Fan Conglomerates Research Articles

Tertiary alluvial fans at the northern margin of the Ebro Basin: a review

Abstract Tertiary alluvial fan deposits along the margin of the Ebro foreland basin north of Huesca, Spain, are remarkable for the range of sedimentological and tectonic features preserved within them. The fan deposits formed after the southern Pyrenean thrust front (the Guarga Thrust) was established in this area, forming steep topography at the basin margin in the mid-Oligocene. Some shortening continued during deposition of the earliest fans resulting in synsedimentary faults, folds and unconformities. Clast compositions in the fan conglomerate beds record unroofing of the thrust front and also reveal differences in bedrock provenance between adjacent, coeval fan deposits. Bedrock provenance also influenced processes of deposition, with fans built up of detritus derived from areas rich in gypsum and mudrock showing more evidence of debris-flow processes. Deposition by debris flows also dominated the smallest fan body, but the majority of the fans were the products of sedimentation from unconfined or poorly confined traction currents. These resulted in sheets of conglomerate in the more proximal areas. Within individual fan bodies the proportion of sandstone increases over a distance of up to 5 km to where the distal fan facies are seen as thin sandstone and mudstone beds. The fringes of the fan bodies interfinger with lacustrine and fluvial facies, which indicate a temperate-semi-aric palaeoclimate. Vertical aggradation of the fan deposits due to rising base level in the Ebro Basin in the Oligocene and early Miocene was followed by deep incision following a late Miocene base level-fall. This led to the partial erosion of the fan deposits and their spectacular exposure in the modern landscape.

Sedimentary documentation of the break-up of Rodinia, Offerdal Nappe, Swedish Caledonides

Late Riphean to early Vendian metasedimentary rocks of the Offerdal Nappe, central Scandinavian Caledonides, were studied by field mapping and measured sections. Even though the rocks have been metamorphosed and tectonically deformed, preserved sedimentary structures and outcrop exposure permit the interpretation of a little-known area with respect to the break-up of Rodinia. This work shows that although the primary sedimentary textures are strongly altered during metamorphism the primary sedimentary structures and bed geometries are commonly preserved. Moreover, the sedimentary structures, where identified, can be used to hydrodynamically estimate the primary grain-size of the metasedimentary rocks. Seven sedimentary facies were identified in three tectonostratigraphic units. The lower tectonostratigraphic unit consists of polymict conglomerates deposited in alluvial fans. The lower part of the middle tectonostratigraphic unit consists of channelled turbidite deposits. The upper part of the middle tectonostratigraphic unit consists of a regressive to transgressive succession with southeast to northwest transition from fluvial deposits, to storm-dominated deposits and into offshore mudstones. The lower part of the upper tectonostratigraphic unit consists of a regressive succession with southeast to northwest transition from channelled turbidites into sheet-like turbidites. The upper part consists of fluvial deposits in updip (southeast) areas, whereas in downdip (northwest) areas the sheet-like turbidites reflect continued deepwater deposition. (1) The ∼1500 m thickness of the Offerdal succession, (2) the occurrence of >300 m thickness of the pebble- to boulder-bearing continental-basement derived alluvial fan conglomerates, (3) the high sediment input rates, and (4) the high subsidence rates suggest an actively subsiding basin and a relatively uplifted hinterland. Furthermore, the arkosic composition of the Offerdal metasandstones, as well as the Precambrian porphyry, syenite and quartzite clast composition of the lithic conglomerates, recognised from the subjacent continental basement of Baltica, suggest a continental sediment source. The rapid accommodation changes, and especially the simultaneous subsidence of downdip areas and uplift of updip areas in UTU2 suggest syn-depositional extensional block rotation. Collectively, this evidence suggests that the Offerdal succession was deposited in a continental rift basin related to break-up of Rodinia.

Clastic sedimentation in a late Archaean accretionary terrain, Midlands greenstone belt, Zimbabwe

Abstract Late Archaean (∼2.68 Ga) clastic sedimentary rocks of the Shamvaian Group form a shear zone-bounded lithotectonic unit in the Midlands greenstone belt (MGB) of Zimbabwe. The sedimentary rocks are wedged in between mid-Archaean granitoid gneisses and late Archaean mafic and bimodal volcanic rocks. The ≥2000-m thick sedimentary sequence consists of marine turbidite deposits overlain by high-energy shelf sandstone, fluvial braid-plain pebbly sandstone and alluvial fan conglomerate. Deposition took place in a northward-deepening basin with fluvial systems draining a source dominated by granitoid gneiss and banded chert. Basin formation was associated with a west-directed thrusting event, and the basin fill was incorporated into the thrust stack soon after deposition. The Shamvaian Group is similar to modern foreland and forearc successions and possibly accumulated during collision of a continental fragment with an island arc-like complex.

Late Archaean clastic sedimentary rocks (Shamvaian Group) of the Zimbabwe craton: first observations from the Bindura-Shamva greenstone belt

The ~2.65 Ga old Shamvaian Group sedimentary rocks occur as a folded succession in the central part of the Bindura–Shamva greenstone belt of Zimbabwe. The strata comprise distinct, shear zone-bounded tectonostratigraphic units which may be stratigraphically arranged as follows. The lower part of the succession is represented by a transgressive, fining-upward sequence of alluvial fan conglomerate, overlain by fluvial braid-plain pebbly sandstone and marine shoreface sandstone. Detritus was derived from a mid-Archaean granitoid-gneiss terrain situated to the east. Sediment supply and subsidence rate must have been high. Shallow shelf sedimentation was followed by deep-water (sub-wave base) deposition by turbidity currents, giving rise to a thick succession of fine to coarse clastic material. The turbidite deposits were locally overlain by shallow-marine sandstone and fluvial to alluvial fan conglomerate. An upward increase in the abundance of intermediate and felsic volcanic clasts suggests an increase in the proximity of a volcanic terrain, such as a volcanic arc. Deposition was followed by layer-parallel shearing during thrust belt-style tectonism. Major shear zones developed preferentially along the contact between shallow- and deep-marine facies associations. Basin initiation may have been related to extensional tectonics, possibly on rifted continental crust, whereas later stages of basin history were characterized by compression, suggesting a foreland or fore-arc basin setting. Sedimentary facies, stratigraphy, and facies distribution are remarkably similar to some late Archaean sedimentary sequences of the Superior Province in Canada.

Growth and linkage of the East Tanka fault zone, Suez rift: structural style and syn-rift stratigraphic response

Abstract: An integrated tectono-stratigraphic analysis of the East Tanka fault zone, Suez rift, indicates fault growth by linkage of initially isolated fault segments that is consistent with fault growth models based on displacement–length (D-L) scaling laws. During the initial 2.4 Ma of rifting, the East Tanka fault zone was composed of two en-echelon fault segments c. 1–1.5 km long, separated by a hanging-wall intrabasin high that controlled the geometry of depocentres filled with continental deposits. Alluvial fan conglomerates were fed through the region between the two fault segments, and form a discrete coarse-grained body, preserved in the immediate hanging wall of the fault zone. Subsequent stratigraphic patterns indicate that the two faults hard-linked to form a single fault zone c. 3.5 km long. Hard linkage of the segments resulted in migration of the zone of maximum displacement and subsidence into the zone of linkage. Uplift due to the migration of activity caused modification of drainage in the footwall of the fault zone that terminated the growth of the alluvial fan. This study demonstrates the need to integrate structural and stratigraphic data when attempting to reconstruct the temporal and spatial evolution of normal fault zones. Additionally, the fault dynamics illustrated have implications for tectono-stratigraphic models of rift basins, and syn-rift stratigraphic evolution.

Rb-Sr chronological characteristics of Jurassic granitoid gravels in north margin of Dabie block, Central China

Granitoid gravels are discovered from alluvial fan conglomerates, the base of Jurassic Fenghuangtai Formation (J2 2), in the north margin of Dabie block and the south part of the Hefei Basin. These granitoid gravels make up 5%–10% of all gravels by volume percent. For the four components, K-feldspar, biotite, amphibole and whole rock divided from two selected granitoid gravel samples, their Rb-Sr isotopes are measured, which gives two formation ages of granitoids: (428±20) Ma and (430±54) Ma, and means that the Early Paleozoic granitoid intrusive rocks once existed in the Dabie Mountain provenances of Jurassic. The granitoids occur in the tectonic setting of post-collision uplift according to lithogeochemical discriminant analysis.

Stratigraphic interplays between igneous and sedimentary events in the early Palaeozoic Jaibaras trough (northeast Brazil)

The Jaibaras Trough is an example of an intraplate province for which both origin of magmatic manifestations and basin generation mechanisms are coeval. The magmatic suites demonstrate closed spatial and temporal relationships and, in turn, appear to be genetically associated. The simple distribution of igneous bodies occurrence constrains the spatial relationship between the whole magmatic activity during the basin infill. Sequential stratigraphic interplays between sediments and magmatic activities are perfectly constrained through thermal-structural features such as contact aureoles, faulted contacts/abutting and cross-cutting. The basin infill congregates continental immature siliciclastic sedimentary deposits. Deposition took place in areas of instable young shoulder relief and problably totals up to 3 km in thickness. Marked variations in thickness, rapid changes in depositional facies, interbedded thick conglomerates, local unconformities and disconformities of variable extent typify the overall sedimentary deposits making a regional palaeogeographic reconstruction difficult. Probably axial basin filling provided the sedimentation of shale and sandstone, whereas the rift flanks laterally sustained the alluvial fan conglomerates. The magmatic evolution of Jaibaras Trough comprises basically four temporal and spatially separated phases. The early phase involved the Vendian Coreau Dike Swarm, and represents the initial tectonic pulse of rift opening. With continued breakup, reactivation of deeper shear zones gave rise to the emplacement of the Mucambo Pluton during Early Cambrian times, which preceeded the main rift infill. The basin sedimentation was accompanied by a huge volume of volcanism, most of which is composed by flood basalts, dikes and sills. The Meruoca Pluton is the last basin-related igneous manifestation during the Upper Cambrian

STRATIGRAPHIC INTERPLAYS BETWEEN IGNEOUS AND SEDIMENTARY EVENTS IN THE EARLY PALAEOZOIC JAIBARAS TROUGH (NORTHEAST BRAZIL)

The Jaibaras Trough is an example of an intraplate province for which both origin of magmatic manifestations and basin generation mechanisms are coeval. The magmatic suites demonstrate closed spatial and temporal relationships and, in turn, appear to be genetically associated. The simple distribution of igneous bodies occurrence constrains the spatial relationship between the whole magmatic activity during the basin infill. Sequential stratigraphic interplays between sediments and magmatic activities are perfectly constrained through thermal-structural features such as contact aureoles, faulted contacts/abutting and cross-cutting. The basin infill congregates continental immature siliciclastic sedimentary deposits. Deposition took place in areas of instable young shoulder relief and problably totals up to 3 km in thickness. Marked variations in thickness, rapid changes in depositional facies, interbedded thick conglomerates, local unconformities and disconformities of variable extent typify the overall sedimentary deposits making a regional palaeogeographic reconstruction difficult. Probably axial basin filling provided the sedimentation of shale and sandstone, whereas the rift flanks laterally sustained the alluvial fan conglomerates. The magmatic evolution of Jaibaras Trough comprises basically four temporal and spatially separated phases. The early phase involved the Vendian Coreau Dike Swarm, and represents the initial tectonic pulse of rift opening. With continued breakup, reactivation of deeper shear zones gave rise to the emplacement of the Mucambo Pluton during Early Cambrian times, which preceeded the main rift infill. The basin sedimentation was accompanied by a huge volume of volcanism, most of which is composed by flood basalts, dikes and sills. The Meruoca Pluton is the last basin-related igneous manifestation during the Upper Cambrian

Esk Trough—Yarraman Block contact, an unconformity: Its nature and implications for regional tectonics

Detailed field study in southeast Queensland has resulted in the interpretation of an unconformity at the base of the Esk Trough sequence at its contact with the Yarraman Block (Maronghi Creek beds and associated intrusions). Previously this contact had been considered to be faulted. The nature of the unconformity is very variable with the Esk Formation resting on freshly eroded surfaces, on mature palaeosols and on an immature palaeosol. Immediately above the unconformity, the Esk Formation variably comprises scree breccia, fluvial conglomerate and arenite, and alluvial fan conglomerate and arenite. North‐northwest‐south‐southeast‐striking faults are associated with the unconformity. Where the unconformity parallels these faults, it retains a relatively constant character, but where it is cut by these faults, it shows greater variability, a relationship interpreted to result from contemporaneous tectonism. The Glen Howden Fault extends into structurally disturbed areas previously described as ‘fractured anticlines’ and ‘complex anticlines’, which are here interpreted as flower structures and associated features. The south‐southeast extension of the Glen Howden Fault strikes obliquely across the Esk Trough to finally pass into the South Moreton Anticline previously interpreted as a positive flower structure, and resolves structural and stratigraphic observations that previously appeared anomalous. Inferred strike‐slip movement in the Esk Trough resulted from Early to Middle Triassic north‐northwest‐south‐southeast oblique transtension followed by Late Triassic transpression, and similar tectonism probably affected adjacent portions of the Yarraman Block.

Stratigraphy of the Kuwait Group in Umm -Gudair and Surrounding areas, Kuwait

Stratigraphy of the Miocene-Pleistocene Kuwait Group is studied on the basis of core samples and bore-hole logs from Umm-Gudair, NW Al-Shagaya, Al-Salmi, Al-Wafra water wells, exposures at Jal Az-Zor escarpment and Ahmadi quarry. The over all stratigraphie sequence of the Kuwait Group bear clues for a gradual regional regression and accompanying increase in clastic material influx. The Kuwait Group starts with marine clays and continues upward as fluvial, river channel sands, and ends with alluvial fan conglomerates. Deposition of the Kuwait group was interpreted frequently by brief periods of quiescence and non-deposition. Pauses in sedimentation are marked by surfaces of carbonate and gypsum cementation in the pore spaces of sands by evaporation of capillary water, and alteration of clays. The lower parts of the Kuwait Group consist mostly of loose sands without cementation. The degree of cementation and frequency of cemented layers increase towards the top of the group. The Kuwait Group is subdivided into three formations in the pre¬ sent study from bottom to top. These are Ghar, Jal Az-Zor and Dibdibba Formations. The fossiliferous sandy limestone layer at the lower part of the group was selected as the boundary between Ghar and the overlying Jal Az-Zor Formations. The boundary between the Jal Az-Zor and Dibdibba Formations is hard to locate. The carbonate-cemented sand layers are included in the Jal Az-Zor, and overlying poorly-bedded and weakly cemented gravels are included in the Dibdibba Formation. Subsurface correlation suggests that the thickness of Ghar and Jal Az-Zor Formations are uniform, but Dibdibba Formation varies in thickness from north to south.

The stratigraphy of the Middle Eocene–Pleistocene sediments in Kuwait

The Dammam Formation and the Kuwait Group are the major aquifers containing useable brackish water in Kuwait. The Dammam Formation is a limestone–dolomite sequence of Middle Eocene age. It is underlain by Middle Eocene Rus evaporites and is overlain unconformably by the clastic sediments of the Kuwait Group. It is subdivided into three members. The lower member, A, is a nummulitic limestone with shale interlayers at its base. The middle member, B, is a highly silicified and dolomitized fossiliferous limestone which includes lignite and silty lignite interlayers. The upper member, C, is a friable, white dolomite which is silicified and karstified at the top, beneath the unconformity with the overlying Kuwait Group. Similar silicification and karstification occurs at the top of the B member, suggesting a minor but significant unconformity between the B and C members. The upper part of the Dammam Formation is dolomite of Lutetian age with fragmented and cherty zone on top. Secondary dolomite was formed from early dolomitization of fossiliferous micrites, pelmicites, intramicrites and fossiliferous pelmicrites. Silification was related to percolation of acidic ground-water during the non-depositional period in Oligocene time when the surface of the Dammam Formation was karstified.The Dammam Formation was deposited on a shallow marine shelf experiencing minor fluctuations from lagoon to tidal flat and swamp environments. This tectonically stable period was interrupted by small pulses in the source land and minor fluctuations in the sea level, which caused alternating transgressive and regressive cycles.Stratigraphy of Miocene–Pleistocene Kuwait Group is studied. The work is based on cores and bore hole logs from Umm-Gudair, NW Al-Shagaya, Al-Salmi and Al-Wafra water wells, and exposures at Jal Az-Zor escarpment and Ahmadi quarry.The Kuwait Group was subdivided into two or three formations. There is no general agreement on the boundaries of the formations and the number of formations. The confusion has persisted until present mainly because there is no clear cut criteria to delineate formations. There are no markers or distinct lithologic boundaries to set formations apart. The Kuwait Group is subdivided into three formations in the present study in agreement with previous work. These are Ghar, Jal Az-Zor and Dibdibba Formations from bottom to top. No clear cut stratigraphic clue could be obtained from wells to identify formations. The fossiliferous sandy limestone layer at the lower part of the group was selected as the boundary between Ghar and overlying Jal Az-Zor Formations. The boundary between the Jal Az-Zor and Dibdibba Formations is hard to locate. The ledge-making, carbonate-cemented sand layers are included in the Jal Az-Zor, and overlying poorly-bedded and weakly-cemented gravels are included in the Dibdibba Formation.The overall stratigraphic sequence of the Dammam limestone and the Kuwait Group bear clues for a gradual regional regression and accompanying increase in clastic material influx. The Kuwait Group starts with marine clays and continues upward as fluvial, river channel sands, and ends with alluvial fan conglomerates. Deposition of the Kuwait Group was interrupted frequently by brief periods of quicence and non-deposition. Pauses in sedimentation are marked by surfaces of carbonate and gypsum cementation in the pore spaces of sands by evaporation of capillary water, and alteration of clays. The lower parts of the Kuwait Group consist mostly of loose sands without cementation. The degree of cementation and frequency of cemented layers increase towards the top of the Kuwait Group.

Miocene barnacle assemblages from southern Spain and their palaeoenvironmental significance

The Tabernas Basin in southeast Spain is a Neogene intermontane basin which filled with sub-aerial and submarine fan conglomerates grading into turbidite sandstones towards the basin centre. The basin history involved the development of Seravallian-Tortonian subaerial fans which coalesced southwards from the basin margin. Increasing water depth in the Tortonian led to a submarine development of these fans. The coarse fan conglomerates are colonized almost solely by barnacles of the species Megabalanus tintinnabulum, known from Miocene-Recent deposits. The barnacles occur in seven preservational-state assemblages, which provide a record of the colonization of stabilized mass-flow conglomerates and of subsequent reworking episodes. M. tintinnabulum is known from intertidal environments today. In situ clusters developed on the fan conglomerates record the transition of the fans from subaerial to submarine environments. Mass-flow deposits rich in barnacle shell debris are representative of basin-wards reworking of intertidal clusters and illustrate relative increase in water depth. A further shallowing episode is identifiable by the recolonization of mass flows by in situ barnacle clusters. This case history is demonstrative of the utility of some barnacles in determining basin history with respect to minor sea level fluctuations.

上部白亜系双葉層群の河川成〜浅海成堆積システム 堆積相と堆積シーケンス

The Futaba Group is the Lower Coniacian to Lower Santonian, fluvial to shallow-marine sedimentary succession distributed in the southern Abukuma Belt in north Honshu. Fifteen depositional facies were discriminated. They include alluvial fan, meandering river, braided river, transgressive lag and upper shoreface to inner shelf sediments. It is divided into three formations, the Ashizawa (the Asamigawa and Obisagawa Members), Kasamatsu and Tamayama Formations in upward sequence. Each formation consists of a third-order depositional sequence (DS).The first depositional sequence (DS 1) has a lowstand systems tract (LST) as the lower main part of the Asamigawa Member distributed in the northern half of the area. It is lithologically characterized by upper alluvial fan conglomerate and mid to lower fan sandstone and siltstone nonconformably overlying lower Cretaceous granite (sequence boundary 1 : SB 1). In the southern periphery of the distributed area the member contains talus or upper fan angular conglomerate. The upper part of the member distributed over the area is interpreted to be transgressive conglomerate with a sharp base called a ravinement surface (RS 1) which was formed by transgressive shoreface erosion. It may represent transgressive systems tract (TST). The following highstand systems tract (HST) is represented by the upper shoreface to inner shelf, medium to fine sandstone of the Obisagawa Member.The Kasamatsu Formation (DS 2) unconformably (SB 2) overlying the preceding depositional sequence (DS 1), is substantially characterized by fluvial upward-fining successions composed of channel sandstone and flood plain siltstone. The third sequence (DS 3) is characterized by thick fluvial (braided river) sandstone of the lower to middle part of the Tamayama Formation interpreted as LST and the upper shoreface to inner shelf sandstone of the upper Tamayama interpreted as HST. The sudden lithofacies change between the Kasamatsu and Tamayama Formations implies some large-scale changes in fluvial depositional systems by sea-level fall (SB 3). The transgressive conglomerate just above RS 2 in the basal part of the upper Tamayama Formation represents a thin TST. The upper sequence boundary (SB 4) of DS 3 shows the conspicuous clinounconformity between the Futaba Group and the Paleogene Shiramizu Group.

An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

Carboniferous calcretes in the Canadian Arctic

ABSTRACTCalcrete palaeosols have been found in the Upper Carboniferous Canyon Fiord Formation of southwestern Ellesmere Island, Canadian Arctic. These calcretes were developed in fluvial and shallow marine sediments that accumulated within two adjacent subbasins, in which the tectono‐sedimentary environment led to the deposition of five different sedimentary facies: (i) floodbasin sandstones; (ii) alluvial fan sandstones; (iii) alluvial fan conglomerates; (iv) braided fluvial sandstones; and (v) shallow marine limestones.Nodular/massive palaeosol profiles, consisting of cryptic, nodular, massive and laminar horizons, occur within the floodbasin sandstone and alluvial fan sandstone facies. Plugged palaeosol profiles, consisting of cryptic, plugged and laminar horizons, are restricted to the alluvial fan conglomerate facies. Massive/brecciated palaeosol profiles, consisting of cryptic, massive/brecciated and laminar horizons, occur essentially within the shallow marine limestone facies.The relationships between calcrete profiles and sedimentary facies suggest that profile types were controlled mainly by the texture and composition of the parent material: nodular/massive profiles are restricted to silicate‐rich sandstone hosts, plugged profiles are restricted to carbonate‐rich conglomerate hosts and massive/brecciated profiles are restricted to limestone hosts. Important relationships also exist between the maturity levels of nodular/massive profiles and sedimentary facies: profiles are generally mature in the floodbasin sandstones, invariably immature in the alluvial fan sandstones and absent from the braided fluvial sandstones. These different maturity levels were probably controlled mainly by exposure time, vegetation and substrate composition.

An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

ABSTRACTThe feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south‐west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (Rw), maximum projection sphericity (δp), oblate‐prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δp and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade‐shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and Rw is observed, hence the abundance of disc‐shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.

Stratigraphy and facies analysis of the Caqueza Group, a sequence of Lower Cretaceous turbidites in the Cordillera Oriental of the Colombian Andes

The thickest and most widely exposed Lower Cretaceous lithostratigraphic unit in the Cordillera Oriental of the Columbian Andes is the Caqueza Group. Along the eastern margin of the Bogota trough, the group is formally subdivided into the Buenavista, Macanal, and Alto de Caqueza Formations. On the basis of ammonite assemblages, the age of the group is late Tithonian-Hauterivian. Six main lithofacies and two subfacies are recognized, based on six detailed cross sections. These form five submarine fan facies associations (inner fan conglomerate, middle fan braided channel, middle fan-mouth channel, outer fan lobe sandstone-mudstone, and outer fan mudstone-siltstone/sandstone). The progression of the fan facies associations shows two main cycles of retrogradation and progradation in the turbidite basin. The Lower Cretaceous turbidites of the Caqueza Group were deposited in different submarine fan environments. The breccia-conglomerates and conglomerates of the Lower Member of the Buenavista Formation were deposited in inner fan environments. During the earliest Cretaceous, basin subsidence increased more rapidly than sediment supply, and channelized massive and pebbly sandstones of the Upper Member of the Buenavista Formation were deposited in braided middle fan environments that developed only in more proximal parts of the area. As a result of retrogradation of the fan system, middle fan-mouth channel deposits were covered by distal outer fan sediments. After the Berriasian, the sediment supply increased more rapidly than basin subsidence, resulting in deposition of an outer fan/outer fan lobe/braided middle fan progradational sequence.

Normal faulting and alluvial-fan deposition, basal Windermere Tectonic Assemblage, Yukon, Canada

In the Ogilvie Mountains, west-central Yukon, the upper Proterozoic lower Mount Harper group (informal name) is stratigraphically equivalent to basal Windermere Supergroup rocks that occur in other areas of the North American Cordillera. A preserved synsedimentary normal fault system forms the southern margin of an east-trending half-graben basin. The basin fill was derived from the south. The fill consists of fault-proximal talus breccia and as much as 1,100 m of debris-flow conglomerate deposited in coalescing alluvial fans. Intermediate and distal alluvial-fan facies include mid-to-lower fan conglomerate and sandstone deposited by sheetfloods, distal debris flows, and braided channel streamflows. A thick, coarsening-upward megasequence in the eastern part of the study area records the change from waterlaid (lacustrine?) red beds to lower alluvial-fan sandstones and conglomerates. Clastic deposition ended soon after volcanism was initiated, resulting in the overlying Mount Harper volcanic complex (ca. 750 Ma). The half-graben basin was formed by the southern margin faulting. Comparison with strike-slip basin depositional patterns indicates significant differences, suggesting that faulting was predominantly dip-slip. Synsedimentary fault movements and minor back-stepping of the fault system controlled development of the megasequences. Volcanism postdates sedimentation in this half-graben, but north-derived coarse clastic rocks which intertongue with the volcanic rocks were deposited to the northwest and are not directly related to lower Mount Harper group deposition. A model is proposed in which diachronous, opposed-symmetry half-grabens formed, with volcanism concentrated in the accommodation zone between half-grabens. Basins in the western arm of the East Africa rift system provide geometric and evolutionary analogues.

Evolution of the Devonian Hornelen Basin, west Norway: new constraints from petrological studies of metamorphic clasts

Abstract The Middle Devonian Hornelen Basin of western Norway is a continental fault-basin which formed during the late stages of the Caledonian orogeny, closely following the Late Silurian crustal thickening and high pressure metamophism responsible for the formation of eclogites in the adjacent Western Gneiss Region. Clasts from alluvial fan conglomerates which fringe the basin provide important constraints on the type of basin tectonism and the uplift history of this part of the orogen. The clasts lack a petrographic linkage with the Western Gneiss Region (from which they are separated by a fault), but show a strong petrographic linkage with units of the allochthon (which lie unconformably below the basin fill). The clast sourceland had a similar metamorphic evolution to the allochthon, and both lack evidence for the high pressures recorded in the Western Gneiss Region. Both clasts and allochthon cooled through muscovite argon-retention temperatures about 40 Ma earlier than the Western Gneiss Region. Depth-time curves derived from isotopic and petrological results show that prior to basin initiation the Western Gneiss Region was uplifting faster than the clast source, but still lay at about 10 km depth during basin formation. A tectonic scenario is suggested in which much of the uplift of the Western Gneiss Region took place before the deposition of the Hornelen Basin sediments, aided by crustal thinning along a major extensional mylonitic shear-zone. However, final exposure of this high-pressure metamorphic terrain only took place after the Hornelen Basin sediments were deposited, aided by uplift in the footwall of the basin sole fault.

Structural and sedimentological evolution of part of the west central Spanish Pyrenees in the Late Tertiary

During the late Oligocene and early Miocene, the frontal thrust complex of one of the south Pyrenean thrust sheets (the Guarga or Gavarnie sheet) became emergent to form part of the northern margin of the Ebro Basin. The principal structures in the frontal thrust complex show a progressive development from a simple recumbent fold in the west to an emergent thrust in the east. Southward movement at the thrust front decreased from east to west and in the west the thrust tip line is buried. These structures influenced Miocene continental sedimentation on two scales. The emergent thrust front formed an orographic barrier to rivers draining the higher Pyrenees. These rivers passed around the barrier rather than across it, and the positions of the apices of fluvial distributary systems in the Ebro Basin were thus controlled by the low points in the structure of the thrust front. On a smaller scale, sedimentary sequences and syn-depositional structures in small alluvial fan bodies at the basin margin were controlled by late thrust movements in the adjacent frontal thrust complex. Syn-depositional faults, folds and unconformities in the alluvial fan conglomerate sequences can be related to movements on individual faults within the thrust complex.