Anne E. Mather

150 million years of climatic stability: evidence from the Atacama Desert, northern Chile

The sedimentary succession in the Atacama Desert records deposition under an arid to semiarid climate from the late Jurassic (150 Ma) to the present day. Palaeomagnetic data indicate no significant latitudinal movement of this area since the late Jurassic. The present-day location of the Atacama within the dry subtropical climate belt is the principal cause of aridity. This situation is likely to have prevailed since the late Jurassic, supplemented by (1) the continentality effect (enhanced by the Gondwanan landmass), and (2) the presence offshore of a cold, upwelling current (from at least the early Cenozoic onwards and possibly earlier), resulting in conditions promoting climatic stability and desert development. Rapid and extreme climatic fluctuations during the Plio-Pleistocene were not sufficient to destabilize the climate within the Atacama. Comparison with other long-lived deserts (e.g. SW USA, Namib, Sahara and Australia) suggests that the Atacama is the oldest extant desert on Earth.

The impact of Quaternary sea-level and climatic change on coastal alluvial fans in the Cabo de Gata ranges, southeast Spain

Conventionally, a fall in base level is seen as stimulating incision into the distal zones of alluvial fans. In the Cabo de Gata ranges of southeast Spain evidence exists to the contrary. Two sets of Quaternary coastal alluvial fans demonstrate the interaction between climatically-driven variations in the supply of sediment and eustatically-driven changes in base level. The fans are supplied from Miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan development. The evolution of the east-coast fans has been affected by variations in sediment supply and changes in sea level. The west-coast fans were buffered from the effects of changes in sea level by coastal barriers. Three phases of past sedimentation can be identified on the fans. These can be differentiated on the basis of field observations of soil profiles (particularly colour of the B horizons and accumulation of CaCO3), and laboratory analyses of sequential iron oxide extractions and magnetic mineral properties. The two earlier (major) sedimentation phases were coincident with global glacials (>ca. 135 ka and ca. 85–10 ka, based on the stratigraphy and uranium/thorium dating of the coastal sediments). High sea levels during the intervening interglacial and during the Holocene caused erosion of the distal zones of the east-coast fans which led to channel incision into the fan surfaces. On the west-coast fans no such incision occurred, simply proximal incision by small fanhead trenches. The youngest (relatively minor) phase of fan sedimentation has occurred during the Holocene. These contrasting contexts have produced differing styles of fans, with telescopic fan morphology on the east-coast and stacked morphology on the west-coast fans. The differences are reflected in the fan profiles, with steeper gradients dominating the east-coast fans, and extensive lower gradient distal surfaces on the west-coast fans. Fan morphometry, based on analysis of the residuals from drainage area to fan area and gradient regressions, also differentiates between the fan contexts. The fan building phases appear to be controlled proximally by climatically-driven pulses of sediment supplied to the fans. These occurred during global glacials coincident with low sea levels, and caused fan progradation onto the exposed foreshore. The intervening global interglacials were times of little fan sedimentation, and on the east coast, where high sea levels were able to erode the fan toes, deep through-fan dissection ensued.

Alluvial fans: geomorphology, sedimentology, dynamics — introduction. A review of alluvial-fan research

This volume presents a series of papers on the geomorphology, sedimentology and dynamics of alluvial fans, selected from those presented at the ‘Alluvial Fans’ Conference held in Sorbas, SE Spain in June 2003. The conference was sponsored primarily by the British Geomorphological Research Group and the British Sedimentological Research Group, both organizations affiliated to the Geological Society of London. It is some time since an international conference has been held that was exclusively devoted to the geomorphology and sedimentology of alluvial fans. The previous such conference was that organized by Terry Blair and John McPherson in 1995, and held in Death Valley, a classic setting for alluvial fans (Denny 1965; Blair & McPherson 1994a). Although many of the papers presented there have since been published, no dedicated volume on alluvial fans as a whole resulted from that meeting, so even longer has elapsed since there has been a specific publication devoted wholly to a series of papers on the geomorphology and sedimentology of alluvial fans (Rachocki & Church 1990). South-east Spain was chosen as the venue for this conference, partly for logistic reasons and partly because it is a tectonically active dry region within which there is a wide range of Quaternary alluvial fans. These fans exhibit differing relationships between tectonic, climatic and base-level controls (Harvey 1990, 2002a, 2003; Mather & Stokes 2003; Mather et al. 2003), core themes in consideration of the dynamics of alluvial fans. An emphasis within the previous alluvial fan literature has been on fans within

Dropstones: their origin and significance

Abstract Dropstones are clasts which pose a hydrodynamic paradox or which have an exotic or extra-basinal lithology with uncertain provenance. Recently, dropstones have been consistently interpreted as the product of ice rafting, and have been used to substantiate the presence of cool climatic phases in the geological record. This review illustrates that not all dropstones are reliable as palaeoclimatic indicators. Dropstones are the product of two distinct entrainment and transport mechanisms: by a rafting agent, or as a projectile. Four main processes of transport and formation are identified: biological rafting, ice rafting, floatation and projectiles. Biological rafting agents include vertebrate gastroliths and other stomach contents, vegetational rafting and anthropomorphic activity. Ice rafting can be the product of ice bergs or seasonal sea, river or lake ice. Floatation of clasts in calm waters has been recorded for stones up to 25 mm in length. Projectiles are most commonly of volcanic origin. Given the range of transport and depositional mechanisms reviewed we argue that great care is required in the interpretation of the origin of dropstones.

Adjustment of a drainage network to capture induced base-level change: an example from the Sorbas Basin, SE Spain

Abstract Quaternary catchments in the south of the Sorbas Basin, SE Spain have been affected by two regionally significant river captures. The river captures were triggered by changes in regional gradients associated with sustained Quaternary uplift in the region of 160 m Ma−1. The first capture occurred in the early Pleistocene and re-routed 15% of the original Sorbas Basin drainage into the Carboneras Basin to the south. The second occurred in the late Pleistocene and re-routed 73% of the original Sorbas Basin drainage to the east. This latter capture had dramatic consequences for base-level in the Sorbas Basin master drainage. Local base-level was lowered by 90 m at the capture site, 50 m at 7 km upstream and 25 m at 13 km upstream of the site. The base-level change instigated a complex re-organisation of the drainage networks in systems tributary to the master drainage over the ensuing period (some 100 ka). After the capture, drainage systems closer to the capture site experienced a tenfold increase in incision rates over most of their network. Those located some 13 km upstream of the capture site experienced a fivefold increase in incision, although in this instance, the changes do not appear to have propagated to the headwater regions of the drainage nets. The sensitivity of individual catchments was largely governed by geological controls (structure and lithology). The detailed network evolution in the most sensitive areas can be traced by reconstructing former drainage pathways using abandoned drainage cols and the alignment and degree of incision of the drainage networks. Three main stages of evolution can be identified which record the progressive spread of base-level changes from the master drainage. These are Stage 1 (pre-capture): original south-to-north consequent drainage; Stage 2 (early stage, post capture): aggressive subsequent southwest-to-northeast and east–west drainage developed along structural lineaments first in the east of the area (Stage 2a), and later in the west of the area (Stage 2b); and Stage 3 (late stage, post capture): obsequent drainage developing on the topography of the Stage 2 drainage. All stages of the network evolution are associated with drainage re-routing as a function of river capture at a variety of scales. The results highlight the complex response of the fluvial system, and the very different geomorphological histories of adjacent catchments, emphasising the need for regional approaches for examining long-term changes in fluvial systems.

Multiphase development of the Atacama Planation Surface recorded by cosmogenic 3He exposure ages: Implications for uplift and Cenozoic climate change in western South America

The Atacama Planation Surface is an extensive west-dipping surface developed between 16°S and 27°S along the Pacific margin of South America. It is considered to have formed between 16 and 7.5 Ma and to have important chronostratigraphic significance. Here we present new cosmogenic 3He exposure dates of boulders on the planation surface using pyroxene and amphibole. Exposure ages display good within- and between-site consistency and range from 22 to 1.2 Ma, with ages of ca. 14.6, 7, and 3 Ma recurring at more than one site on the planation surface. The 14.6 Ma peak records the cessation of the main period of planation surface development, but, contrary to popular opinion, the younger ages reflect subsequent modification of the planation surface by alluvial activity. Comparison with other climate proxies for western South America suggests that since 14.6 Ma, a predominantly hyperarid climate, interspersed with short-lived phases of more intense runoff driven by global climate change, has prevailed. The longevity and composite nature of the Atacama Planation Surface suggest that regionally extensive planation surfaces may have a multiphase history, are unlikely to have any chronostratigraphic significance, and cannot be used to reconstruct uplift histories.

Tectonic origin and evolution of a transverse drainage: the Rı́o Almanzora, Betic Cordillera, Southeast Spain

Abstract The Rio Almanzora forms one of the larger drainage systems within the Betic Cordillera, Southeast Spain. In its distal reaches, prior to joining the Mediterranean Sea, the Rio Almanzora cuts across a major topographic barrier, the Sierra Almagro, forming drainage that is transverse to the regional geological structure. The long-term drainage evolution of the Rio Almanzora and its creation as a transverse drainage across the Sierra Almagro has been examined by reconstruction of an evolving basin-scale drainage network using a combination of the geological and geomorphological record. The early stages of drainage evolution (Late Miocene–Pliocene and Plio-Pleistocene) reveals the emergence of marine basins to the north and south of the Sierra Almagro and the development of unconnected drainage systems. This emergence records the development of the proto Rio Almanzora on the south side of the Sierra Almagro as a prograding fan-delta and alluvial fan system. Expansion of this drainage network via incision and headward erosion across the Sierra Almagro took place during the Pleistocene. During the Early–?Mid-Pleistocene, the proto Rio Almanzora became fully transverse across the Sierra Almagro and linked the endorheic Almanzora/Huercal-Overa basin with the Vera basin. A tectonically induced lowering of regional base level is proposed for the incision, headward erosion and drainage network expansion that has resulted in the creation of the Rio Almanzora as a transverse drainage. Differential uplift between the Huercal-Overa/Almanzora and Vera basins has resulted in southwards tilting and the creation of a regional gradient. The fluvial response to tilting was for incision into the steepened regional surface gradient via an increase in stream power. The incision, headward erosion and drainage network expansion by the proto Rio Almanzora and its creation as a transverse drainage appears to be part of a regional response by fluvial systems to differential uplift recorded throughout the Betic Cordillera of Southeast Spain during the Plio-Pleistocene.

Quantifying long-term catchment changes of alluvial fan systems

In mountain areas affected by uplift, significant reorganization of drainage networks can occur through river capture. This modification can dramatically affect sediment flux and routing into adjacent sedimentary basins. It is thus important to obtain information on rates and direction of changes in catchment areas in such environments. This can be achieved where the original, precapture catchments can be compared with the postcapture scenario. This paper examines three modern mountain catchments from southeast Spain that were affected by river capture, and that fed relict Pliocene–Pleistocene alluvial fan systems. Morphometric data have been collated for complete Quaternary mountain catchment piedmont fan systems. These data are used to establish regressions to determine catchment area and basin relief from a combination of fan characteristics (area and gradient). These relationships are then used to predict the original catchment characteristics for the three Pliocene–Pleistocene relict fans and compared with the modern catchments to quantify the magnitude and direction of changes, which have affected the drainage basins since fan abandonment in the early Pleistocene. The figures suggest that the catchment areas are responding to regional uplift, most significantly by drainage net expansion facilitated by river capture rather than lowering of surface relief. As a result, sediment routing into the sedimentary basins has been radically altered.

Response of Plio‐Pleistocene alluvial systems to tectonically induced base‐level changes, Vera Basin, SE Spain

Tectonics are perceived to be a major control on the positioning and long‐term evolution of alluvial systems. By increasing slope gradients through uplift and tilting, or by changing local base‐level, incision can be stimulated leading to a switch in the active area of sedimentation. An example of such a sedimentary response to tectonic activity is provided by well‐exposed late Pliocene/early Pleistocene alluvial sediments of the Salmerón Formation from the western margins of the Vera Basin, SE Spain. Early stage palaeogeographic reconstructions demonstrate the occurrence of two alluvial fan bodies with distinct palaeocurrent and provenance signatures that suggest sediment source areas from the north (Sierra Lisbona) and south (Sierra de Bédar) of the study area. Late stage reconstructions suggest fan abandonment and indicate the occurrence of a braided river system sourced from the Sierra de Bédar in the south. Proximal parts of this braided river are incised by up to 100 m into underlying fan sediments sourced from the Sierra de Bédar. In distal areas, incision is negligible and the braided system forms a conformable sedimentary succession with underlying fan sediments sourced from the Sierra Lisbona. The switch from alluvial fan to braided river sedimentation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected the Vera Basin during the early Pleistocene. Extensional faulting resulted in uplift and subsidence, leading to localized tilting of depositional surfaces in distal areas of the southern fan. Increased stream power resulted in headward incision through mid and proximal fan areas. Once the system became fully trenched distal base‐level controls became effective in generating an enlarged catchment area and continuing incision. The resultant changes dramatically increased both sediment and water discharge to the alluvial system and a switch to braided river sedimentation.

Quantification of river-capture-induced base-level changes and landscape development, Sorbas Basin, SE Spain

Abstract The Aguas/Feos river system of the Sorbas Basin, SE Spain was captured by an aggressive subsequent stream c. 100 ka. The consequence of the capture event was twofold: (1) basin-scale drainage reorganization via beheading of the southward flowing Aguas/Feos system and re-routing the drainage eastwards into the Vera Basin; and (2) the creation of a new, lower base level and associated upstream propagation of a wave of incision. The sequence of pre- and post-capture events are well established from previous studies of the Quaternary terrace record. Using these studies, this paper makes the first attempt to quantify the impact of river capture in terms of spatial and temporal variations in rates of incision, sediment flux and surface lowering. This was carried out through construction of 43 valley cross-sections from the ‘captured’ (Upper Aguas), ‘beheaded’ (Feos) and ‘capturing’ streams (Lower Aguas) within the central-southern parts of the Sorbas Basin. Dated pre- and post-capture terrace and corresponding strath levels were plotted on to the valley cross-sections enabling incision amounts, rates and valley cross-sectional areas to be calculated. Sediment fluxes were calculated using a mean valley section method. Surface lowering calculations were made through reconstruction of the top basin-fill surface and subtraction from the modern contour values. The lowering of base level has resulted in a dramatic increase in incision upstream of the capture site by a factor of 4 to 20. This in turn has been associated with significant pre- and post-capture changes in valley shape. The increased incision resulted in dramatic post-capture increases in valley erosion upstream and downstream of the capture site by a factor of 2 to 9 which can be related to changes in associated stream power as a function of increased gradient and discharge. In excess of 60% of the landscape change can be accounted for by valley-constrained erosion as opposed to overall surface lowering.