Labuan Outcrop Revisited: New Findings on Belait formation Facies Evolution
M. Rapi M. SomM Fauzi B A KadirSiti Syareena M AliShamsuddin JirinW.M. Khairul W. SulaimanNorshida MohsinShahriza Salwani M Shah
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Abstract:
The sedimentary successions of the Belait formation exposed across the northern side of the Labuan Island has been studied by various workers such as Hazebroek, 1993; Levell, 1983, 1987; Tate, 1994. Based on his work, Mazlan Madon (1994, 1997) concluded that the basal Belait Formation was deposited in fluvial system developed over an eroded Temburong landscape in an overall transgressive regime. Facies development in the basal Belait reflects a quick change transition from fluvial systems (braided to meandering) to shallow marine successions represented by coarsening-upward offshore shales to shoreface sandstones. The presence of two (2) new outcrops provide the opportunity to further study the lateral continuity and vertical facies succession within the Belait Formation. A total of nine (9) outcrop sites including two (2) new locations were studied and logged and 142 samples were taken and analysed for biostratigraphic information. Results showed that the fluvial succession within the Belait Formation is not presence above the Temburong Formation at the new outcrop and replaced by coastal plain, fresh/brackish water estuarine successions. The fluvial succession thickened away from the new outcrop in the direction of Layang-layangan in the west and Tg. Kubong to the east. Furthermore, the fluvial succession in Tg. Kubong is also thinner than previously reported (Mazlan, 1994). Rapid change form fluvial to estuarine environment was observed based on biostratigraphic data. Interms of vertical facies development, we proposed that there are two (2) incised valleys developed where the fluvial succession was deposited and rapidly overlain by brackish water fluvial-estuarine deposits. The new outcrop area is interpreted as an interfluve and appears to be where the center of the anticline is located. The relatively thin fluvial to shallow marine transition above the sequence boundary, implying rapid deepening due to the steepening depositional surface, coupled with rising sea level and uplifting in the new outcrop area. This finding will help us in understanding the relationship between sea level, tectonic activity and vertical/laterar\l facies development.Keywords:
Outcrop
ABSTRACT The Lower Cretaceous Hensel Formation in central and north-central Texas consists of predominantly terrestrial deposits. Study of Hensel outcrops in Kimble, Gillespie, and Blanco Counties, south of the Llano Uplift, reveals a facies evolution that is associated with a major marine transgression. This evolution is expressed in both a north-south facies tract and in the overall stratigraphic succession. Four major depositional systems are recognized. (1) Basal, valley-fill deposits are limited to localities proximal to the source area. (2) Low-sinuosity, bedload channel-facies overlie the valley-fill and are widely distributed. (3) Low-sinuosity channel-facies evolved into more distal, somewhat finer grained, coastal-plain fluvial systems. (4) Both the fluvial and coastal-plain facies are characterized by extensive flood-basin muds and small, ephemeral arroyos. The pervasive development of calcrete, or caliche, within the overbank deposits, and the overall depositional style of the unit imply a semi-arid, seasonal climate. As the marine trangression progressed, the sediment supply decreased, and channel gradients lowered. Deposition of the shallow marine or lagoonal carbonates that overlie the Hensel resulted from the final inundation of the source area.
Marine transgression
Overbank
Outcrop
Sinuosity
Alluvial fan
Progradation
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ABSTRACT Kilometer-scale prograding clinoforms associated with deltas are rarely seen in outcrop; however, one such example is found in a Miocene sand-shale sequence exposed along the Jerudong anticline in Brunei Darussalam. Regional sequence stratigraphic interpretation shows that large clinoforms at the base of the Miocene Belait delta represent a succession of at least three major sand-shale sequences. The stratigraphically highest and best exposed sequence exhibits large slumps and sharp-based detached sand bodies at its base. Accumulation of these units most likely occurred during a relative sea-level lowstand. An overlying 1-1.5 km thick shale unit is interpreted to have developed during subsequent transgressive and early highstand conditions. Rapid progradation of thick sand-dominated shoreface deposits characterizes the late highstand systems tract. The clinoforms below show similar depositional geometries: slumps and thin blankets of shallow-marine sandstones mark the individual bases, shales and mudstones succeed, and progradational shoreface and tidal deposits form the top of each clinoform. New sedimentological and micropaleontological data document that all sediments (regardless of whether sand- or shale-dominated) formed in a shoreface to shelfal setting in front of a mud-rich delta. This differs from previous studies interpreting a continental-slope to deep-marine depositional environment for all shale-dominated units, and FIG. 1. A) Landsat satellite composite image of NE Brunei Darussalam (after Sandal 1996) and B) geological map (after Wilford 1961). The Belait Formation is dominated by shallow-marine shoreface and tidal sandstones. The Setap Shale Formation (Setap Shales) is mainly composed of shelfal mudstones and shales. Note pronounced progradational geometries at the base of the Belait Formation on the eastern margin of the Belait Syncline. Black boxes indicate locations of Figures 2 and 4. End_Page 913------------------------ indicates that kilometer-scale clinoforms can develop entirely on the continental shelf in water depths less than 200 m.
Anticline
Sequence Stratigraphy
Sequence (biology)
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The Point Lookout Formation, which is well exposed along the northwest margin of the San Juan basin in northwestern New Mexico, includes nearshore sediments deposited during a regression of the Late Cretaceous epicontinental sea in earliest Montanan time. The unit is composed of sandstone and siltstone with sand percentage increasing up-section. Principal outcrop lithofacies include a lower interbedded, highly bioturbated, very fine sandstone and shale that represents the transition from innershelf to shoreface environments. The middle part of the unit gradationally overlies the lowermost lithofacies and represents a complex depositional history in the nearshore zone. These progradational units are thickly bedded and coarsen upward. This simple sequence is interrupted by he occurrence of hummocky stratified storm deposits and by broad surfaces of nondepositional scouring in the lower shoreface. A medium-grained upward-fining sandstone lithofacies that caps the entire formation has an erosional base and large-scale lateral accretionary bedsets. Measured sections from the outcrop of the Point Lookout closely correspond with electric-log patterns from subsurface data east of the outcrop belt. The mapped distribution of SP-pattern facies (representing sandstone textural characteristics) depicts the primary depositional elements of the progradations. Correlation of genetically related sand packages permits the evaluation of changing sedimentation patterns through time. Seven regressive events (time-stratigraphic units) are recognized based on subsurface identification of the transgressive boundaries that rise stratigraphically away from the basin and obliquely traverse lithofacies boundaries. Each unit is composed of three depositional phases (progradation, transgression, and aggradation) that occur in regular succession. Discrete distributary and interdistributary areas were maintained in the initial depositional phases throughout the history of the Point Lookout. In the broad areas between depositional axes the shoreline prograded by the seaward accretion of beach ridges until sediment sources became insufficient to maintain the shoreface advance. Transgressive reworking of the seaward part of the unit followed and dominated the arrangement of net-sandstone thicks by redistributing the sands into a strike-alignment. Each time-stratigraphic sedimentary unit is therefore the product of a progradational-transgressive depositional couplet. Whereas periodic transgressions were mainly erosive, they did cause the formation of coalesced shallow-shelf bars analogous to estuarine-shoal retreat massifs found on the modern continental margin of the Mid-Atlantic Bight. During periods of shoreline stability following transgression a End_Page 449------------------------------ channeled estuarine system developed landward of the retrograded shoreline trend. During this tide-dominated aggradational phase, channels migrated over the back-barrier area and produced the relatively coarser sand facies capping the Point Lookout in the study area. After the estuarine system was infilled, coastal plain facies were established in the former back-barrier zone and progradation was renewed. With the repetition of this depositional pattern through time, the coastal plain advanced in a step-wise fashion. As a consequence of the progradational-transgressive cyclicity, a significant degree of stratigraphic rise was attained during the Point Lookout regression. Each time-stratigraphic coastal sand body acts as a discrete reservoir that interfingers landward with impermeable sediments of the coastal plain facies. Given the necessary present-day structural configuration, major stratigraphic rises corresponding to aggradational phases can act as updip migration boundaries to gas derived from the center of a basin. End_of_Article - Last_Page 450------------
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Paleocurrent
Conglomerate
Marine transgression
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ABSTRACT In this paper we describe little-studied exposures of the Upper Cretaceous Cardium Formation in the northern Foothills and Peace River Plains, located north of Twp. 58. We correlate sixteen principal outcrop sections along the Foothills and across the northern escarpment of the Cardium as far east as the Smoky River. Correlations are based on tracing bounding erosion surfaces, initially observed in well logs and cores, and later extended into outcrop. The Kakwa Member is conglomeratic in many of these sections. The member displays a spectrum of conglomerate types, of both shallow marine and fluvial origin. Depositional environments are differentiated on the basis of stratigraphic context, conglomerate texture, sedimentary structures, stratification style and paleogeographic setting. We describe in detail four localities exemplifying the main paleoenvironments. At Bay Tree, a 12 m thick section consists primarily of clast-supported conglomerates representing a wave-dominated shoreface and beachface. This environment is dominated by broadly horizontal stratification with horizontal facies heterogeneity on a metre scale. At Cutbank Lake, swaley-stratified shoreface sandstones contain pebble-lined scours that show an upward increase in thickness and lateral continuity and represent rip current deposits. At Mount Niles, conglomerate forms a 2-6 m thick, >200 m wide, channelized body consisting of heterolithic matrix-supported conglomerates and pebbly sandstones capped by gravel wave ripples. Deposition may have been in a river-mouth setting. At 'Horseshoe Mountain', the upper part of the Kakwa Member consists of metre-scale crossbedded pebbly sandstone and matrix-supported conglomerate deposited in a pebbly fluvial system lacking marine influence. For the entire study area, paleocurrent observations indicate a strong NW to SE alongshore transport of gravel, with storm waves approaching from the NE. Fluvial crossbedding is directed towards the NE. Some of the outcrop facies can also be recognized in core from the adjacent subsurface. Our descriptions of conglomerate facies in outcrop facilitate interpretation of the depositional environment of comparable units in subsurface. Unfortunately, the limited extent of exposures does not allow us to determine with precision the geometry of conglomeratic units. However, a broad conglomerate-rich zone >90 km in dip extent and 10-20 km in strike extent can be traced across the northern outcrop belt and probably records the position of a major and long-lasting pebbly fluvial system. Subsurface analogs would be exploration fairways. At the reservoir scale, conglomerate bodies showing greater influence of wave reworking would tend to be more elongate, and have greater facies continuity/permeability, in a shore-parallel direction. The outcrops display lateral textural heterogeneity at scales of metres to 10s of metres. This heterogeneity would affect reservoir performance, but could not be captured (for reservoir modeling) using typical development well spacing. End_Page 437------------------------
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In the Falher Member of the Mannville Group (Aptian-Albian) of western Canada, two shoreline successions contain the reservoir conglomerates for the giant Elmworth gas field. The Falher B succession has a basal sheetlike shoreface unit of hummocky cross-stratified sandstone that thins seaward and terminates about 30 km north (seaward) of the landward limit of the transgression. Another 25 km farther basinward, the succession shows a 20-30-m-thick sandstone, unattached to the prograding shoreface, and an overlying coarsening-upward shoreface succession with thin muds and coals, interpreted as back-barrier deposits. These basinward facies are the results of a relative sea level fall and the early stage of the subsequent rise. In the upper (Falher A) succession, immediately andward (south) of the barriers, fluvial valleys were incised into nonmarine mudstones and coals during the base-level fall. As relative sea level subsequently rose, in nonmarine areas the valleys were filled by estuarine and fluvial sands, then a widespread sheet of fine-grained nonmarine sediment was deposited. At the same time, the shoreline migrated back across the shelf. As it reached the original shorezone (structurally controlled), reworking of underlying deposits successively generated three gravelly barrier islands superimposed on the sandy shoreface succession. The conglomeratic reservoirs all rest above the unconformities, in the transgressive depositional system. Because this sequence is essentially a set of linked facies and not a composite of stacked individual facies successions, it is affected by sediment partitioning between facies. During relative sea level rise, little marine sedimentation occurred because sediment was trapped mainly in nonmarine areas. Conversely, during sea level fall, most deposition occurred in marine areas because of the absence of nonmarine accommodation. Westward (alongshore) toward the thrust belt, no falling or lowstand sea level succession developed. Instead, a wide regressive shoreface sandstone with a transgressive cap occurs. Subsidence rates were higher in this area, and relative sea level appears always to have risen, but at varying rates. The surface under the transgressive facies changes from a type 1 unconformity in eastern, lower subsidence areas to a type 2 unconformity in western, higher subsidence areas. Any two-dimensional sequence stratigraphic model, therefore, is inadequate to describe the lateral variation of the sequence and distribution of shoreface sandstones, because the subsidence gradient was not parallel to the direction of shoreface progradation.
Sequence (biology)
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ABSTRACT Facies belts have been recorded using conventional field methods in the Lower Cretaceous Helvetiafjellet Formation in Spitsbergen, Svalbard. The formation was deposited upon a regional subaerial unconformity (SU) that developed downdip from an incised valley complex to the W, NW and possibly north of Spitsbergen during fall in relative sea level and incision into the marine Rurikfjellet Formation. The unconformity likely resulted from crustal tectonics related to the formation of the proto‐Amerasian basin. The unconformity was initially covered by braided stream facies sand and locally by small bay head delta deposits. Further rise in relative sea level resulted in transgression, and the initial braidplain turned into a low‐sloping shelf ramp of coastal plain and paralic to marine environments. Balance between increase in accommodation space and rate of sediment supply gave rise to aggradational stacking architecture of the middle and upper part of the Helvetiafjellet Formation, until further rise in sea level turned the shelf again into an open marine setting with deposition of the overlying Carolinefjellet Formation. Architectural pattern of fluvial distributary channel sandstone bodies in the aggradational succession is explained within the framework of the Boreal basin, not only the Spitsbergen outcrop domain as in previous models. The revised depositional model for the Helvetiafjellet Formation implies that fluvial systems in similar emerged epicontinental basins may have large potential to carry sand several hundreds of kilometres out into the basin with formation of fluvial sandstone intervals of large regional extent, bounded by semi‐parallel stratigraphic surfaces transecting time‐lines at very low angles. Sandstone bodies with clinoform geometry are in this setting restricted to local small‐scale paralic deltas and shoreface deposits.
Marine transgression
Aggradation
Subaerial
Outcrop
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The Ness Formation (Bajocian) consists of the alluvial distributary plain deposits of the Middle Jurassic Brent delta. The formation comprises fluvial channel sandstones intercalated with fine-grained floodplain deposits. The sandstone bodies are significant hydrocarbon reservoirs in the Oseberg field and surrounding smaller structures in the Norwegian North Sea, to which the present study pertains. The alluvial succession shows significant thickening across normal faults, reflecting syndepositional differential subsidence. The thickness proportion of fluvial sandstones varies with the succession thickness. Where the succession is relatively thin, it is characterized by a large variation in the content of fluvial sandstones. Where thicker, the succession shows less variation in the proportion of fluvial sandstones. The proportion of fluvial sandstones tends to stabilize or even decrease in the thickest profiles of the formation. These findings are in contrast to theoretical alluvial stratigraphy models, which predict greater sandstone body proportions in areas of greater subsidence. The stacking pattern of fluvial sandbodies shows widespread temporal changes that can be correlated throughout the study area, and are independent of variations in thickness of the formation. These facies changes are approximately chronostratigraphic and allow for the definition of two sequences, each comprising a low-accommodation systems tract, succeeded by a high-accommodation systems tract.
Progradation
Sequence Stratigraphy
Overbank
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The Bida Basin is located in central Nigeria and it is perpendicular to the main axis of the Benue Trough. Due to its large areal extent and facies variation, the basin is often geographically divided into northern and southern Bida Basins. Whereas, aspects of the mineral resource and sedimentation history of the sediments in NW and SE extremes have been consistently studied, the present study area (Share-Lafiagi-Shonga areas) remains either unknown or under-reported. In the study area, fifteen vertical profiles of the Campanian-Maastrichtian sediments were studied along road cuts, erosional channels and Cliff sides with special attention focused on their internal physical and biogenic attributes. The sedimentological analysis permitted recognition of five distinct depositional facies; alluvial fan, braided channel, floodplain, tidal channel and shoreface in the lithostratigraphic units mapped. In Unit I, the proximal alluvial fan facies were preserved as conglomeratic facies which overly nonconformably, the Pre-Cambrian weathered schists and granites. Both the matrix and grain supported subfacies are indicative of gravity induced alluvial processes. The braided channel facies comprising of conglomeratic sandstone, medium-coarse grained sandstone subfacies are wide spread and their fluvial origin is supported by unidirectional flow pattern and absence of marine biogenic features. The sequence grades into claystone facies which probably formed in localized non marine floodplains. The younger Unit II comprises of conglomerate, sandstone and claystone facies. The conglomerate facies is moderately sorted and mature showing evidence of reworking and recycling. Association of this facies with herringbone cross stratifified beds probably indicates tidal channel lag origin. The sandstone facies are commonly compositionally mature, bioturbated and contain clasts of reworked clays and clay drapes suggesting high energy tidal channels and shoreface subenvironments. The depositional model for the Upper Cretaceous sediments in the study area is strongly dominated by alluvial processes which in places evolved into shallow marine processes and frequently incised by fluvial channels. The clay deposits of the floodplain may offer economic resource potential in the area.
Conglomerate
Alluvial fan
Trough (economics)
Paleocurrent
Marine transgression
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Detailed facies analysis and sequence stratigraphic principles applied to outcrop and subsurface data have aided in the development of a reservoir geological
model for the Pacoota P3B Unit at the Mereenie Field, Central Australia. Mereenie is a linear Northwest/southeast trending oil and gas field 4 km wide and 35 km long, and covers an area of approximately 130 km2. In this field, oil and gas are produced from
some of the oldest known petroleum reservoirs in the word: reservoirs approximately 500Ma.
The Ordovician Pacoota P3B Unit, is part of an overall transgressive succession which records the transition from non-marine to marine environments in the northeastern margin of the Amadeus Basin. This transgression was punctuated by episodic events of rapid sea level rise and periods of sea level fall. The resulting vertical succession consists of three Fourth-order deltaic sequences formed by the regular alternation of sand-prone, non-marine sediments with marine mud/sand-prone deposits that prograded northeast as the basin subsided. Unlike previous investigations, this study recognizes four distinct types of sandstone facies associations within the broad braid delta system that characterizes the Pacoota P3B Unit. Facies Association 1 records the depositional characteristics of a
distal braid plain that was dominated by episodic sheetflood events. Facies Association 2 reflects a sudden change in fluvial style from fine-grained sheetflood
lobes to a coarse to pebbly-grained braid-delta system during a short-lived regressive phase. With time, this basal braid-delta system evolved into a tide-influenced braid plain indicating a transgressive phase. Facies Association 3 records the abrupt change from fluvial to tidal processes. This association is interpreted as the product of a tide-dominated delta front that prograded northeast. The palaeoenvironment of Facies Association 4 is interpreted as the fill of a wide incised fluvial valley system,
which marked the end of fluvial sedimentation at the margin of the Amadeus Basin
during the Ordovician. This association is capped by the transgressive marine deposits
of the Pacoota P3A Unit. These four facies associations represent a complex network
of depositional environments that results from the deposition of superimposed sandy, deltaic systems affected by tidal currents. The vertical facies evolution is punctuated by erosional sequence boundaries.
The development of a detailed stratigraphic framework allows the Pacoota P3B Unit to be subdivided into five correlative intervals that define reservoir
compartments in the Mereenie Field. These reservoir compartments are bounded by key stratigraphic surfaces and represent the lowstand (LST), transgressive (I'ST) and highstand (HST) systems tracts of the Fourth-order sequences defined within the P3B Unit. Maximum reservoir quality is associated with amalgamated fluvial sandstones
that define the LST of each sequence. Marginal to impermeable reservoir characteristics occur within the tidally-influenced TST and HST. From base to top
reservoir intervals are: P3-250, P3-230, P3-190, P3-150 and P3-120/130. Of these, the lowstand P3-120/130, P3-230 and P3-250 Reservoir Intervals are the most
prolific producers. The transgressive to highstand P3-150 and P3-190 Reservoir Intervals are considered as not economically profitable for hydrocarbon exploitation.
Petrophysical characterization of lithofacies types observed in the succession indicate that within each compartment, depositional facies exert the primary control on reservoir properties. Flow units are associated with tabular, cross-bedded sandstones. Permeability barriers are associated with bidirectional cross-beds, parallellaminated sandstones, soft-sediment deformed sandstones and bioturbated beds. During transgression the upper part of the lowstand fluvial system was sheared off resulting in a transgressive surface capping the fluvial deposits. Reworked fluvial
sediments were redeposited as reversing tidal flows above the lowstand intervals. These deposits, interpreted as neap-spring tidal cycles, consist of alternating sand and
silt/mud and bioturbated beds. In this setting, intense bioturbation generate sediment mixing destroying the reservoir properties of this interval. Additionally the areally continuous and impermeable silt/shale intervals of the tidal deposits contributed to the vertical barriers to flow in the reservoir. This study illustrates how facies analysis and high resolution sequence
stratigraphy can be applied to improve reservoir characterization in fluvio-marine successions deposited before the existence of land vegetation. In the Mereenie Field, these concepts have been successfully applied to: i) recognize with confidence all correlative reservoir intervals and ·ii) identify, orientate and map the LST of the Fourth-order sequences which represent the major reservoir intervals of the P3B Unit.
Marine transgression
Transgressive
Stratigraphic unit
Outcrop
Sequence Stratigraphy
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