Fan Complex Research Articles

A point‐sourced calciclastic submarine fan complex (Eocene Anotz Formation, western Pyrenees): facies architecture, evolution and controlling factors

AbstractA complete transect of a fossil carbonate ramp slope has been reconstructed using outcrop data from the Lower‐Middle Eocene Anotz Formation in the western Pyrenees. The Anotz Formation contains four calciclastic (mostly bioclastic) members encased within hemipelagic marl/limestone alternations, each calciclastic member representing a submarine fan system. Individual fans are composed of a gullied upper slope, a leveed feeder channel, a channelized lobe area, an unconfined lobe zone and a peripheral lobe fringe that grades downcurrent into basinal deposits. Quantitative data on the dimensions and degree of lateral continuity and vertical connectivity of the Anotz calciclastic fan elements are presented. These data provide information for a better understanding of the intrinsic nature of calciclastic submarine fans and to model their reservoir potential. The long‐term evolution of the Anotz carbonate slope was generally progradational, as evidenced by the four discrete episodes of calciclastic‐fan development. The location of the fans was controlled by the synsedimentary tectonic activity of the Pamplona fault, which created a slope valley along which the reworked shallow‐water calciclastic sediments were funnelled. In addition, episodic basinward tilting of the shallow‐water carbonate ramp, linked to the development of the South Pyrenean foreland basin, switched on and off the process of calciclastic resedimentation and determined the growth or abandonment of the fan systems.

AVA simultaneous inversion of partially stacked seismic amplitude data for the spatial delineation of lithology and fluid units of deepwater hydrocarbon reservoirs in the central Gulf of Mexico

This paper describes the successful application of amplitude-versus-angle (AVA) inversion of prestack-seismic amplitude data to detect and delineate deepwater hydrocarbon reservoirs in the central Gulf of Mexico. Detailed AVA fluid/lithology sensitivity analysis was conducted to assess the nature of AVA effects in the study area based on well-log data. Standard techniques such as crossplot analysis, Biot-Gassmann fluid substitution, AVA reflectivity modeling, and numerical simulation of synthetic gathers were part of the AVA sensitivity analysis. Crossplot and Biot-Gassmann analyses indicate significant sensitivity of acoustic properties to fluid substitution. AVA reflectivity and angle-gather modeling indicate that the shale/sand interfaces represented by the top and base of the M-10 reservoir are associated with typical Class III AVA responses caused by relatively low-impedance gas-bearing sands. Consequently, prestack seismic inversion provided accurate and reliable quantitative information about the spatial distribution of lithology and fluid units within the turbidite reservoirs based on the interpretation of fluid/lithology-sensitive modulus attributes. From the integration of inversion results with analogous depositional models, the M-series reservoirs were interpreted as stacked terminal turbidite lobes within an overall fan complex. This interpretation is consistent with previous regional stratigraphic/depositional studies.

Sedimentology of rift climax deep water systems; Lower Rudeis Formation, Hammam Faraun Fault Block, Suez Rift, Egypt

In most marine rift basins, subsidence outpaces sedimentation during rift climax times. Typically this results in sediment-starved hangingwall depocentres dominated by deep-marine mudstones, with subordinate local development of coarser clastics in the immediate hangingwall derived from restricted catchments on the immediate footwall scarp. To highlight the spatial variability of rift climax facies and the controls upon them, we have investigated the detailed three-dimensional geometry and facies relationships of the extremely well exposed Miocene, rift climax Lower Rudeis Formation in the immediate hangingwall to the Thal Fault Zone, Suez Rift, Egypt. Detailed sedimentological analyses allows the Lower Rudeis Formation to be divided into two contemporaneous depositional systems, (1) a laterally continuous slope system comprising, hangingwall restricted (< 250 m wide) slope apron, slope slumps, fault scarp degradation complex and laterally extensive lower slope-to-basinal siltstones, and (2) a localized submarine fan complex up to 1 km wide and extending at least 2 km basinward of the fault zone. Interpretation of individual facies, facies relationships and their spatial variability indicate that deposition in the immediate hangingwall to the Thal Fault occurred via a range of submarine concentrated density flows, surge-like turbidity flows, mass wasting and hemipelagic processes. Major controls on the spatial variability and stratigraphic architecture of the depositional systems identified reflect the influence of the steep footwall physiography, accommodation and drainage evolution associated with the growth of the Thal Fault. The under-filled nature of the hangingwall depocentre combined with the steep footwall gradient result in a steep fault-controlled basin margin characterised by either slope bypass or erosion, with limited coastal plain or shelf area. Sediment supply to the slope apron deposits is controlled in part by the evolution and size of small footwall drainage catchments. In contrast, the localized submarine fan is interpreted to have been fed by a larger, antecedent drainage network. The structural style of the immediate footwall is also believed to exert a control on facies development and stratigraphic evolution. In particular, fault scarp degradation is enhanced by fault propagation folding which creates basinward-dipping bedding planes in the pre-rift footwall strata that large pre-rift blocks slide on.

Comment on “Linking the Mesoproterozoic Belt-Purcell and Udzha basins across the west Laurentia–Siberia connection”

Abstract Paleogeographic reconstruction of the northern Siberian craton against southwestern Laurentia, in accordance with paleomagnetic data, basement piercing points, and the best fit of the rift margins, aligns Mesoproterozoic dike and sill swarms in northeast Siberia with correlative ones in Montana and Wyoming. It also juxtaposes the Mesoproterozoic Belt-Purcell basin of west Laurentia against the Mesoproterozoic Udzha basin of the northern Siberian craton. We review the veracity of this hypothetical Belt-Purcell–Udzha basin as a test of our Siberia–west Laurentia reconstruction. Various elements of the structural framework of the basins are closely aligned in the reconstruction. The Udzha trough is aligned with the St. Mary–Moyie fault zone and Vulcan basement structure of southwestern Canada. The Khastakh trough is aligned with the Helena embayment of central Montana. Parts of the Belt-Purcell Supergroup appear to correlate with parts of the Riphean section in northeastern Siberia. However, the Siberian section appears to be much thinner than the Belt-Purcell section, and precise correlation is not possible with present stratigraphic and geochronological data. The reconstruction leads to the prediction that the Udzha rift channeled sediment from a cratonic pediment into the delta and alluvial fan complex in the deep Belt-Purcell rift-basin. Distributary channels may have shifted within the Udzha basin to feed shifting depocenters in the Belt-Purcell basin. Details of age and tectonic evolution for the Udzha basin are less clear than for the Belt-Purcell basin, but we outline specific geological relationships predicted by the reconstruction model, and suggest tests for future research.

Plain Fan Complexes in Jurassic-Cretaceous Grabens of the Western Transbaikal Region

The paper deals with peculiarities of structure, composition, and formational environment of Jurassic-Lower Cretaceous coaliferous-terrigenous rocks in the Western Transbaikal region. These rocks are commonly considered intermontane molasses. It is proven that, in most cases, these rocks are fan complexes, which fill narrow troughs (grabens) against the background of a plain morphology rather than molasses (indicators of mountain landscape). Models of structure and sedimentation of fan complexes and the reasons favoring the origination of thick coal deposits in them are elaborated. Distinctions of fan complexes from the molasses are revealed. It is shown the discrimination of fan complexes is important for the reconstruction of paleogeographic and paleotectonic environments.

Vegetation ecology of Early Pennsylvanian alluvial fan and piedmont environments in southern New Brunswick, Canada

The vegetation ecology of Pennsylvanian upland/dryland regions is poorly known, despite its evolutionary significance. Here, fossil plant assemblages are described from well-drained alluvial fan/piedmont deposits in the uppermost Boss Point and Tynemouth Creek formations (late Yeadonian–Langsettian), southern New Brunswick. Beds record the northward building of a large alluvial fan complex over alluvial plain deposits in response to near-continuous sourceland uplift. Proximal alluvial fan environments, characterized by sheetfloods and braided streams, were dominated by large cordaitalean trees, medullosan pteridosperms, ferns, and calamiteans. Distal alluvial fan environments, where braided stream and leveé/splay sedimentation predominated, were covered by similar vegetation, together with lycopsids in localized poorly drained depressions. Calamitean thickets were particularly widespread in rapidly aggrading settings on the distal fan. Well-drained alluvial plains beyond the fan toe were characterized by axial braided rivers containing cordaitalean trunks transported from proximal settings. Piedmont vegetation is otherwise poorly resolved. All studied plant assemblages are of low- to medium-diversity, and dominated by the remains of a single group, cordaitalean seed plants. Such dominance-diversity characteristics, together with the presence of charcoal, the product of wildfire, imply that Pennsylvanian upland/dryland vegetation experienced water-stress and that the seed habit was integral to successful colonization.

Lithostratigraphy of the Mérida (Wisconsinan) glaciation and Pedregal interstade, Mérida Andes, northwestern Venezuela

At Pedregal, more than 40 m of sediments are exposed within a ‘fan complex’ formed between lateral moraines of the adjacent Mucuchache and El Caballo valleys. Early and late Mérida (Wisconsinan) glaciations are represented by till and till plus proglacial sediments, respectively. A middle Wisconsinan interstadial event, here termed the Pedregal interstade, began at the end of the Early Mérida glaciation at approximately 60 ka BP. Following the retreat of ice from the small Pedregal Basin, a lake formed when the local drainage was blocked due to movement of the Mesa de Caballo along the Boconó Fault. Shallow lake or no-lake phases lasted approximately a few hundred to, at most, 2000 years, and each lake phase was marked by peat accumulation. Four of seven peats identified formed during sufficiently long intervals for soil profiles (incipient to mature Spodosols) also to develop. The Spodosol with the strongest development (Eb/Bsb/Coxb/Cub horizons) is found adjacent to the lowest peat and reflects ongoing early Mérida stadial (MIS 3) conditions; the youngest peats, associated with weak podzolic soils (Eb/Bsb horizons), formed under slightly warmer interstadial conditions, presumably with less soil water. Cyclic lacustrine deposition is related to lake level and relative depth fluctuations, due in part to variable shoreline/delta progradation and shallowing as the lake deepened in general. Whereas final drainage of the lake is related to movement of the Boconó Fault and breach of the moraines that form the Mesa de Caballo, earlier lake level fluctuations appear related to climate change. Radiocarbon dating of the peats suggests they are related to warmer periods and may tentatively correlate with small ‘interstadials’ or ‘D-O events’ detected in the oxygen-isotope record of Greenland ice cores and North Atlantic marine sediments.

Proterozoic sedimentation and volcanism in the Singhbhum crustal province, India and their implications

The Proterozoic volcano-sedimentary succession comprising successively younger Dhanjori, Chaibasa, Dhalbhum, Dalma and Chandil Formations of the Singhbhum crustal province, India records sedimentation and volcanism in a rapidly changing tectonic scenario. Cooling of the vast volume of Archaean Singhbhum granite possibly induced an isostatic readjustment. The associated tensional regime and deep-seated fractures controlled the formation of the Proterozoic Singhbhum basin. The Dhanjori Formation unconformably overlies the Singhbhum granite and is entirely terrestrial, dominantly fluvial. At the base, the conglomerate deposits, coarse-grained sandstone, and shale represent the distal fringe of an alluvial fan complex. The rest of the formation including the volcaniclastic rocks consists almost entirely of fining-upward fluvial cycles. The base of the Dhanjori Formation is a sequence boundary and the formation itself represents a lowstand systems tract. The Dhanjori volcano-sedimentary succession displays evidence of having passed through passive, as well as, active phases of continental rifting with an increasingly important influence of volcanism on the sedimentation through time. The Chaibasa Formation sharply overlies the Dhanjori Formation and a transgressive lag demarcates their contact. The basal part of the Chaibasa Formation immediately overlying the transgressive lag deposit represents a transgressive systems tract while offshore shales that sharply overlie the shallow marine sandstones up-section represent marine flooding surfaces. The shallow marine subtidal sandstones bear an excellent record of sandwave migration and provide a rare opportunity to unlock the Late Palaeoproterozoic lunar orbital periodicities. Unlike the Chaibasa, the overlying Dhalbhum Formation is entirely terrestrial (fluvial-aeolian) indicating that the Chaibasa-Dhalbhum contact is a sequence boundary. The Dalmas, overlying the Dhalbhum Formation, represents concordant lava outpourings without any break in sedimentation; these lavas are genetically related to mantle plume upwelling in an intracontinental rift setting. The volcano-sedimentary package lying north of the Dalma volcanic belt (Chandil Formation) is of Mesoproterozoic age. The entire Late Palaeoproterozoic volcano-sedimentary package displays post-depositional compressional deformation and greenschist to amphibolite facies metamorphism dated at ca. 1600 Ma, forming the so-called North Singhbhum fold belt. The volcano-sedimentary package lying south of the Dalma volcanic belt was pushed further south towards the Singhbhum granite batholith complex as a result of uplift related to the Dalma plume. The Singhbhum granite batholith acted as a rigid body. This gave rise to a compressional stress regime that induced shearing/thrusting at ca. 1600 Ma along the Singhbhum Shear Zone. The Dalma plume magmatism was possibly part of a ∼1600 Ma global tectono-thermal event.

Morphology and slip rate of the Hurunui section of the Hope Fault, South Island, New Zealand

The Hurunui section of the Hope Fault is a newly defined, 42 km long geomorphic fault section which extends from Harper Pass to the Hope‐Boyle River confluence. Reconnaissance mapping along the Hurunui section from Hope Shelter to Harper Pass provided new data on its location, geomorphology, displacement, and slip rate. More than 200 previously published field observations of dextrally and vertically displaced landforms along the fault provide data on the distribution of displacement along the fault trace. Five radiocarbon dates found in association with offset geomorphic features are presented and two new measures of dextral slip rate are calculated. At McKenzie Stream, a late Holocene fan complex is cut by the Hope Fault. Young abandoned and active channels on this surface show dextral offsets of up to 22 ± 2 m along a south‐facing scarp with a height of up to 5 m. Woody litter from a unit in this complex has yielded a radiocarbon age of 2331 ±55 yr BP and a corresponding minimum horizontal slip rate of 8.1–11.0 mm/yr. At Macs Knob, large dextral deflections of stream catchments are linked to episodes of glacial resetting of the landscape. Correlation of the offset of “Macs stream” (166 ± 17 m) with a post‐Aranuian age peat (10 782 ± 60 yr BP) yields a maximum horizontal slip rate of 13.0 ± 1.5 mm/yr. The single‐event dextral displacement, based on offset stream channels at McKenzie fan, is 3.2–3.8 m (av. c. 3.4 m). The ratio of dextral to vertical slip is c. 7 ± 2:1, indicating that the Hope Fault has a dominantly strike‐slip sense of motion. The average recurrence interval for the last 5–7 events (i.e., to produce 19–24 m slip at McKenzie fan) is 310–490 yr. The age of the most recent surface‐rupturing earthquake at this site is not known, though felt effects, fault scaling, and landscape arguments indicate it was not the AD 1888 North Canterbury earthquake.

Recent Deformation along the Offshore Malibu Coast, Dume, and Related Faults West of Point Dume, Southern California

Offshore faults west of Point Dume, southern California, are part of an important regional fault system that extends for about 200 km, from near the city of Los Angeles westward along the south flank of the Santa Monica Mountains and through the northern Channel Islands. This boundary fault system separates the western Transverse Ranges, on the north, from the California Continental Borderland, on the south. Previous research showed that the fault system includes many active fault strands; consequently, the entire system is considered a serious potential earthquake hazard to nearby Los Angeles. We present an integrated analysis of multichannel seismic- and high-resolution seismic-reflection data and multibeam-bathymetric information to focus on the central part of the fault system that lies west of Point Dume. We show that some of the main offshore faults have cumulative displacements of 3–5 km, and many faults are currently active because they deform the seafloor or very shallow sediment layers. The main offshore fault is the Dume fault, a large north-dipping reverse fault. In the eastern part of the study area, this fault offsets the seafloor, showing Holocene displacement. Onshore, the Malibu Coast fault dips steeply north, is active, and shows left-oblique slip. The probable offshore extension of this fault is a large fault that dips steeply in its upper part but flattens at depth. High-resolution seismic data show that this fault deforms shallow sediment making up the Hueneme fan complex, indicating Holocene activity. A structure near Sycamore knoll strikes transversely to the main faults and could be important to the analysis of the regional earthquake hazard because the structure might form a boundary between earthquake-rupture segments.

Holocene and Latest Pleistocene Oblique Dextral Faulting on the Southern Inyo Mountains Fault, Owens Lake Basin, California

The Inyo Mountains fault (imf) is a more or less continuous range-front fault system, with discontinuous late Quaternary activity, at the western base of the Inyo Mountains in Owens Valley, California. The southern section of the imf trends ∼N20°–40° W for at least 12 km at the base of and within the range front near Keeler in Owens Lake basin. The southern imf cuts across a relict early Pliocene alluvial fan complex, which has formed shutter ridges and northeast-facing scarps, and which has dextrally offset, well-developed drainages indicating long-term activity. Numerous fault scarps along the mapped trace are northeast-facing, mountain-side down, and developed in both bedrock and younger alluvium, indicating latest Quaternary activity. Latest Quaternary multiple- and single-event scarps that cut alluvium range in height from 0.5 to 3.0 m. The penultimate event on the southern imf is bracketed between 13,310 and 10,590 cal years b.p., based on radiocarbon dates from faulted alluvium and fissure-fill stratigraphy exposed in a natural wash cut. Evidence of the most recent event is found at many sites along the mapped fault, and, in particular, is seen in an ∼0.5-m northeast-facing scarp and several right-stepping en echelon ∼0.5-m-deep depressions that pond fine sediment on a younger than 13,310 cal years b.p. alluvial fan. A channel that crosses transverse to this scarp is dextrally offset 2.3 ± 0.8 m, providing a poorly constrained oblique slip rate of 0.1–0.3 m/k.y. The identified tectonic geomorphology and sense of displacement demonstrate that the southern imf accommodates predominately dextral slip and should be integrated into kinematic fault models of strain distribution in Owens Valley.

Hydraulic Properties of a Desert Soil Chronosequence in the Mojave Desert, USA

Desert pavements are prominent features in arid environments and consist of a surface layer of closely packed gravel that overlies a thin, gravel‐poor, vesicular A (Av) soil horizon. Well‐developed Av horizons form distinct and highly structured columnar peds. These structures, along with their silt‐ and clay‐rich texture, are hypothesized as controlling infiltration and hence the overall hydrologic conditions in the soil profile. The objectives of this study were to (i) evaluate how pedological development in near‐surface soil horizons in an arid alluvial fan complex affects the soil hydraulic characteristics, and (ii) to compare the use of Wooding's equation and inverse modeling for evaluating hydraulic conductivity in highly layered, near‐surface soils. These objectives were approached through field tension infiltrometer studies, soil sampling, and laboratory analyses of soil texture, water content, and soluble salt concentrations. Soils at five sites were studied at the Mojave National Preserve, California, representing a soil chronosequence (50–100000 yr) with varying degrees of desert pavement development. Results indicated 100‐fold and threefold declines, respectively, in saturated hydraulic conductivity (Ks) with both analytical methods, and αw using Wooding's method, as the soils aged. No clear trends in Ks or αw were detected in the underlying horizon, indicating that the controlling feature at these sites, in terms of water entry, was the Ks of the surface (Av) horizon. Soluble salt concentrations within the profile indicated reduced infiltration with increased pavement development. Results showed that surface age can be used as an excellent predictor of saturated hydraulic conductivity (r2 = 0.9254). Further, results suggest that Av horizon development represents a key process controlling water cycling, potentially influencing ecosystem function in arid lands.

Hydraulic Properties of a Desert Soil Chronosequence in the Mojave Desert, USA

Desert pavements are prominent features in arid environments and consist of a surface layer of closely packed gravel that overlies a thin, gravel-poor, vesicular A (Av) soil horizon. Well-developed Av horizons form distinct and highly structured columnar peds. These structures, along with their silt- and clay-rich texture, are hypothesized as controlling infiltration and hence the overall hydrologic conditions in the soil profile. The objectives of this study were to (i) evaluate how pedological development in near-surface soil horizons in an arid alluvial fan complex affects the soil hydraulic characteristics, and (ii) to compare the use of Wooding's equation and inverse modeling for evaluating hydraulic conductivity in highly layered, near-surface soils. These objectives were approached through field tension infiltrometer studies, soil sampling, and laboratory analyses of soil texture, water content, and soluble salt concentrations. Soils at five sites were studied at the Mojave National Preserve, California, representing a soil chronosequence (50–100000 yr) with varying degrees of desert pavement development. Results indicated 100-fold and threefold declines, respectively, in saturated hydraulic conductivity (Ks) with both analytical methods, and αw using Wooding's method, as the soils aged. No clear trends in Ks or αw were detected in the underlying horizon, indicating that the controlling feature at these sites, in terms of water entry, was the Ks of the surface (Av) horizon. Soluble salt concentrations within the profile indicated reduced infiltration with increased pavement development. Results showed that surface age can be used as an excellent predictor of saturated hydraulic conductivity (r2 = 0.9254). Further, results suggest that Av horizon development represents a key process controlling water cycling, potentially influencing ecosystem function in arid lands.

Planktonic foraminiferal biostratigraphy of the Neogene sequence in the Adana Basin, Turkey, and its correlation with standard biozones

The aim of this study is to describe the biostratigraphic, chronostratigraphic and ecostratigraphic relationships of the Neogene sequence in the Adana Basin. The Adana Basin is located in southern Turkey, and bordered by the Tauride Orogenic Belt to the north, the Amanos Mountains to the east, the Mediterranean coast to the south and the Ecemiş Fault Zone to the west. From base to top, the Neogene sequence consists of the Gildirli Formation (continental redbeds), the shallow marine Kaplankaya Formation, the reefal limestones of the Karaisalı Formation, the shales of the Güvenç Formation (slope to deep marine), a thick submarine fan complex (Cingöz Formation), the shallow marine and fluvio-deltaic Kuzgun Formation and the shallow marine lagoonal–continental Handere Formation. The planktonic foraminiferal biozones identified within the Neogene sequence of the Adana Basin are Globigerinoides trilobus and Praeorbulina glomerosa curva (Burdigalian), Globorotalia fohsi peripheroronda/Orbulina suturalis (Langhian), and Globorotalia mayeri (Serravallian). The Late Tortonian is characterized by the first occurrence of Globorotalia suterae. There are no planktonic foraminiferal zones in the Messinian, but this level may be correlated with a non-distinctive zone in the Mediterranean region. The Pliocene is represented by the Sphaeroidinellopsis Acme Zone.

Sedimentation history of the Palaeoproterozoic Dhanjori Formation, Singhbhum, eastern India

Abstract The Palaeoproterozoic Dhanjori Formation, eastern India is a terrestrial (dominantly fluvial) volcano-sedimentary succession. Basal conglomerate, coarse-grained sandstone, and shale rests directly on the granite basement and represents the distal fringe of an alluvial fan complex. Sieve, sheet flood, mass flow and alluvial channel and overbank deposits comprise the fan segment. The rest of the formation is entirely constituted by fining upward fluvial cycles. An event of basin tilting and volcanic eruption intervened and resulted a second phase of Dhanjori sedimentation, although the general fluvial depositional framework remained unaltered. The two members of the Dhanjori Formation display different paleocurrent trends related to fluvial response to basin tilting. In the lower member spatial variability in paleocurrent directions occurs as a consequence of fan development. Unlike the first phase, the second phase of fluvial deposition incorporates profuse volcanics and accompanying volcaniclastic deposits. These volcanic and volcaniclastic rocks might have locally blocked the river courses resulting in short-lived lacustrine deposition. Composition of the volcanics varies upward from ultramafic to mafic. The Dhanjori volcanism took place in an intracontinental rift setting as is evident from the interbedded terrestrial deposits. Interbedded volcanic and volcaniclastic rocks in different stratigraphic levels suggest episodic volcanic eruption. A semiarid paleoclimate is proposed and is consistent with development of an alluvial fan comprised of coarse clastic deposits.

Development of tumuli in the medial portion of the 1983 aa flow-field, Mount Etna, Sicily

A number of tumuli formed on the aa-dominated lava fan complex which developed in the medial zone of the 1983 flow-field of Mount Etna during the later stages of the eruption. This complex flow-field formed on shallow sloping ground below a scarp between 1900 and 1700 m asl. A major tube system fed a branching tube network in the fan complex. Numerous tumuli and break-outs of lava formed in the fan. Three main types of tumulus are identified: (1) Focal tumuli, which are formed from the break-up and uplift of ‘old’, thick lava crust and themselves become sustained sites for the distribution of lava both as flows and within distributary tubes. These focal tumuli are significant centres associated with major tubes. (2) Satellite tumuli, which are typically elongate, whale-back shaped features that branch out from focal tumuli. These satellite tumuli were initially lava flows erupted from a focal tumulus. The crust of the flow slowed or came to a halt and the rigid crust became uplifted and fractured, forming a dome-shaped ridge feature. These satellite tumuli continued to be fed from the focal tumulus and became sites of lava emission with numerous break-outs. (3) Distributary tumuli formed on the fan associated with short-lived break-outs from tubes and are relatively simple structures formed from limited effusion of toey lobes and pahoehoe lava. The major tumuli on the fan complex show distinct dilation fractures. The fracture surfaces provide good exposure of the crust and three distinct zones are recognised – an upper zone showing columnar jointing, a middle zone consisting of planar fracture surfaces and a basal zone with distinctive banded planar fracture surfaces showing evidence of both brittle and ductile formation. Using these data a model is proposed for tumulus growth. Field analysis of the fan complex shows how it was fed by a branching tube system, leading to flow thickening, formation of tumuli and numerous ephemeral boccas.

Three-dimensional geological models from outcrop data using digital data collection techniques: an example from the Tanqua Karoo depocentre, South Africa

Abstract Recent technological advances have made the collection of digital geological data from outcrops a realistic and efficient proposition. The world-class exposures of Permian basin-floor turbidite fans of the Tanqua depocentre, Karoo Basin, South Africa have been the focus of one such study. These outcrops are faulted at a subseismic scale (displacements of up to 40 m), with continuous exposures of up to 40 km in depositional dip and 20 km strike directions. Digital data collection has been undertaken using a variety of methods: differential global-positioning systems (DGPS) mapping, surveying using laser total station and laser rangefinders, ground- and helicopter-based digital photography and photogrammetry, and digital sedimentary outcrop logging as well as geophysical data from boreholes. These data have then been integrated into several 3-D geological models of the study area, built using a subsurface reservoir-modelling system. The integrated dataset provides insights into the stratigraphic evolution of a deep-water fan complex by allowing true 3-D analysis and interpretation of data collected in the field. The improved understanding of these deep-water fan systems will improve existing models of offshore analogues by enhancing understanding of geometries and trends not resolvable from existing offshore data and by identifying potential problematic areas for fluid flow. Initial results from the application of this approach have been successfully applied to the conditioning of stochastic geological models of a subsurface deep-water reservoir from the North Sea.

Large‐scale glaciotectonic deformation in the Great Lakes basin, USA‐Canada

Investigation of the sedimentology, stratigraphy and deformation structures of an exposed sedimentary sequence associated with an isolated large ridge along the eastern shore of Lake Michigan reveals that it is part of a broad grounding‐line fan complex that was subjected to extensive glaciotectonic deformation. It is concluded that the sequence formed following melting back of the margin of the Lake Michigan lobe and was subsequently overridden and deformed during advance of the ice margin probably to the Port Huron moraine. The lack of reported major glaciotectonic structures in the Lake Michigan basin compared to that in the North Sea and Baltic Sea basins is most likely due to paucity of soft and incompetent bedrock and/or lack of continuous permafrost conditions during deglaciation.

Evidence of iceberg‐ploughing in a subaqueous ice‐contact fan, glacial Lake Rinteln, NW Germany

The Coppenbrügge subaqueous ice‐contact fan complex of early Saalian age is about 10 km long and up to 10 km wide and is composed of offset‐stacked fan clinothems that are transgressive‐regressive sequences formed during an overall lake level rise. The individual fan bodies consist of coarse gravel in the ice‐proximal part, passing distally into sandy facies and showing large‐scale foreset bedding. The iceberg scour recognized in an open‐pit outcrop is up to 1.5m deep, up to 2m wide and cut in undisturbed mid‐fan deposits. The scour‐fill can be traced laterally for about 15m and consists of sheared sand and, in the frontal zone, of downbent overlying strata surrounded by a zone of deformed sediments. The deformed sediment produced by the iceberg keel's shearing of the trough walls is a sand mass containing angular soft‐sediment clasts that show internal folds and fractures. The basal surface of the deformed sediment is a nearly horizontal shear plane, steepening up laterally as a discrete thrust and showing a flat‐ramp‐flat geometry. The scour was formed by the iceberg keel's ploughing the substrate and pushing the sediment sideways and frontally, forming a ridge of deformed sediments at the trough end. This ridge was concurrently eroded by an accompanying meltwater underflow which apparently developed a horseshoe system of scouring vortices around the grounded iceberg. The current's scour was filled with massive, non‐stratified sand deposited rapidly from turbulent suspension. The iceberg eventually broke up and its keel part was buried. As these ice fragments gradually melted, the space was closed by normal faulting and downbending of overlying strata. The collapsing scour‐fill became partly liquified, and the resulting water‐escape structures cut the normal faults and the overlying deposits. Though produced chiefly by tangential shear strain, iceberg‐ploughing features are readily distinguishable from other glaciotectonic deformations. They can serve as a diagnostic criterion for glaciolacustrine or glaciomarine environments and the distinguishing of ice‐contact subaqueous fans from ice‐contact deltas in the stratigraphic record.

The impacts on groundwater development on shallow aquifers in the lower Okavango Delta, northwestern Botswana

The Okavango Delta is a large alluvial fan complex in the northwest corner of Botswana. Both surface and groundwater in this area are important to wildlife, flora, and human settlements. A groundwater exploration study covering an area of over 14,000 km2 was undertaken to assess the potential of groundwater to supply a major town and associated settlements. The primary control on groundwater development potential is water quality, with much of the area characterised by brackish/saline aquifers. This largely reflects the high rate of potential evapotranspiration. As a consequence, freshwater aquifers are strongly dependent on recharge from surface water during annual flooding. In the absence of river flow, water quality in the fresh aquifers begins to deteriorate and the thickness of the fresh water lens reduces through upward migration from underlying saline aquifers. Lateral inflow from surrounding saline aquifers does not appear to be a significant. The impacts of evapotranspiration indicate the importance of regular surface water recharge to sustainable development of groundwater resources. Groundwater development strategies are changing as a result of the understanding of salinification processes in these aquifers.