We build understanding of these results, in turn using scaling analyses of appropriate physical models. We show that rivers, glaciers, and groundwater sapping erode the landscape in rigorously distinguishable ways. Whereas rivers are characterized by nearly constant minimum width, variable aspect ratio, and high stream orders, glaciers have highly variable minimum widths and aspect ratios and much smaller stream orders. Erosion by subglacial meltwater remains poorly understood, and we argue that we require an additional metric to fully characterize these systems. Our methodology can more generally be applied to identify the contributions of different processes involved in carving a channel network. In particular, we are able to identify transitions from fluvial to glaciated landscapes or vice versa.

Submarine canyon-head morphologies and inferred sediment transport processes in the Alías-Almanzora canyon system (SW Mediterranean): On the role of the sediment supply

Canyon head regions are key areas for understanding the shelf-to-canyon sedimentary dynamics and assessing the predominant hydrodynamic and sedimentary processes shaping their morphology. High-resolution multibeam bathymetry surveys conducted on the shelf, upper slope and along the main thalwegs of the Alías-Almanzora submarine canyon system were used to recognize the specific morphological features of the various canyon heads and infer the main sediment transport mechanism operating through them. Off the Almanzora River, different canyon head morphologies were observed at 65–90m water depth, incising the outer prodeltaic deposits. A direct connection from the river supply is evidenced by the presence of a large radial field of sediment waves on the prodelta, presumably related to hyperpycnal flows during flash flood events, and their continuation towards trains of cyclic steps that merge into the Almanzora canyon head. Processes linked to seepage and groundwater sapping also seems apparent in the prodeltaic deposits, which results in pockmark alignments and formation of elongated narrow canyon heads. At the southern limit of the Almanzora prodelta, another type of canyon head results from the formation and merging of linear gullies. These have been interpreted as the morphological expression of the distal off-shelf transport of flood-related hyperycnal flows. However, most of the canyon heads of the main tributaries of this canyon system (i.e., Garrucha, Cabrera and Alías) are found at much shallower water depths (7–20m). They generally show a continuation in land with rivers or intermittent creeks (known as “ramblas”) that also experience flash flood events, but some of them are disconnected from any river source. These canyon heads cut into the infralittoral prograding wedge, displaying feeder channels that tend to oriented against the direction of the prevalent long-shore current. Some canyon head tributaries show sediment waves along their axis interpreted as the result of hyperpycnal flood events or storm-induced turbidity flows. This study focusses on the diversity of sediment transport processes that can co-exist in the various canyon tributaries of a given submarine canyon system, stressing on the importance of the canyon heads location with respect to the principal sedimentary source, which ultimately determines their main geomorphological traits.

Seafloor terraces and semi-circular depressions related to fluid discharge in Stockholm Archipelago, Baltic Sea

Terraces and semi-circular depressions in the seafloor along islands in the southern Stockholm Archipelago, Sweden, were mapped in the 1990s with single-beam echo sounder and side-scan sonar equipment (Soderberg & Floden 1991, 1995). After inspection by divers, these features were interpreted to be formed by processes related to groundwater that disharged at the seafloor after flowing through permeable layers in glacial clay (Soderberg & Floden 1991, 1995). Erosional features related to submarine groundwater discharge (or sapping/piping) in the seafloor have been identified in many places and at a number of scales. For example, terraced walls and irregular courses of valleys in the continental slope of New Jersey are interpreted to be formed by groundwater sapping during periods of lower sea level (Robb 1984). The contribution of fresh groundwater to the sea is identified as an important component of the global hydrological cycle (Church 1996; Post et al. 2013) and also may contribute as much as 6–7% of the total freshwater discharge to the World Ocean (Zekster 2000 …

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 (87Sr/86Sr, [Sr], δ13C, and δ18O) 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 (<1 Ma) and speleothems (2–4 Ma) yield 87Sr/86Sr and δ13C and δ18O 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 87Sr/86Sr, δ13C, and δ18O that are similar to Grand Canyon springs and travertines, suggesting a long-lived spring-fed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87Sr/86Sr and δ13C and increase in δ18O 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 87Sr/86Sr, δ13C, and δ18O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover.

Groundwater sapping as the cause of irreversible desertification of Hunshandake Sandy Lands, Inner Mongolia, northern China

In the middle-to-late Holocene, Earth's monsoonal regions experienced catastrophic precipitation decreases that produced green to desert state shifts. Resulting hydrologic regime change negatively impacted water availability and Neolithic cultures. Whereas mid-Holocene drying is commonly attributed to slow insolation reduction and subsequent nonlinear vegetation-atmosphere feedbacks that produce threshold conditions, evidence of trigger events initiating state switching has remained elusive. Here we document a threshold event ca. 4,200 years ago in the Hunshandake Sandy Lands of Inner Mongolia, northern China, associated with groundwater capture by the Xilamulun River. This process initiated a sudden and irreversible region-wide hydrologic event that exacerbated the desertification of the Hunshandake, resulting in post-Humid Period mass migration of northern China's Neolithic cultures. The Hunshandake remains arid and is unlikely, even with massive rehabilitation efforts, to revert back to green conditions.

Origin of collapsed pits and branched valleys surrounding the Ius chasma on Mars

Abstract. Chasma is a deep, elongated and steep sided depression on planetary surfaces. Several hypothesis have been proposed regarding the origin of chasma. In this study, we analysed morphological features in north and south of Ius chasma. Collapsed pits and branched valleys alongwith craters are prominent morphological features surrounding Ius Chasma, which forms the western part of the well known Valles Marineris chasma system on Martian surface. Analysis of images from the High Resolution Stereo Camera (HRSC) in ESA’s Mars Express (MEX) with a spatial resolution of 10 m shows linear arrangement of pits north of the Ius chasma. These pits were initially developed along existing narrow linear valleys parallel to Valles Merineris and are conical in shape unlike flat floored impact craters found adjacent to them. The width of conical pits ranges 1–10 km and depth ranges 1–2 km. With more subsidence, size of individual pits increased gradually and finally coalesced together to create a large depression forming a prominent linear valley. Arrangement of pits in this particular fashion can be attributed to collapse of the surface due to l arge hollows created in the subsurface because of the withdrawal of either magma or dry ice. Branched valleys which are prominent morphologic features south of the Ius chasma could have been formed due to groundwater sapping mechanism as proposed by previous researchers. Episodic release of groundwater in large quantity to the surface could have resulted in surface runoff creating V-shaped valleys, which were later modified into U-shaped valleys due to mass wasting and lack of continued surface runoff.

Origin and development of theater-headed valleys in the Atacama Desert, northern Chile: Morphological analogs to martian valley networks

Understanding planetary landforms, including the theater-headed valleys (box canyons) of Mars, usually depends on interpreting geological processes from remote-sensing data without ground-based corroboration. Here we investigate the origin and development of two Mars-analog theater-headed valleys in the hyperarid Atacama Desert of northern Chile. Previous workers attributed these valleys to groundwater sapping based on remote imaging, topography, and publications on the local geology. We evaluate groundwater sapping and alternative hypotheses using field observations of characteristic features, strength measurements of strata exposed in headscarps, and estimates of ephemeral flood discharges within the valleys. The headscarps lack evidence of recent or active seepage weathering, such as spring discharge, salt weathering, alcoves, or vegetation. Their welded tuff caprocks have compressive strengths multiple times those of the underlying epiclastic strata. Flood discharge estimates of cubic meters to tens of cubic meters per second, derived using the Manning equation, are consistent with the size of transported clasts and show that the ephemeral streams are geomorphically effective, even in the modern hyperarid climate. We interpret that headscarp retreat in the Quebrada de Quisma is due to ephemeral flood erosion of weak Miocene epiclastic strata beneath a strong welded tuff, with erosion of the tuff facilitated by vertical jointing. The Quebrada de Humayani headscarp is interpreted as the scar of a giant landslide, maintained against substantial later degradation by similar strong-over-weak stratigraphy. This work suggests that theater-headed valleys on Earth and Mars should not be attributed by default to groundwater sapping, as other processes with lithologic and structural influences can form theater headscarps.

Catastrophic ice lake collapse in Aram Chaos, Mars

Hesperian chaotic terrains have been recognized as the source of outflow channels formed by catastrophic outflows. Four main scenarios have been proposed for the formation of chaotic terrains that involve different amounts of water and single or multiple outflow events. Here, we test these scenarios with morphological and structural analyses of imagery and elevation data for Aram Chaos in conjunction with numerical modeling of the morphological evolution of the catastrophic carving of the outflow valley. The morphological and geological analyses of Aram Chaos suggest large-scale collapse and subsidence (1500m) of the entire area, which is consistent with a massive expulsion of liquid water from the subsurface in one single event. The combined observations suggest a complex process starting with the outflow of water from two small channels, followed by continuous groundwater sapping and headward erosion and ending with a catastrophic lake rim collapse and carving of the Aram Valley, which is synchronous with the 2.5Ga stage of the Ares Vallis formation. The water volume and formative time scale required to carve the Aram channels indicate that a single, rapid (maximum tens of days) and catastrophic (flood volume of 9.3×104km3) event carved the outflow channel. We conclude that a sub-ice lake collapse model can best explain the features of the Aram Chaos Valley system as well as the time scale required for its formation.

Geoarchaeology and Paleoenvironmental Context of the Burntwood Creek Rockshelter, High Plains of Northwestern Kansas, U.S.A

The first systematic geoarchaeological study of a rockshelter in Kansas yielded information about rockshelter formation, Holocene paleoenvironments, archaeological site formation processes, and prehistoric human habitation and site use. The rockshelter was carved by groundwater sapping; colluvium, alluvium, and roof‐fall debris subsequently accumulated on its floor. An assemblage of Late Archaic (∼2000–4000 14C years before present [yr B.P.]) cultural deposits (lithic material, bone, and charcoal) is preserved in a 2‐m thick package of colluvium and alluvium in front of the shelter. Chipped stone analysis indicates that Late Archaic occupants of the shelter were using mostly local materials and resharpening their stone tools at the shelter. Stable carbon isotope and phytolith analyses indicate that a mixed C3 and C4 plant community existed in the study area and that it changed in composition through the late Holocene. Based on the phytolith assemblage from an unlined fire pit at the front of the shelter, Late Archaic people used local hackberry trees for fuel and/or food.

Rate and processes of river network rearrangement during incipient faulting: The case of the Cahabon River, Guatemala

Deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics. Although the reorganization of deeply incised drainage systems has been documented, the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood. To address this problem, we have explored the rapid drainage reorganization that affected the Cahabón River in Guatemala during the Quaternary. Sediment-provenance analysis, field mapping, and electrical resistivity tomography (ERT) imaging are used to reconstruct the geometry of the valley before the river was captured. Dating of the abandoned valley sediments by the ¹⁰Be-²⁶Al burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes, such as the rate of tectonic deformation of the paleovalley, the rate of propagation of post-capture drainage reversal, and the rate at which canyons that formed at the capture sites have propagated along the paleovalley. Transtensional faulting started 1 to 3 million years ago, produced ground tilting and ground faulting along the Cahabón River, and thus generated differential uplift rate of 0.3 ± 0.1 up to 0.7 ±0.4 mm · y⁻¹ along the river9s course. The river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence. Then, the river experienced two captures and one avulsion between 700 ky and 100 ky. The captures breached high-standing ridges that separate the Cahabón River from its captors. Captures occurred at specific points where ridges are made permeable by fault damage zones and/or soluble rocks. Groundwater flow from the Cahabón River down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges. Valley-fill formation and capture occurred in close temporal succession, suggesting a genetic link between the two. We suggest that the aquifers accumulated within the valley-fills, increased the head along the subterraneous system connecting the Cahabón River to its captors, and promoted their development. Upon capture, the breached valley experienced widespread drainage reversal toward the capture sites. We attribute the generalized reversal to combined effects of groundwater sapping in the valley-fill, axial drainage obstruction by lateral fans, and tectonic tilting. Drainage reversal increased the size of the captured areas by a factor of 4 to 6. At the capture sites, 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm · y⁻¹ while deepening at a rate of 0.7 to 1.5 mm · y⁻¹. At this rate, 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley. It is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field, which involved transient sedimentation along the river9s course. If the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium, then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced.

Equilibrium form of horizontally retreating, soil-mantled hillslopes: Model development and application to a groundwater sapping landscape

[1] We present analytical solutions for the steady state topographic profile of a soil-mantled hillslope retreating into a level plain in response to a horizontally migrating base level. This model applies to several scenarios that commonly arise in landscapes, including widening valleys, eroding channel banks, and retreating scarps. For a sediment transport law in which sediment flux is linearly proportional to the topographic slope, the steady state profile is exponential, with an e-folding length, L, proportional to the ratio of the sediment transport coefficient to the base level migration speed. For the case in which sediment flux increases nonlinearly with slope, the solution has a similar form that converges to the linear case as L increases. We use a numerical model to explore the effects of different base level geometries and find that the one-dimensional analytical solution is a close approximation for the hillslope profile above an advancing channel tip. We then compare the analytical model with hillslope profiles above the tips of a groundwater sapping channel network in the Florida Panhandle. The model agrees closely with hillslope profiles measured from airborne laser altimetry, and we use a predicted log linear relationship between topographic slope and horizontal distance to estimate L for the measured profiles. Mapping 1/L over channel tips throughout the landscape reveals that adjacent channel networks may be growing at different rates and that south facing slopes experience more efficient hillslope transport.