Debris Flow Facies Research Articles

Reservoir Quality Under the Control of Gravity Flows in the C7 Member of Yanchang Formation in the Jinghe Oilfield, Ordos Basin, China

The late Triassic sandstone reservoir in the C7 member of the Jinghe oilfield southern Ordos basin is a typical deep-water gravity flow tight oil reservoir. Sedimentary microfacies, physical properties, and petrographic analysis were being examined for quality determination. Pore structure and physical properties data together combined with, thin sections, and scanning electron microscope and core images were used to identify factors controlling reservoir physical properties. The depositional system under debates of different gravity flows including debris flow, seismite slumping, and turbidity flows. Among which sandy debris flow facies shows a better distribution of porosity and permeability followed by seismite-slump, where turbidity facies are the poorest. The petrophysical analysis shows that the study oil interval is a typical tight sandstone reservoir with an average porosity of 9% and permeability average being 0.025mD. The rock classification criteria of the C7 sandstone reveal the sub-categories of lithic feldspar sandstone and feldspar lithic sandstone. Average quartz sandstone contents of 48.25%, average feldspar sandstone content being 25%, and lithic fragments content of 29%. The formation lithology comprises mostly fine-grained sandstone and small pore size, which disclose that the porosity-permeability distribution increases proportional to the average and median pore throat radius, and decreases with average and median pressure. The microfacies distribution shows that the depositional facies controlled physical properties. The sandstone primary pores are affected by the mineral composition of quartz, feldspar, illite, smectite, kaolinite, calcite, and dolomite. Features such as dissolved pores and intergranular pore filling by feldspar, silky-like aggregates of illite-smectite intergranular pore filling and most diagenetic minerals influenced the sandstone pores beside the compaction.

Enhanced debris flow activities in the subtropical mountainous catchment of South China during early Marine Isotope Stage 3

Geomorphic processes and their responses to climatic change in subtropical mountainous catchments are not yet well understood. In this study, a unique sequence of alluvial sediments (>33 m thick) from the Nanxu karst valley, Guilin, South China, was investigated to characterize the nature of erosion–deposition processes in humid, subtropical catchments and decipher any climatic constraints on the earth surface dynamics and landscape evolution. Erosional outcrops show that the Nanxu valley fill sequence mainly comprises m-scale sheets of matrix-supported gravel (a debris flow facies), which, in the distal part of the valley, are intercalated with beds of fine-grained sediments (facies of horizontally interbedded sand and mud or massive mud). The fine-grained sediments were likely deposited in a subaqueous environment, which was probably formed when the karst sinkhole was contemporarily blocked by a large volume of debris flows. Optically stimulated luminescence (OSL) as well as radiocarbon dating constrained a chorology framework of ca. 62–42 ka for the valley fill sequence. Comparisons of the sedimentation phase with local and regional climate proxy records revealed that enhanced debris flow activities and sediment aggradation occurred under generally warm and wet but unstable climatic conditions during early Marine Isotope Stage 3. Under these climatic conditions, there was likely a subtropical landscape with high vegetation cover in the catchment at the time of sedimentation, as supported by our palynological data. Thus, it is suspected that enhanced debris flow activities were triggered by the recurrence of extreme monsoonal rainfall events. Since ∼ 42 ka, the stream-channel process dominated the valley, and the valley fill was deeply dissected, which might be related to changes in the overall intensity and internal variability of monsoonal rainfall over a period. These findings identify the role of high-intensity monsoonal precipitation in controlling geomorphological activities in humid, subtropical landscapes of South China.

Early Oligocene gravity-flow deposits at the base of the Meiji Reef system in the South China Sea, and their palaeoclimate significance

In this paper, we describe gravity flow facies that form the base of the Meiji Reef system in the South China Sea, and discuss their palaeoclimate significance. These Early Oligocene (26.1–23.5 Myr) deposits occur in Well NK1 and comprise a 110.6 m thick succession developed on top of volcanic basement. The deposits can be divided into two intervals, both of which are dominated by debris flow facies. The lower 17.6 m thick interval has a gravel content of 50–65% with parent rocks including dacite, basalt, tuff, and coral reef fragments. From base to top, these units coarsen upward and show increasing gravel contents. Gravel clasts are well-rounded but poorly sorted (diameter 10–350 mm), which, together with their complex composition, indicate transportation by the gravity flows from the near-shore volcanic rock basement to the slope. The upper 93 m thick interval comprises abundant bioclastic materials, including benthic foraminifera, crustal red algae, and red algae detritus, with a few volcanic gravel clasts. The content of volcanic rock gravels decreases towards the top, which may be attributed to the reduction or disappearance of volcanic rocks in the provenance area due to the establishment of coral reef facies that cap the succession and dominate the overlying interval from a core depth of 885.6 m to the seafloor. Based on U-Pb dating, the cap-shaped red algae laminae at the facies transition were formed at 24.6 ± 1.1 Ma, dating the earliest initiation of reef building to end-Oligocene times. The emergence of gravity flow sediments during the early Oligocene may be attributed to the onset of the Antarctic glaciation and concurrent regional tectonic activities that led to a shallowing of the sea. The origin of debris with high roundness characteristics is believed to be associated with transportation along the slope triggered by tectonic and climatic events, resulting in the formation of debris flow deposits. Studying the gravity flow facies at the Meiji Reef system is crucial for determining the onset of reef-building and helps establish the relationship between gravity flow deposition, paleoclimate, and regional tectonic evolution in the South China Sea.

Geological investigation of river terraces along the Modi Khola valley, Parbat district, western Nepal Lesser Himalaya

Fluvial terraces are remnants of the former floodplain of a stream or river. Wide river valley with spectacular terraces are the well-known places of the Himalaya because of their unique morphology and history of origin. The Modi Khola and Kali Gandaki valleys in west Nepal, Lesser Himalaya are characterized by a number of spectacular river terraces. In the present study, the river terraces along the Modi Khola valley were mapped at 1:25,000 scale and their lithological and sedimentological characteristics were investigated. The river terraces along the Modi Khola are distributed in at least three levels. They are named as Godam Formation (Higger Terrace), Badagaun Formation (Middle Terrace) and the Modibeni Formation (Lower Terrace). The Godam Formation is standing out at an elevation between 860 m and 1100 m. The upper surface of the terrace is extremely rugged with karstic features. The Godam Formation is characterized by matrix-supported, calcite cemented conglomerate with angular to sub-angular clasts of calcareous shale, laminated siliceous limestone and quartzite. The Badagaun Formation (Middle Terrace) is distributed at an altitude from 700 to 860 m on both sides of the Modi Khola valley. It is deeply weathered forming a very thick (almost 3-5 m) residual soil (mostly lateritic soil). Karst features such as caves and sink holes are common in this terrace. This formation is mainly composed of matrix-supported conglomerate with granule-, cobble- and boulder-sized clasts of calcareous shale, siliceous limestone and rarely quartzite. The Lower Terrace (Modibeni Formation) is fluvial deposit consisting of rounded and subrounded clast-supported conglomerate. Both the Upper and Middle Terraces have been interpreted as cohesive debris flow facies derived from the Annapurna Range. Presence of three levels of terraces indicates at least three phase of uplift in the area in the Holocene time. The debris flow deposit in the upper and Middle Terraces indicates catastrophic outbursts from natural dams or large rock-ice avalanches in the Annapurna Range triggered probably by megaearthquakes as in Pokhara valley as suggested by Schwanghart et al. (2015).

Facies analysis, depositional environments and paleoclimate of the Cretaceous Bima Formation in the Gongola Sub – Basin, Northern Benue Trough, NE Nigeria

Facies analysis of the Cretaceous Bima Formation in the Gongola Sub –basin of the Northern Benue Trough northeastern Nigeria indicated that the Lower Bima Member is composed of alluvial fan and braided river facies associations. The alluvial fan depositional environment dominantly consists of debris flow facies that commonly occur as matrix supported conglomerate. This facies is locally associated with grain supported conglomerate and mudstone facies, representing sieve channel and mud flow deposits respectively, and these deposits may account for the proximal alluvial fan region of the Lower Bima Member. The distal fan facies were represented by gravel-bed braided river system of probably Scot – type model. This grade into sandy braided river systems with well developed floodplains facies, forming probably at the lowermost portion of the alluvial fan depositional gradient, where it inter-fingers with basinal facies. In the Middle Bima Member, the facies architecture is dominantly suggestive of deep perennial sand-bed braided river system with thickly developed amalgamated trough crossbedded sandstone facies fining to mudstone. Couplets of shallow channels are also locally common, attesting to the varying topography of the basin. The Upper Bima Member is characterized by shallow perennial sand-bed braided river system composed of successive succession of planar and trough crossbedded sandstone facies associations, and shallower channels of the flashy ephemeral sheetflood sand – bed river systems defined by interbedded succession of small scale trough crossbedded sandstone facies and parallel laminated sandstone facies. The overall stacking pattern of the facies succession of the Bima Formation in the Gongola Sub – basin is generally thinning and fining upwards cycles, indicating scarp retreat and deposition in a relatively passive margin setting. Dominance of kaolinite in the clay mineral fraction of the Bima Formation points to predominance of humid sub – tropical to tropical climatic conditions. This favors pedogenic activities which are manifested in the several occurrences of paleosols. Pronounced periods of arid climatic conditions are also notable from the subordinate smectite mineralization. Chlorite mineralization at some localities is indicative of elevation of the provenance area, and this is synonymous with deposition of the Bima Formation, because of its syn – depositional tectonics. The absences of lacustrine shales in the syn – rift stratigraphic architecture of the Bima Formation indicates that the lower Cretaceous petroleum system that are common in the West and Central African Rift basins are generally barren in the Gongola Sub – basin of the Northern Benue Trough.

New interpretation of the Gran Dolina-TD6 bearing Homo antecessor deposits through sedimentological analysis

Gran Dolina is a cavity infilled by at least 25 m of Pleistocene sediments. This sequence contains the TD6 stratigraphic unit, whose records include around 170 hominin bones that have allowed the definition of a new species, Homo antecessor. This fossil accumulation was studied as a single assemblage and interpreted as a succession of several human home bases. We propose a complete stratigraphic context and sedimentological interpretation for TD6, analyzing the relationships between the sedimentary facies, the clasts and archaeo-palaeontological remains. The TD6 unit has been divided into three sub-units and 13 layers. Nine sedimentary facies have been defined. Hominin remains appear related to three different sedimentary facies: debris flow facies, channel facies and floodplain facies. They show three kinds of distribution: first a group of scattered fossils, then a group with layers of fossils in fluvial facies, and third a group with a layer of fossils in mixed fluvial and gravity flow facies. The results of this work suggest that some of these hominin remains accumulated in the cave by geological processes, coming from the adjacent slope above the cave or the cave entry, as the palaeogeography and sedimentary characteristics of these allochthonous facies suggest.

THE PROBLEMS OF IDENTIFICATION OF THE NEO-PLEISTOCENE GLACIAL MEGA-FLOOD DEPOSITS IN THE ALTAI MOUNTAINS REGION

Similarities and differences of glacial megaflood deposits and deposits of other genetic types of Gorny Altai are considered in the paper. Diluvial sedimentary complex includes (from bottom to top): debris flow facies of boulder-pebbles with giant boulders; floodplain facies of parallel laminated sands and gruss, fluvial cross-bedded pebbles facies, suspension facies of sands and silts; mud flow facies, facies secondary-dammed lake is thin parallel-laminated silts and sands. The deposits of different genetic types may appear similar in facies, textural and structural characteristics, but the geological structure and sedimentary facies architecture of the sediment complexes of various origins (the number of co-observed lithotypes and geologic nature of their relationship) have a specific and recognizable.

Submarine landslide deposits of the historical lateral collapse of Ritter Island, Papua New Guinea

The March 13th 1888 collapse of Ritter Island in Papua New Guinea is the largest known sector collapse of an island volcano in historical times. One single event removed most of the island and its western submarine flank, and produced a landslide deposit that extends at least 70 km from the headwall of the collapse scar. We have mapped and described the deposits of the debris avalanche left by the collapse using full-coverage multibeam bathymetry, side-scan sonar backscatter intensity mapping, chirp seismic-reflection profiles, TowCam photographs of the seafloor and samples from a single dredge. Applying concepts originally developed on the 1980 Mount St. Helens collapse landslide deposits, we find that the Ritter landslide deposits show three distinct morphological facies: large block debris avalanche, matrix-rich debris avalanche and distal debris flow facies. Restoring the island's land and submarine topography we obtained a volume of 4.2 km3 for the initial collapse, about 75% of which is now forming the large block facies at distances less than 12 km from the collapse scar. The matrix-rich facies volume is unknown, but large scale erosion of the marine sediment substrate yielded a minimum total volume of 6.4 km3 in the distal debris flow and/or turbidite deposits, highlighting the efficiency of substrate erosion during the later history of the landslide movement. Although studying submarine landslide deposits we can never have the same confidence that subaerial observations provide, our analysis shows that well-exposed submarine landslide deposits can be interpreted in a similar way to subaerial volcano collapse deposits, and that they can in turn be used to interpret older, incompletely exposed submarine landslide deposits. Studying the deposits from a facies perspective provides the basis for reconstructing the kinematics of a collapse event landslide; understanding the mechanisms involved in its movement and deposition; and so providing key inputs to tsunami models.

Halokinetic-sequence stratigraphy, fluvial sedimentology and structural geometry of the Eocene Carroza Formation along La Popa salt weld, La Popa Basin, Mexico

Abstract The Eocene Carroza Formation in La Popa Basin, Mexico, represents fluvial sedimentation in a shortening-influenced salt-withdrawal minibasin, termed the Carroza Syncline. The Carroza Syncline lies adjacent to the La Popa salt weld, which was formerly a passively-rising salt wall that was shortened during the Hidalgoan Orogeny in Late Cretaceous and Palaeogene time. The Carroza Formation displays distinct upsection changes in fluvial facies distribution and geometry of halokinetic drape folding. Fluvial channel distribution changes upwards from widespread thin, broad channels with variable palaeocurrents in the lower part of the formation to thick, stacked channels concentrated in the hinge of the Carroza Syncline with weld-parallel palaeocurrent directions in the upper part. The upper and middle members of the Carroza contain debris-flow facies derived from diapir roof strata and the diapir itself. The style of halokinetic drape fold upturn and thinning towards the weld changes upsection from a broad (800–1500 m) to a narrow (50–200 m) zone, where upper Carroza strata are overturned and in direct contact with remnant gypsum along the weld. The upsection changes in fluvial facies distribution and geometry reflect an overall decrease in local sediment-accumulation rates relative to salt-rise rates controlled by both Hidalgoan shortening and passive diapirism.

Quaternary sedimentology of the Rose Creek fan delta, Walker Lake, Nevada, USA, and implications to fan‐delta facies models

AbstractThe 3·2 km long Rose Creek fan delta of west‐central Nevada is prograding from an active rift margin into the 32 m deep Walker Lake. A case study of the forms, processes and facies of this fan delta reveals that the proximal and medial zones mainly are of sub‐aerial origin, and the distal zone is of lacustrine origin. Pebbly to bouldery rock‐avalanche mounds >100 m thick (Facies A) and muddy to bouldery debris flow levées 0·5 to 2·0 m thick (Facies B) dominate the proximal zone, whereas mostly matrix‐supported cobbly pebbly debris flow lobes 0·1 to 1·0 m thick (Facies C) typify the medial zone. Surficial pebble lags and gully fills (Facies D) are widespread in both zones but, in exposures, comprise only partings or lenticles between debris flow units. The distal fan delta mainly consists of lakeshore to lake‐bottom tracts formed by extensive wave reworking of debris flow facies. Nearshore deposits include erosional cobbly boulder lag beaches (Facies E), pebbly constructional beaches attached at headcuts or on barrier spits (Facies F), pebbly upper shoreface (Facies G) and sandy lower shoreface (Facies H) tracts positioned lakeward of the beach, and pebbly landward‐dipping foresets (Facies I) and backshore‐pond sand and mud (Facies J) present landward of the spits.Erosional lag beaches fringe the windward north side of the fan‐delta front, attached constructional beaches characterize the central zone, and southward‐elongating barrier spits typify the leeward south side, extending from the zone of greatest projection of the fan delta into the lake. Shoreline facies asymmetry results from largely unidirectional longshore drift caused by high fetch to the north and minimal fetch to the south, combined with the arcuate shape of the fan‐delta front. The spits overlie a platform deposited below common wave base consisting of south‐east‐trending cones of pebbly Gilbert foresets (Facies K) and sandy toesets (Facies L). Typically slumped silt and mud (Facies M) fringe both this platform and lower shoreface sand in deeper water. This case demonstrates facies types and patterns that are inconsistent with the widely promoted fan‐delta facies model having a front consisting of an apron of radially directed Gilbert foresets deposited where sub‐aerial flows enter the lake. The Rose Creek fan‐delta front instead features a sharp contact between sub‐aerial and lakeshore facies formed where waves erode, sort and redistribute heterogeneous debris flow sediment into the various shallow‐to‐deep lake facies. Gilbert foresets are present only in the lee of the fan delta where sediment moving by longshore drift reaches the brink of the spit front. This facies scenario results from the infrequency of fan‐building events versus nearly constant wind‐induced waves, a scenario that, in contrast to the popular Gilbert model, probably is the norm for fan deltas. The level of Walker Lake, and thus the position of wave reworking on the Rose Creek fan delta, fluctuated over a range of ∼157 m during the last 18 kyr, producing complex interfingering between sub‐aerial and lakeshore facies across a 1700 m wide radial belt, typifying a wave‐modified, freestand lacustrine fan delta.

Identification of stratigraphic traps with subtle seismic amplitude effects in Miocene channel/levee sand systems, NE Gulf of Mexico

Abstract Stratigraphic traps created and preserved on the unconfined slope of the ancestral Mississippi submarine fan in the northeastern Gulf of Mexico have been found to contain substantial and profitable hydrocarbon reserves. Nearly 2 trillion cubic feet of gas (350 million barrels of oil equivalent) have been produced in the last 13 years in less than 1400 feet (425 m) of water. Drilling results have yielded 31 fields from 45 drilled prospects for a success rate of 69%. The main phase of exploration lasted eight years. The first 3D survey in 1994 sparked a major increase in drilling and success rates until the last field was discovered in 2001. This play has now essentially been exhausted of sizeable opportunities. The underlying Mesozoic section sources hydrocarbons to the play for middle Miocene slope sands. The unconfined (non mini-basin) slope sands are turbidites and debris flows that were deposited between 5 and 25 miles (8 and 40 km) from their coeval shelf margins. Stratigraphic traps in this environment are created at the margins of the levees as they interfinger with slope shales. Additional geometric modifications of the stratigraphic traps result from post-depositional erosion or ‘cannibalization’. The erosion left ‘monadnocks’, or remnant patches of porous sands, that are encased in low permeability shales. Turbidite channel/levee deposits are the dominant reservoir facies in these ‘patches’; however, they may consist of channel, levee, or debris flow facies. Detailed mapping of these dominantly gas charged erosional remnants shows little to no fit of seismic amplitude effects to structure and the hydrocarbon accumulations often appear as amplitude anomalies ‘floating in space’. This work was done to provide an analogue study, as well as to document any remaining prospects that were initially overlooked. Almost every discovery and dry hole in the trend was interpreted and included in this study. Integrating the well control into the geological model by using detailed seismic stratigraphy, whole core description, dipmeter logs, seismic to well tie synthetics, Gassmann fluid substitution and AvO analysis, provided the necessary insight to prosecute this play. Within the play area, seismic velocities of sands and shales are very similar. As a consequence of this, seismic reflections are generally weak and, due to the stratigraphic variability, they are discontinuous. Hence, standard sequence stratigraphic mapping techniques alone are not enough to define these subtle traps. The key to successful exploration is a complete understanding of the rock properties. All of the fields examined displayed subtle amplitude anomalies associated with the presence of hydrocarbons. The ability to understand and quantify both the amplitude is essential to increasing the probability of success in this play.

Source area and tectonic control on alluvial-fan development in the Miocene Fohnsdorf intramontane basin, Austria

Abstract Middle Miocene alluvial fans in the intramontane Fohnsdorf Basin of the Eastern Alps originated along normal faults and linked strike-slip faults in a continental half-graben setting. The fans display considerable facies differences. Debris flows of the Rachau fan are characterized by a sandy matrix and large boulders, whereas debris flows of the Apfelberg fan are characterized by higher silt and clay content and smaller clasts. Key control of debris-flow facies is the lithology contrast in the fan source areas. Sand, pebbles and large outsized boulders originated predominantly from the resistant augengneiss- and amphibolite-dominated hinterland of the Rachau fan, whereas a significant higher proportion of mud and silt and smaller boulders have been derived from the Apfelberg fan catchment, which was dominated by mica schists and marble.

Upper Oligocene to Lowermost Miocene Strata of King George Island, South Shetland Islands, Antarctica: Stratigraphy, Facies Analysis, and Implications for the Glacial History of the Antarctic Peninsula

The Cape Melville Formation (CMF), exposed on southeastern King George Island, South Shetland Islands, provides rare evidence of extensive earliest Miocene glaciation in the Antarctic Peninsula region. The formation records the presence of regional marine-based grounded ice on the continental shelf. It overlies disconformably the upper Oligocene Destruction Bay Formation, which consists of sandstones recording nonglacial shallow marine conditions. Four units have been identified within the approximately 150 m thickness of the CMF. The basal unit (A) consists of coarse glacigenic debris-flow facies interbedded with glaciomarine mudstone and sandstone. The overlying unit (B) is mainly fine-grained. This succession may represent relatively ice-proximal deposition followed by glacial retreat and/or relative sea-level rise. The upper CMF (units C and D) was deposited in an ice-distal marine environment, with intermittent input of coarse glacigenic debris, mainly from ice rafting. Thin beds of pelagic carbonate ooze within unit C indicate periods of low terrigenous sediment input and high productivity. Lithologically diverse glacigenic gravel clasts (mainly ice-rafted debris) in the CMF had a wide regional source area, suggesting that ice cover was widespread regionally and included calving ice margins. For a small proportion of clasts the nearest known source is the mountains fringing the southern Weddell Sea. Such clasts were presumably transported north in debris-laden icebergs by a strong, cold Weddell Sea surface current. A temperate glacial setting is tentatively inferred from the CMF. Palynological results confirm and enhance the paleoenvironmental interpretation from the sedimentology, and include the first early Miocene dinoflagellate cyst assemblages recorded on the Antarctic Peninsula. This reappraisal of the glacial record from the CMF provides valuable constraints on the Antarctic cryosphere and regional paleoenvironments in the mid-Cenozoic.

Debris-flow deposits in an alluvial-plain succession; the Upper Triassic Callide Coal Measures of Queensland, Australia

The Carnian-Rhaetian Callide Coal Measures are preserved in a small (22.5 km by 8 km), partially fault-bounded basin remnant in east-central Queensland, Australia. The <150 m thick coal-measure succession is interpreted to have accumulated during a phase of mild crustal extension that formed a series of discrete, intermontane basins in eastern Australia. The succession fines upward from a conglomerate-rich lower part into a finer-grained and coal-bearing upper section (including coal seams <34 m thick), and is interpreted as the deposits of an alluvial-plain environment. Anomalous, matrix-rich diamictites, breccias, and conglomerates have been recognized within the succession at several localities, in many cases interbedded with coals. These are interpreted as the product of debris flows. Two debris-flow lithofacies are recognized: (1) mixtures of fine carbonaceous material, clay, silt, sand, gravel, and volcaniclastic debris, and (2) breccias consisting principally of coal clasts in a coaly matrix with minor clastic and volcaniclastic debris. Facies 1 is found in sharp-based, simple or composite bodies (<15 m thick) that are elongate to lobate in plan (with some multi-lobate), and as much as at least 1500 m long and 600 m wide. Facies 2 is less common and occurs in sharp-based units as much as 1.5 m thick, at least 200 m in length, and 60 m in width. Both facies are characterized by abundant fine matrix and a very poorly sorted clast population, and show little internal organization or sedimentary structure other than variably developed imbrication of coarse clasts. Parts of some bodies show an erosional base (with up to several meters of coal removed), although a flat, apparently non-erosional contact is more common. The distribution of debris flows in the Callide Coal Measures shows a coincidence with mapped faults and interpreted structural lineaments. The debris flows may have been triggered by fault movements, which formed rupture topography on the flat alluvial plain, and caused destabilization of water-saturated clastic and organic sediments. Some debris-flow bodies may have been mounded, such that subsequent peat formation was restricted until those bodies were buried. The preservation of debris-flow units at different stratigraphic levels along mapped structures suggests multiple paleoseismic events or multiple debris-flow events along those structures. The mixing of volcaniclastic debris into debris-flow facies suggests that seismic events were coincident with (or perhaps caused by) nearby, explosive volcanic activity. The close relationship between debris-flow deposits and thick coal bodies on the inferred downthrown sides of faults at Callide further suggests that periodic, tectonic subsidence may have facilitated thick coal accumulation.

A preliminary history of Holocene colluvial (debris‐flow) activity, Leirdalen, Jotunheimen, Norway

A stratigraphic succession of alternating peat and minerogenic sediments at the foot of a steep mountain slope provides the basis for the reconstruction of a preliminary colluvial history from the alpine zone of Jotunheimen, southern Norway. Layers of silty sand and sandy silt, typically 5–10 cm thick and interpreted as distal debris-flow facies, are separated by layers of peat that have been radiocarbon dated. Deposition from at least 7500 to about 3800 14C yr BP of predominantly minerogenic material suggests relatively infrequent but large-magnitude debris-flow events in an environment warmer and/or drier than today. Particularly low colluvial activity between about 6500 and 3900 14C yr BP was terminated by a succession of major debris-flow events between about 3800 and 3400 14C yr BP. Unhumified peats indicative of higher water tables, separate six debris-flows that occurred between about 3300 and 2300 14C yr BP and signify a continuing high frequency of colluvial activity. Uninterrupted peat accumulation between about 2400 and 1600 14C yr BP indicates reduced debris-flow activity; subsequent renewed activity appears to have culuminated in the ‘Little Ice Age’ after about 600 14C yr BP. This pattern of colluvial deposition demonstrates a long history of natural Holocene low-alpine landscape instability, suggests an increase first in the magnitude and then in the frequency of debris-flow activity coincident with late Holocene climatic deterioration, and points to the potential of debris-flow records as a unique source of palaeoclimatic information related to extreme rainfall events. © 1997 John Wiley & Sons, Ltd.

A preliminary history of Holocene colluvial (debris-flow) activity, Leirdalen, Jotunheimen, Norway

A stratigraphic succession of alternating peat and minerogenic sediments at the foot of a steep mountain slope provides the basis for the reconstruction of a preliminary colluvial history from the alpine zone of Jotunheimen, southern Norway. Layers of silty sand and sandy silt, typically 5–10 cm thick and interpreted as distal debris-flow facies, are separated by layers of peat that have been radiocarbon dated. Deposition from at least 7500 to about 3800 14C yr BP of predominantly minerogenic material suggests relatively infrequent but large-magnitude debris-flow events in an environment warmer and/or drier than today. Particularly low colluvial activity between about 6500 and 3900 14C yr BP was terminated by a succession of major debris-flow events between about 3800 and 3400 14C yr BP. Unhumified peats indicative of higher water tables, separate six debris-flows that occurred between about 3300 and 2300 14C yr BP and signify a continuing high frequency of colluvial activity. Uninterrupted peat accumulation between about 2400 and 1600 14C yr BP indicates reduced debris-flow activity; subsequent renewed activity appears to have culuminated in the ‘Little Ice Age’ after about 600 14C yr BP. This pattern of colluvial deposition demonstrates a long history of natural Holocene low-alpine landscape instability, suggests an increase first in the magnitude and then in the frequency of debris-flow activity coincident with late Holocene climatic deterioration, and points to the potential of debris-flow records as a unique source of palaeoclimatic information related to extreme rainfall events. © 1997 John Wiley & Sons, Ltd.

Continental slope sedimentation adjacent to an ice-margin. II. Glaciomarine depositional facies on labrador slope and glacial cycles

Eight depositional facies have been identified in Labrador Slope and Basin cores on the basis of sedimentary structures and textures. These are combined into three genetic groups: (1) hemipelagic facies HI with ice-rafted debris (IRD) and H without IRD (with a combined thickness of about 53% of the total core length); (2) debris-flow facies D (7%) and three turbidite facies (34%) T (thin, fine-grained bodyspill turbidites), TH (headspill turbidites) and TI (turbidites interlayered with laminae of IRD); and (3) contourite (<3%) and nepheloid-layer deposits (3%) of facies C and N. The latter and facies TI are characteristic of proximal Heinrich layers. The intercanyon regions of the slope are mainly covered by hemipelagic sediment with intercalated spillover turbidites. The canyons, if filled, contain debris-flow deposits and turbidites. The dominant sediment-delivery and transport mechanisms change from vertical settling and bedload transport by glacial stream-discharge on the upper slope to down-slope mass-flow processes, which predominate over along-slope processes on the lower slope and rise.Up to 15 lithostratigraphic units have been differentiated in the slope cores, based on the recognition of various combinations of the 8 facies types and short-distance correlations of AMS 14C-dated intervals. Very dark hemipelagic muds with low detrital carbonate content, little ice-rafted debris, a low degree of bioturbation and abundant sinistral-coiling, cold-water foraminifera (Facies H of units 3,5,7,9,11 and 13) are tentatively ascribed to tunes of the Mid-to Late Wisconsinan glaciation with extensive seasonal sea-ice cover in the Labrador Sea. Turbidites and debris-flow deposits cannot be related to specific phases of the glacial cycle and result largely from redeposition of upper and mid-slope sediments. Ice-rafted debris dispersed in nepheloid-flow deposits or interlayered as separate laminae between thin mud-turbidites are related to short-lived Heinrich events during times of partial ice-cap collapse, but are not restricted to these events. The youngest slope sediments, which were deposited since the early part of marine oxygenisotope stage 2 (<32 ka) correlate chronologically and lithologically with adjacent shelf, rise and basin sediments. The oldest AMS 14C date of 54,530 ± 2090 yr B.P. was obtained for the unit 11/12 boundary at 1008 cm subsurface depth in core 90-26.

Sandy Slump and Sandy Debris Flow Facies in the Pliocene and Pleistocene of the Gulf of Mexico: Implications for Submarine Fan Models: ABSTRACT

Sandy Slump and Sandy Debris Flow Facies in the Pliocene and Pleistocene of the Gulf of Mexico: Implications for Submarine Fan Models: ABSTRACT

Core-Based Evidence for Sandy Slump and Sandy Debris Flow Facies in the Pliocene and Pleistocene of the Gulf of Mexico: Implications for Submarine Fan Models: ABSTRACT

Core-Based Evidence for Sandy Slump and Sandy Debris Flow Facies in the Pliocene and Pleistocene of the Gulf of Mexico: Implications for Submarine Fan Models: ABSTRACT

Stratigraphy and Diagenesis of Sonora Canyon Deep-Water Sandstones, Val Verde Basin, Southwest Texas

Several 'Canyon Sands' (Virgilian-Wolfcampian) intervals occur in the Val Verde basin, but the authors' study focuses on the Sonora Canyon, which is centered in western Sutton County. Sonora Canyon sandstones formed in continental-slope and basin-floor depositional systems basinward of a northwest-trending shelf margin. Well-log analysis revealed an overall wedgeshape geometry, thickening southwesterward from 500 ft at the shelf margin to 2000 ft where sandstone grades into basinal shale. The Sonora Canyon comprises coalesced submarine-fan systems forming a slope apron. Deltaic sandstones occur locally along the shelf margin, but mudstone covers most of the outer shelf. On the upper slope, channel-fill sandstones are discontinuous at 0.5- to 1.5-mi well spacing, and are enclosed in thick slope mudstones. Turbidites are the most prominent sedimentary features identified in Sonora Canyon cores. Channel-fill sandstones are massive to normally graded (Bouma divisions A and B) and are associated with slump and debris-flow facies. Sonora Canyon sandstones are mostly fine-grained to very fine-grained sublitharenites and litharenites. Chert, mudstone, sandstone, and low-rank metamorphic rock fragments are the predominant lithic grains. Major diagenetic events were (1) siderite and chlorite cementation, (2) mechanical compaction, (3) quartz cementation, (4) feldspar dissolution and illite and kaolinite precipitation, and (5) ankerite cementation. more » Intergranular porosity and permeability are higher in samples with abundant siderite cement because early precipitation of siderite inhibited later mechanical compaction and quartz cementation. « less