Basin Research - Volume 15, Issue 3, 2003

Two types of depositional sequences can be defined within the sequence stratigraphic framework: the parasequence and the high‐frequency sequence. Both sequences consist of stacked regressive and transgressive deposits. However, a parasequence forms under conditions of overall sea‐level rise, whereas a high‐frequency sequence forms as the sea level oscillates which results in typical forced regressive deposits during sea‐level fall. Both depositional sequences may develop over comparable temporal (10–100 kyr) and spatial (1–20 km wide and 1–40 m thick) scales. Numerical modelling is used to compare the architecture, preservation potential, internal volumes, bounding surfaces, condensed and expanded sections and facies assemblages of parasequences and high‐frequency sequences. Deposits originating from transgression are less pronounced than their regressive counterparts and consist of either preserved backbarrier deposits or shelf deposits. Shoreface deposits are not preserved during transgression. The second half of the paper evaluates in detail the preservation potential of backbarrier deposits and proposes a mechanism that explains the occurrence of both continuous and discontinuous barrier retreat in terms of varying rates of sea‐level rise and sediment supply. The key to this mechanism is the maximum washover capacity, which plays a part in both barrier shoreline retreat and backbarrier‐lagoonal shoreline retreat. If these two shorelines are not balanced, then the retreat of the coastal system as a whole is discontinuous and in time barrier overstep may take place.

The distribution of detrital mineral cooling ages in river sediment provides a proxy record for the erosional history of mountain ranges. We have developed a numerical model that predicts detrital mineral age distributions for individual catchments in which particle paths move vertically toward the surface. Despite a restrictive set of assumptions, the model permits theoretical exploration of the effects of thermal structure, erosion rate, and topography on cooling ages. Hypsometry of the source‐area catchment is shown to exert a fundamental control on the frequency distribution of bedrock and detrital ages. We illustrate this approach by generating synthetic ⁴⁰Ar/³⁹Ar muscovite age distributions for two catchments with contrasting erosion rates in central Nepal and then by comparing actual measured cooling‐age distributions with the synthetic ones. Monte Carlo sampling is used to assess the mismatch between observed and synthetic age distributions and to explore the dependence of that mismatch on the complexity of the synthetic age signal and on the number of grains analysed. Observed detrital cooling ages are well matched by predicted ages for a more slowly eroding Himalayan catchment. A poorer match for a rapidly eroding catchment may result from some combination of large analytical uncertainties in the detrital ages and inhomogeneous erosion rates within the basin. Such mismatches emphasize the need for more accurate thermal and kinematic models and for sampling strategies that are adapted to catchment‐specific geologic and geomorphic conditions.

The South Caspian Basin has accumulated a sedimentary succession ∼20 km thick. Roughly half of this was deposited in the last 5.5 Ma, mainly in the largely lower Pliocene, fluvio‐lacustrine Productive Series, which is also the principal hydrocarbon reservoir succession in the basin. Heavy mineral data identify different sediment sources for both Productive Series sandstones and modern river sands. Lesser Caucasus sediment was supplied by the Palaeo‐Kura into the western part of the South Caspian Basin. Productive Series strata in the north of the basin were supplied by the Palaeo‐Volga, and represent a mixture of sediment from the Greater Caucasus and Russian Platform/Urals. Greater Caucasus sand input to the Palaeo‐Volga increased at the start of deposition of the Pereriva Suite, which is an important reservoir subunit of the Productive Series. We interpret this provenance shift as indicating enhanced uplift and exhumation of the Greater Caucasus within the Pliocene, during regional re‐organization of the Arabia–Eurasia collision, although late Cenozoic climate changes may have played a role.

Analysis of physical properties measured on cores and on discrete samples collected by the Ocean Drilling Programme (ODP) Leg 178 on the Pacific margin of the Antarctic Peninsula reveals anomalous down‐hole curves of porosity, density, water content, and P‐wave velocity. These indicate an overall trend of increasing porosity with depth and suggest that the drifts are mostly undercompacted. In one of the two boreholes analysed, a sharp decrease in porosity, matching increasing bulk sediment density and increasing compressional velocity occurs towards the base of the hole, which corresponds to a bottom‐simulating reflector in the seismic section. Analysis of seismic reflection, down‐hole logging, geotechnical and mineralogical data from two drilling sites indicates that the observed anomalous consolidation trends are a consequence of the presence of biogenic silica (diatom and radiolarian skeletons) even with a small to moderate amount. Above the bottom‐simulating reflector, intergranular contacts among whole or broken siliceous microfossils prevent normal sediment consolidation. Diagenetic alteration of biogenic opal‐A to opal‐CT causes a dramatic reduction of intra‐ and interskeletal porosity allowing sediments to consolidate at depth. This results in overpressuring and a decrease in the effective stress. Excess fluids are expelled towards the sediment surface through near vertical, small throw normal faults extending from the diagenetic front to the seafloor and affecting the stability of the submarine slope in the form of gravitational creep along a weakened surface.

This work shows how physical properties of shallow fine‐grained marine sediments can be analysed as basin‐wide indicators of biogenic silica abundance. The diagenetic alteration of siliceous microfossils is a possible cause of slope instability along world continental margins where bottom‐simulating reflectors related to silica diagenesis are present at a regional scale.

In areas of broadly distributed extensional strain, the back‐tilted edges of a wider than normal horst block may create a synclinal‐horst basin. Three Neogene synclinal‐horst basins are described from the southern Rio Grande rift and southern Transition Zone of southwestern New Mexico, USA. The late Miocene–Quaternary Uvas Valley basin developed between two fault blocks that dip 6–8° toward one another. Containing a maximum of 200 m of sediment, the Uvas Valley basin has a nearly symmetrical distribution of sediment thickness and appears to have been hydrologically closed throughout its history. The Miocene Gila Wilderness synclinal‐horst basin is bordered on three sides by gently tilted (10°, 15°, 20°) fault blocks. Despite evidence of an axial drainage that may have exited the northern edge of the basin, 200–300 m of sediment accumulated in the basin, probably as a result of high sediment yields from the large, high‐relief catchments. The Jornada del Muerto synclinal‐horst basin is positioned between the east‐tilted Caballo and west‐tilted San Andres fault blocks. Despite uplift and probable tilting of the adjacent fault blocks in the latest Oligocene and Miocene time, sediment was transported off the horst and deposited in an adjacent basin to the south. Sediment only began to accumulate in the Jornada del Muerto basin in Pliocene and Quaternary time, when an east‐dipping normal fault along the axis of the syncline created a small half graben. Overall, synclinal‐horst basins are rare, because horsts wide enough to develop broad synclines are uncommon in extensional terrains. Synclinal‐horst basins may be most common along the margins of extensional terrains, where thicker, colder crust results in wider fault spacing.

Most authors suggest that the main contraction phases in the southern Central Andes started in the Late Miocene. Along the flat‐slab segment, deformation has progressively involved basement and a broken foreland has developed. Recent work suggests that construction of the Andes by late Neogene shortening may have been controlled by lithospheric thinning and crustal structure generated during mid‐Tertiary times in the Southern and Central Andes. Exposures at the eastern border of the Famatina Ranges in western Argentina in the flat‐slab segment document basement involved extension of approximately this age. The Del Abra Formation, the lower unit of a major Andean synorogenic cycle (Angulos Group), reveals a distinct and previously unrecognized early Middle Miocene tectonic event. This is suggested by a 505‐m‐thick thinning‐ and fining‐upward megasequence. Dominantly conglomeratic facies record a continuous progression from fault‐scarp‐related high‐gradient colluvium to relatively distal terminal‐fan facies. The fining–thinning upward megasequence characterizes progressive scarp backstepping and decreasing relief after active extension. Interpretation of the stratigraphic fill and the associated structure (high‐angle hinterland‐dipping fault) favours tectonic inversion of an originally normal fault. This allows reappraisal and new understanding of the early‐stage architecture of the Central Andean foreland. Early Middle Miocene extension may have had an important bearing on the later evolution of the broken foreland.

The China–Mongolia border region contains many late Mesozoic extensional basins that together constitute a regionally extensive basin system. Individual basins within the system are internally composed of a family of sub‐basins filled with relatively thin sedimentary piles mostly less than 5 km in thickness. There are two types of sub‐basins within the basins, failed and combined, respectively. The failed sub‐basins are those that failed to continue developing with time. In contrast, the combined ones are those that succeeded in growing by coalescing adjacent previously isolated sub‐basins. Thus, a combined sub‐basin is bounded by a linked through‐going normal fault that usually displays a corrugated trace on map view and a shallower dip on cross‐section. Along‐strike existence of discrete depocenters and alternation of sedimentary wedges of different types validate the linkage origin of combined sub‐basins. Localized high‐strain extension resulted in large‐amount displacement on linked faults, but contemporaneously brought about the cessation of some isolated fault segments and the formation of corresponding failed sub‐basins in intervening areas between active linked faults. Some combined sub‐basins might have evolved into supradetachment basins through time, concurrent with rapid denudation of footwall rocks and formation of metamorphic core complexes in places. A tectonic scenario of the broad basin system can be envisioned as an evolution from early‐stage distributed isolated sub‐basins to late‐stage focused combined or/and supradetachment sub‐basins bounded by linked faults, accompanied by synchronous cessation of some early‐formed sub‐basins.

Initiation of the late Mesozoic extension is believed to result from gravitational collapse of the crust that had been overthickened shortly prior to the extension. Compression, arising from collision of Siberia and the amalgamated North China–Mongolia block along the Mongol–Okhotsk suture in the time interval from the Middle to Late Jurassic, led to significant shortening and thickening over a broad area and subsequent extensional collapse. Pre‐ and syn‐extensional voluminous magmatism must have considerably reduced the viscosity of the overthickened crust, thereby not only facilitating the gravitational collapse but enabling the lower‐middle crust to flow as well. Flow of a thicker crustal layer is assumed to have occurred coevally with upper‐crustal stretching so as to diminish the potential contrast of crustal thickness by repositioning materials from less extended to highly extending regions. Lateral middle‐ and lower‐crustal flow and its resultant upward push upon the upper crust provide a satisfying explanation for a number of unusual phenomena, such as supracrustal activity of the extension, absence or negligibleness of postrift subsidence of the basin system, less reduction of crustal thickness after extension, and non‐compression‐induced basin inversion, all of which have been paradoxical in the previous study of the late Mesozoic basin tectonics in the China–Mongolia border region.

Isopach and sedimentary facies maps of Upper Devonian (upper Frasnian and lower Famennian) strata deposited in a part of the central Appalachian foreland basin (eastern United States) during the Acadian orogeny show a significant change in depositional style over time. Maps of two successive upper Frasnian intervals show steady thickening to the east towards the hinterland. Coarser‐grained sediment was deposited in distinct tongues in front of the Augusta lobe, a previously recognized locus of sediment input in the central Appalachian basin. Maps of two subsequent lower Famennian stratigraphic intervals show distinct depocentres in the study area. Famennian strata thin eastward (by about 50%) over a distance of about 90 km from these depocentres to the limit of mapping at the Allegheny structural front. This is towards the Acadian sediment source and in contrast to general Upper Devonian thickening in that direction. The axes of these lower Famennian depocentres are stacked on top of each other. Also, coarser‐grained lower Famennian sediment is concentrated in strike trends just east of the axes of the depocentres, and no coarser tongues exist in front of the Augusta lobe, in contrast to the underlying (upper Frasnian) strata.

The duration of each of the four study intervals is estimated to be between 0.5 and 3.0 Myr. The early Famennian depocentres may be in a back‐bulge basin, with a forebulge uplifted to the east of the study area. Earlier deposition may have occurred in a basin with a subtle, subdued, and longer wavelength forebulge (perhaps located west of the study area). Previously published regional isopachs of Upper Devonian strata suggest that the main axis of subsidence of the Acadian foreland basin (foredeep depozone) at this time was over 350 km east of the study area. Examination of published quantitative flexural models of other foreland basins with flexural rigidities close to published rigidities calculated for the Appalachian basin suggests that the proposed back‐bulge basin is in the correct location, relative to the suggested position of the foredeep at that time. Several previously recognized structural features of the northern Appalachian basin support the interpretations presented herein. Much of the Acadian foreland basin may be eroded in the central Appalachian basin. The present study demonstrates the difficulties in recognizing foreland basin depozones in partially preserved orogens.