Dan Bosence

Marine Palaeoenvironmental Analysis from Fossils

The approach of the book is analytical rather than taxonomic, concentrating on a range of techniques. The common thread, however, is that it is palaeontological material that is being considered, whether it be identifiable body fossils, trace fossils, distinctive fossil associates, diagenetically unaltered material or organic compounds. Using a number of methods, and comparing their results, allows di f ferent environmental controls to be isolated and provides more information on the record of past environmental parameters.

Geological evolution of the southeastern Red Sea Rift margin, Republic of Yemen

The tectonic evolution of the southeastern margin of the Red Sea Rift in western Yemen has been investigated using a multi-disciplinary field study of an east-west transect between Al Hudaydah and Sana9a. Slow subsidence of up to 1 km occurred over the area during a 100 m.y. period before rifting. There was a major episode of flood volcanism between ca. 30 and 20 Ma, and important extensional faulting began after the eruption of the volcanic rocks and ceased before middle to late Miocene sediments and volcanic rocks were deposited unconformably on top of rotated fault blocks on the coastal Tihama Plain. Surface uplift has produced the Yemen highlands, whose highest peak reaches an elevation of 3660 m. This is attributed to plume heating and eruption of >3000 m of volcanic rocks. Apatite fission-track ages indicate early to middle Miocene exhumational cooling ages, postdating the major volcanic phase and contemporaneous with rifting. Volcanism was accompanied by emplacement of subvertical dike swarms, which generally strike north-northwest to northwest, broadly parallel to the Red Sea coastline. Major faults indicate northeast-southwest-directed extension. Large granitic sheets and plutons (up to 25 km wide) intruded the volcanic rocks. Approximately 30 km of extension has taken place across a 75-km-wide zone (β = 1.7) in 6-8 m.y. The relative timing of volcanism followed by extension and uplift does not fit conventional models of passive or active rifting. We suggest that the proto-Red Sea Rift was caused by regional plate stresses that exploited lithospheric weakening caused by the Afar plume. Appreciable doming only occurred after the main episode of volcanism, which suggests that magmas extruded before maximum thermal expansion of the lithosphere took place.

Quantitative approaches to palaeozonation and palaeobathymetry of corals and coralline algae in Cenozoic reefs

Abstract The value of quantitative surveys of ancient reef slopes for palaeobathymetric analysis is reviewed. Reefs are selected for palaeobathymetric analysis because they are characterized by in situ preservation of benthic communities which are often depth related. In addition, if the reef crest and slope are preserved then ancient water depths can be measured. The zonation of living reefs has been measured using semi-quantitative phytosociological methods as well as a range of plot and plotless techniques, but there is little agreement as to a single best method. For this study a modified line intercept transect method is selected as it may be used on both living reefs and on various types of outcrops of fossil reefs. This method minimizes problems arising from outcrop conditions, is faster than quadrat methods, can be used to assess different frame-building taxa, matrix, cement and porosity along ancient reef surfaces, and is also frequently used in studies of living reefs. The zonation of corals and coralline algae on living and fossil reefs can be characterized by three parameters: framework density, diversity and species abundance. The framework density of reef corals is normally greatest on external reef slopes between 1 and 30 m and decreases into shallow reef crest environments, and towards shorelines. Coralline algal frameworks are highest in reef crest environments and may also replace coral frameworks in deeper reef-slope environments. Variations in diversity are complex and relate to biogeographical constraints as well as local variations of physical factors and biological factors. The distribution of reef-building assemblages is often expressed as a zonation which may relate to depth but also to hydraulic energy. Examples of coral and coralline algal zonation are presented and discussed from Indo-Pacific and Caribbean reefs. Three case studies are presented based on the authors’ work on reef zonation. The Seychelles reefs (Indian Ocean) are used to illustrate depth and water energy related zonation of present-day living corals. Coralline algal zonation is illustrated from present-day St Croix reefs (Caribbean), together with preserved zones in sections through these reefs. Coral and coralline algal zonation on fossil reefs are characterized using our modified line intercept transect technique on exposures from late Miocene reefs of Mallorca (Spain). This review of reef zonation and introduction of a new technique for surveying ancient reef communities to establish depth related zones has important advantages over previous qualitative approaches. Direct measurements of palaeodepths in the field make subjective palaeobathymetric interpretations unnecessary and avoids the problems of assuming uniformitarianism. In situ organism assemblages are sampled which minimizes the effects of taphonomic changes on community composition. The technique allows the quantification of framework density, diversity, species abundance and zonation to be compared between different fossil reefs and between living and fossil reefs for the first time.

Computer modeling the internal architecture of carbonate platforms

A numerical computer model is described that calculates the internal architecture of carbonate platforms in response to varying values of carbonate production, subaerial and submarine erosion, sediment redeposition, and sea-level changes. The computer-generated sections closely resemble large-scale outcrops and interpreted seismic profiles through carbonate platforms. Stillstand and transgressive sequences have prograding and downlapping platform geometries with lagoons developing in transgressive systems. Regressive sequences have downlapping clinoforms and erosional upper surfaces. Glacioeustatic scale cycles have a major control on platform geometry with erosional sequence boundaries developing during low stands and platform drowning occurring during transgressive periods. Lowstand downlapping wedges are minor features when compared with clastic systems, and major progradation and downlap of slope deposits develop with transgressions and flooding of platform tops. Carbonate erosion rates are varied and have an important effect on the morphology of foodback surfaces, which have a major control on platform top production. The computer program contributes to the analysis of carbonate systems in two ways: it gives a visual picture of the quantitative effects of the many parameters controlling carbonate geometries, and it aids quantitative analysis of the architectures and time scales of ancient outcrop or seismic sequences.

Stratigraphy and structure of the Maltese graben system

The Maltese graben system is a series of Miocene–Quaternary extensional basins located in the foreland of the Sicilian Apennine–Maghrebian fold and thrust belt. The tectono-sedimentary development of the graben was determined from the first fully integrated analysis of the area to include onshore field and hydrological borehole data, together with offshore reflection seismic data and exploration wells. The NW–SE- and ENE–WSW-trending rifts have a coeval, four phase tectono-sedimentary evolution revealed by a succession of platform and pelagic carbonates. The pre-rift phase (>21 Ma) is followed by a early syn-rift phase (21–6 Ma) characterized by evidence for relatively minor extensional faulting including the development of neptunian dykes. Major fault activity occurred during the succeeding late syn-rift phase (<5 Ma) which is characterized by growth faults and fault control on facies and the areas of deposition/non-deposition. This was followed by passive graben infilling during the post-rift phase (probably < 1.5 Ma). Despite low measured extension factors (β = 1.03–1.17) across the rift structures, major basin bounding extensional faults have throws of up to 2.2 km. Fault slip data indicate that both NW–SE and ENE–WSW rift trends were generated in response to N–S stretching. These results are in accord with Argnanis (1990) description of a major N–S orientated transfer fault located 50 km west of the present study area.

Architecture and depositional sequences of Tertiary fault-block carbonate platforms ; an analysis from outcrop (Miocene, Gulf of Suez) and computer modelling

Abstract This study combines outcrop with seismic data, and tectonostratigraphic modelling to characterise the stratigraphy of Tertiary fault-block carbonate platforms. Seismic sections from the Middle East and Southeast Asia indicate that carbonate platforms commonly develop on fault-blocks in the late syn-rift to post-rift stage of basin evolution, they are characterised by retrogradational or drowning morphologies with deep water clastics/evaporites sealing the platforms. A new tectono-stratigraphic modelling program simulates the development of carbonate platforms on domino-style fault blocks and predicts a distinctive tectonic control on the evolving syn-rift stratigraphy. Detailed field work from a three-dimensionally exposed Miocene syn-rift platform from the Gulf of Suez (Gebel Abu Shaar, Egypt) shows the existence of unconformity-bound depositional sequences which are predicted by the program and are interpreted to have formed in response to fault-block rotation. The sequences are characterised by synchronous hangingwall subsidence and footwall uplift and erosion. The seismic images, the modelling and the outcrop study provide different levels of information on fault-block platforms and their characteristic stratigraphy. The scale of the modelling is of particular value as it provides a link between the seismic-scale imaging and outcrop data.

The timing of magmatism, uplift and crustal extension: preliminary observations from Yemen

Abstract The Red Sea and the Gulf of Aden form young, oceanic rift basins, situated between the diverging African and Arabian plates and bordered by highly elevated, volcanic margins. Yemen in the southeastern Red Sea, was once centred over the Afar plume/triple-junction (c. 30 Ma) forming part of the Arabian ‘passive’ margin. The present high elevation of the Afro-Arabian rift-flanks (up to 3.6 km as in Yemen), is the combined result of a number of endogenic rift processes which served to generate both the initial crustal uplift and also preserve the elevated topography. A further isostatic response generating uplift is likely to have been driven by differential erosion of the rift-flanks. However, the sedimentary record of the pre-Jurassic to early Tertiary period provides little evidence for major changes in relief or elevation. Furthermore, structural and volcanological observations indicate that most of the crustal extension occurred during mid-late Tertiary. The voluminous Oligo-Miocene basalt-rhyolite magmatism of Yemen was not apparently associated with pre-volcanic (> 30 Ma) uplift despite the commonly held belief that the Afar plume existed beneath the region 30 Ma ago. Geological data point to an episode of uplift that occurred after the initiation of magmatism. Fission track data indicate that uplift related exhumation postdates magmatism by some 10–15 Ma, perhaps the amount of time needed to change the thermal character of the Pan-African lithosphere above the Afar plume. A sequence of magmatism followed by synchronous crustal extension and uplift for Yemen does not fit with the traditional categories of active (uplift-magmatism-rifting) and passive (rifting-uplift-magmatism) rifting. Clearly such end-member models do not simply apply to the Red Sea or the Great Basin of the western USA where a period of tectonic quiescence, followed by post-volcanic extension and uplift (1 km), post-dated the Oligo-Miocene ignimbrite flare-up.

Carbonate production and offshore transport on a Late Jurassic carbonate ramp (Kimmeridgian, Iberian basin, NE Spain): evidence from outcrops and computer modelling

Abstract The complex interplay between shallow-water carbonate production, pelagic sedimentation, and sediment erosion and redeposition on a Kimmeridgian carbonate ramp is analysed from field data and computer modelling. Field data come from reconstructed cross-sections near Zaragoza and near Teruel, NE Spain. Inner ramp areas are dominated by coral patch reefs, ooid shoals and bioclastic packstones and grainstones, whereas middle ramp areas are typified by tempestites and pinnacle reefs. Outer ramp areas are dominated by carbonate muds and marls. The large-scale stratigraphic features of the ramp are simulated using the forward modelling computer program carbonate 6. A good match is found between real and simulated stratigraphic thicknesses along the two sections, large-scale internal stratigraphic geometries, amounts of aggradation and progradation, and the location of sequence boundaries, flooding surfaces and systems tracts in the two modelled cross-sections. To investigate the origin of the offshore carbonate mud, two synthetic stratigraphies, which both closely replicate the overall ramp geometries, were generated by computer simulation. These two simulations are based on two different hypotheses that had not been resolved from field investigation: (1) that most of the outer ramp mud was derived from erosion and redeposition from inshore areas; (2) that most of this mud originated from pelagic sedimentation. Detailed comparison between the simulated occurrence of sedimentary facies (characterized in the program by depth and process of deposition) and the real facies occurrence indicates that the closest match is given by hypothesis (1) where most of the offshore mud is produced by resedimentation from inshore areas. Offshore transport and resedimentation is thought to have been favoured by the windward orientation of the ramp, and offshore transport was highest during periods of sea-level highstands.

Deformation and sedimentation around active Miocene salt diapirs on the Tihama Plain, northwest Yemen

Abstract The Al Salif and Jabal al Milh salt diapirs of Miocene age cut through a 4–5 km thick overburden of Miocene to Recent sedimentary rocks in the southern Red Sea. The Al Salif diapir is part of a north-south oriented diapiric wall which has caused updoming of the overburden and active extensional faulting. The halite is folded into tight to isoclinal subvertical folds, with largescale rafts of interbedded gypsum-anhydrite boudinaged on the fold limbs. There is no evidence for large-scale overburden stoping or injection of salt into overlying faults and fractures, and the upward movement of the salt dome is probably caused by the overburden being forced aside and sliding off the dome. The upper surface of the salt is an undulating smooth surface which was exposed at the sea-floor during the Quaternary. Coral reefs with 14C ages of 3700 a have been raised up to an elevation of 17 m above present-day sea-level, giving a surface uplift rate of 4.6 mm a−1. At the Jabal Al Milh diapir, recumbent folding and thrusting are the main deformation features observed in the siltstones and gypsum layers of the overburden. The overburden has been rotated to the vertical at least 200 m from the diapir walls. The folding and thrusting is interpreted to be caused by overflow of salt in a namakier. This study shows that overburden sediments above the Al Salif diapir extended to allow sediments to slide from the salt dome crest by listric faulting, whereas sediments underlying the Jabal al Milh salt glacier were sheared and shortened in the horizontal direction.

The Tertiary evolution of South Sulawesi: a record in redeposited carbonates of the Tonasa Limestone Formation

Abstract South Sulawesi, situated at the junction of three major plates and with an almost complete Tertiary sequence, is an ideal location in which to study syntectonic sedimentation. Redeposited carbonate facies of the lower/middle Eocene to middle Miocene Tonasa Limestone Formation in the Barru area prove to be reliable indicators of tectonic activity. South of the Barru area contemporaneous carbonate sediments formed on a relatively stable shallow-water platform, known as the Tonasa Carbonate Platform. Redeposited carbonate facies and interbedded marls from the Barru area are described and interpreted in this study. The immaturity and provenance of clasts indicate that the redeposited facies were derived from the faulted northern margin of the Tonasa Carbonate Platform. A relay ramp between at least two major NW-SE trending faults is the inferred configuration of this margin. Three main phases of faulting are indicated by the redeposited facies: late Eocene to early Oligocene, middle Oligocene and early to middle Miocene. This is consistent with other outcrop and seismic data from the region and with the inferred plate tectonic situation during the Tertiary.