Stefan Back

The deep-water fold-and-thrust belt offshore NW Borneo: Gravity-driven versus basement-driven shortening

The deep-water region offshore NW Borneo is an active fold-and-thrust belt that hosts a signifi cant number of proven hydrocarbon accumulations. In the past, two mechanisms have been discussed as primary control for Neogene to Holocene folding and thrusting in this deep-water province: (1) basement-driven crustal shortening and (2) gravity-related delta tectonics. In this study, new, balanced interpretations of regional, crustal-scale, depth-migrated, twodimensional (2-D), multichannel seismicrefl ection profi les are presented that provide for the fi rst time quantitative data on tectonic shortening throughout the entire deep-water fold-and-thrust belt of NW Borneo. We use our tectonic restorations to compare the amount of deep-water shortening on the NW Borneo slope to the amount of extension across the NW Borneo shelf. A key result of this balancing study is the observation that Pliocene to Holocene gravity-driven shortening decreases from south to north, while the total amount of shortening increases slightly to the north. Consequently, the amount of purely basement-driven compression along NW Borneo is strongly inferred to increase toward the north. Because most of the shortening is late Pliocene and younger, we interpret the tectonic shortening to be ongoing.

Estimating hinterland exhumation from late orogenic basin volume, NW Borneo

A new approach for restoring sediment volumes onto the sediment source area to estimate uplift timing and magnitude is discussed and used in the context of late Tertiary basin and topographic development in NW Borneo. Sediment volumes for the Baram Deltaic Province were estimated for four time periods (latest Early, Middle and Late Miocene, and Pliocene–Recent), using 2D and 3D seismic horizons, wells and outcrops. Volume restoration onto the palaeo-sediment source area determined exhumation amount at the drainage divide (c. 5 km from 17 Ma to Recent) and provided a reasonable match with other denudation estimation methods (cooling ages and an older, regional sediment restoration study using 1979 vintage sediment isopach maps). Restoration took into account: (1) changing sediment densities during the erosion–deposition cycle; (2) changing area of the sediment source province with time (e.g. changing shoreline location as a result of eustacy, uplift and delta progradation, tectonic shortening); (3) uplift and partial erosion of Baram Deltaic Province sediment. Denudation at the drainage divide for the Middle Miocene, Late Miocene and Pliocene–Recent has proceeded at a similar rate for each period. Initial uplift of central Borneo has been attributed to buoyancy of thinned continental crust that jammed the subduction zone under NW Borneo in the Early Miocene. However, the absence of decay in the erosion rates with time from the Middle Miocene to Recent suggests operation of an additional uplift mechanism that may be related to delamination of mantle lithosphere; slab detachment is the favoured explanation.

Quaternary Depositional Systems in Northern Lake Baikal, Siberia

New high‐resolution seismic reflection data from northern Lake Baikal and detailed land‐based morphological and sedimentological data reveal a variety of coarse‐grained deposits in the northern Baikal Rift. Each depositional facies is assigned to a specific structural domain. Alluvial fans (onshore) and small fan deltas (offshore) dominate the western border‐fault flank. Glacial deposits (onshore) and large glacio‐lacustrine fans (offshore) characterize the eastern flexural margin. In the north, the axial rift termination comprises a large fluvial delta. This distinct pattern of depositional environments reflects the pronounced asymmetry of the rift and emphasizes the role of rift structure in controlling drainage and the location and type of rift‐basin fill. Climate, however, exerts a profound influence on the generation and availability of coarse‐grained material. This is best documented along the eastern rift margin where Pleistocene valley glaciers advanced at >50, 40–35, and 26–13 ka from the high rift shoulders down to, and beyond, the present shore into the Baikal Basin. During these phases, large volumes of glacial sediment were introduced into the basin, accumulating in extensive glacio‐lacustrine outwash fans. About one‐third of the seismically penetrated rift fill is glacial‐outwash material derived from the eastern shoulder. Under present‐day nonglacial conditions, however, the fans are inactive because of insufficient sediment supply; this documents the importance of major climatic fluctuations with respect to the magnitude of sediment discharge and accumulation. The sedimentary history also underscores the modifying effects of climate‐driven processes on the typical structure‐dominated aspects of rift sedimentation.

Stratigraphic development of synkinematic deposits in a large growth-fault system, onshore Brunei Darussalam

Kilometre-scale synsedimentary faults associated with well-preserved sedimentary successions on both hanging-wall and footwall sides are rarely seen in outcrop. However, one such example is found in a Middle Miocene deltaic sand–shale sequence exposed along the Jerudong Anticline in onshore Brunei Darussalam. Integration of detailed outcrop information with regional geophysical subsurface data documents that: (1) initiation of the Jalan Tutong growth fault was related to delta-front instability and failure; (2) sediment loading under shallow-marine conditions maintained syndepositional faulting; (3) growth-fault abandonment coincided with a significant basinward shift of deposition and subaerial exposure. High-resolution analysis of facies characteristics, stratal expansion and throw development within the studied growth sequence indicates that differential fault movement strongly influenced facies composition, sedimentary geometry and bedding characteristics: sandstone-poor shelf to lower-shoreface successions developed during periods of rapidly increasing fault movement, whereas sandstone-rich, amalgamated upper-shoreface deposits preferentially formed during intervals of constant or decreasing throw development. The data and interpretations presented may help to focus attention on the complex interplay between tectonics and sedimentation associated with the growth and decay of deltaic faults by providing for the first time an outcrop-based view on a feature documented in similar scale only by geophysical subsurface data.

Asymmetric late Pleistocene glaciations in the North Basin of the Baikal Rift, Russia

New morphological, sedimentological, and radiocarbon data of Quaternary deposits from the northern Baikal Rift document at least three extensive late Pleistocene glaciations at >50 ka, 40–35 ka, and 26–13 ka, respectively. During these periods valley glaciers advanced from the high rift shoulders (>2500 m) down to the present-day coast of Lake Baikal (456 m); in places, the glaciers advanced beyond the present-day shoreline into the lake basin as indicated by large moraines exposed in steep cliffs along the lakeshore. Furthermore, stratigraphic relations between lacustrine sequences and glacial deposits, as well as 14C-ages of two palaeo-shorelines 10 and 4 m above the present lake level, show that the last and the penultimate glaciations were followed by relative lake highstands. These land-based observations document a significant influence of late Pleistocene glaciations on the sedimentary development of this lacustrine rift basin. The northern Baikal Rift is characterized by an asymmetric half-graben geometry that controls river drainage and sedimentation. This asymmetry greatly influenced the development and spatial distribution of Pleistocene glaciers on both rift margins. On the western flank, tilted fault blocks parallel to the rift served as topographic barriers for valley glaciers advancing toward the lake basin; only in two locations glaciers could reach the present-day coast. On the eastern rift flank, in contrast, structure and drainage conditions allowed the development of numerous consequent streams providing thoroughfares and space for recurrent valley glaciations. Thus, the glaciers on the eastern flank could advance beyond the present-day lakeshore into the lake basin in numerous locations. These observations indicate that asymmetric rift structure in combination with the effects of global cooling were the important controls on the Pleistocene glacial development of the Baikal Rift.

Autotracking of faults on 3D seismic data

Manual fault mapping in 3D seismic interpretation is labor-intensive and time-consuming. Complex fault geometries and the distortion of the seismic signal close to faults complicate full automation of the fault-mapping process. We present a semiautomatic fault-tracking method for 3D seismic data that consists of fault highlighting followed by model-based fault tracking. Fault highlighting uses log-Gabor filters for emphasizing oriented amplitude discontinuities at faults in the presence of noise. Subsequent fault tracking fits an active contour to the highlighted fault voxels. The active contour searches for a connected, smooth curve which fits the data and disambiguates misleading or missing information. The fault tracker requires the interpreter to place the active contour close to a fault on one initial seismic inline (2D pick). The active contour deforms to the closest amplitude dis-continuity highlighted. This tracking result is then projected forward to the next inline, providing an initial fault pick on this section that is again optimized by the active contour. Tracking results on a series of successive seismic sections, finally, constitute a 3D fault surface. User interaction is solely required for an approximate fault pick on the first inline, and in cases where the fault line is lost due to insufficient signal. Use of the autotracker prototype provides a fast solution for the mapping of complete 3D fault surfaces of constant dip, and for the automated tracking of fault portions within distinct dip domains, if fault surfaces are curved (i.e., listric). The method was applied to a series of high-quality reflectivity sections extracted from a 3D seismic volume from shallow-offshore Nigeria, with the tracking results (generated within seconds) comparing well with manually interpreted fault surfaces.

Identification of a fossil wood specimen in the Red Sandstone Group of southwestern Tanzania: stratigraphical and tectonic implications

A piece of silicified fossil wood was found in the middle part of the Red Sandstone Group of the Songwe-Kiwira area, north of Lake Malawi (Nvasa) in the Western Branch of the East African Rift System. It is identified as Pahudioxylon Chowdhury et el., 1960. Until now, Pahudioxylon has been strictly restricted to the Cenozoic. The occurrence of Pahudioxylon in the Red Beds Formation supports a Cenozoic (Miocene) age rather than a Mesozoic age for this formation. Comparison with a collection of modern species gives information about the environmental conditions of deposition of the Red Sandstone Group. A discussion follows on the significance of the Red Sandstone Group during the Rukwa-Malawi Rift development.

3D seismic geomorphology and sedimentology of the Chalk Group, southern Danish North Sea

Abstract: Classically, the North Sea Chalk is interpreted as having been deposited under quiet, homogeneous pelagic conditions with local redeposition in slumps and slides. Recent observations of highly discontinuous reflection patterns on 2D and 3D seismic reflection data from the NW European Chalk Group have led to a revision of some general ideas of chalk deposition, with the suggestion that long-lived, contour-parallel bottom currents exerted a primary influence on the development of intra-chalk channels, drifts and mounds. This study proposes an alternative explanation for the formation of selected intra-chalk seismic and stratal discontinuities, interpreting these as being caused by gravity-driven processes that developed in response to intense syndepositional tectonics. Submarine mass-transport systems identified in the study area include large-scale slumps, slides, debris flows and turbidites. The last occur in sinuous channel systems flanked by large master levees, with the channel fill exhibiting well-developed secondary banks and overbanks on the outer bends of the channel thalweg. This first documentation of channelized density-flow deposits in the North Sea Chalk has important consequences for the interpretation and prediction of redeposited chalk units, emphasizing at the same time the strength of detailed 3D seismic discontinuity detection for subsurface sedimentary-systems analysis.

Depositional Environment and Sequence Stratigraphy of Miocene Deltaic Cycles Exposed along the Jerudong Anticline, Brunei Darussalam

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.

Temporal and lateral variation in the development of growth faults and growth strata in western Niger Delta, Nigeria

This study examines eight syndepositional faults and syntectonic sediments in five major fault blocks in western Niger Delta, offshore Nigeria, on three-dimensional seismic data. The initiation, lateral growth and retreat, periods of activity and quiescence, and decay of faulting around these blocks can be ascertained by analyzing a series of time-structure and isopach maps. The study area can be subdivided into three structural zones. The first structural zone is a northwestern zone characterized by a major counterregional growth fault in the deep subsurface. This deep-seated structure is superposed by an array of younger regional growth faults displacing a kilometer-thick sedimentary overburden that accumulated on the former footwall. The second structural zone is a central to eastern zone that seems mostly unaffected by young deltaic faulting. This zone is characterized by the thinnest sedimentary record of the study area. The third structural zone is a southeastern zone that is dominated by a large, listric, backstepping master fault zone associated with a kilometer-scale rollover system. Regional structural and stratigraphic analyses document an apparently strong relationship between syntectonic sedimentation and syndepositional fault activity in that phases of significant fault activity, lateral fault growth, and fault migration concur with major depositional phases; in turn, areas and intervals characterized by the least sediment accumulation also record the lowest fault activity. However, one particularity of the studied system is that it underwent at least one period of seaward fault progression that coincided with a backstepping of faulting on the landward side. Although the forward stepping of faulting near the delta front can be interpreted as the consequence of the progressive loading during delta progradation, the contemporaneous backstepping of faulting further inboard likely reflects the sustained lateral growth of mature deltaic faults into previously undeformed proximal parts of the depocenter. The results of this study, thus, document that although an apparent correlation with the superimposed depositional system exists on a regional scale, inboard deltaic faults may persist to grow irrespective of sedimentary loading. The recognition of such fault trends is particularly important for estimating the influence of late-stage fault movement on hydrocarbon migration or the discovery of subtle fault-controlled hanging-wall reservoirs.