ALBERT

Description: The contributions in this volume originally formed a set of presentations at a conference on the same theme held in Mallorca, Spain in 2006. The goal of this conference was to investigate the potential to develop age or architecture specific reference models for carbonate systems and reservoirs similar to those successfully developed for siliciclastic systems. The conference focused on the Mesozoic and Cenozoic carbonate sequences of the Mediterranean and Middle East. These sequences were chosen for a number of reasons. Firstly, they represent sequence development in a variety of basin settings within a contiguous geographical entity, the former NeoTethys Ocean (Fig. 1). The sequences were also formed predominantly within tropical or sub-tropical climatic zones (cf. Schlager 2003). Finally, the high levels of industry and academic interest in the region have generated many excellent multidisciplinary studies of these sequences, based on both the comprehensive datasets of hydrocarbon-bearing strata and the excellent surface exposures in the region. In general, all Earth models underestimate the complexity of the subsurface and hence are intrinsically inaccurate. The value of developing such models, however, lies in the improved understanding of the processes controlling sequence development gained from their application (e.g. Ahr 1973; Read 1985; Burchette & Wright 1992; Handford & Loucks 1993; Pomar 2001; Bosence 2005). Extrapolating from data rich examples into areas where data coverage is poorer obliges us to distil out the generic from the specific and to propose appropriate subsurface analogues...

Description: A regional sequence stratigraphic model is proposed for the Oligo-Miocene Asmari and Pabdeh Formations in the Dezful Embayment of SW Iran. The model is based on both new detailed sedimentological observations in outcrops, core and well logs, and an improved high-resolution chronostratigraphic framework constrained by Sr isotope stratigraphy and biostratigraphy. A better understanding of the stratigraphic architecture distinguishes four, geographically separated types of Asmari reservoirs. Three Oligocene sequences (of Rupelian, early Chattian and late Chattian age) and three Miocene sequences (of early Aquitanian, late Aquitanian and early Burdigalian age) have been distinguished, representing a period of 15.4 Ma. The stratigraphic architecture of these sequences is primarily controlled by glacio-eustatic sea-level fluctuations, which determined the distribution of carbonates, sandstones and anhydrites in this sedimentary system. Tectonic control became important in the Burdigalian with a regional tilt down towards the NE. The lithological heterogeneity, the complex geometries, and both early and late diagenetic alterations are the basis for a classification of four main stratigraphic reference types for the Asmari Reservoirs: Type 1, sandstone dominated; Type 2, mixed carbonate-siliciclastic; Type 3, mixed carbonate-anhydrite; and Type 4, carbonate dominated. The sequence stratigraphic model predicts how and when these types change laterally from one to another.

Description: The combination of sedimentological and diagenetic data is important for the characterization of carbonate pore systems. This is particularly true for carbonates that were affected by meteoric diagenesis during sub-aerial exposure, for instance at sea-level lowstands. This diagenetic environment is commonly believed to be associated with increases in porosity, permeability and pore-throat diameters. Using data from three localities, improvement or deterioration of reservoir parameters below karst unconformities were analysed with a three-fold approach. In the first step, meteoric dissolution was characterized and early to late diagenetic products were described. In the second step, sedimentological and diagenetic data were converted to petrophysical data. In the third step, modelled climate data, in particular the occurrence of monsoon cells, in conjunction with other control mechanisms, were considered to understand the processes that controlled meteoric dissolution and later pore infill. Three case studies were analysed: (1) Lower Triassic oolites (sedimentary rocks dominated by ooids) and microbialites of the Calvorde Formation (Buntsandstein Group, Germany); (2) stacked shallowing-upward cycles of carbonate platform deposits in the Middle-Upper Triassic Mahil Formation (Arabian plate, Oman), capped by palaeosols; and (3) an Upper Triassic coral patch reef and overlying strata (Adnet, Salzburg region, Austria). Data integration allowed the establishment of three scenarios of significantly different processes related to meteoric diagenesis below unconformities: (1) increase of porosity and permeability and their preservation through time; (2) increase of porosity and permeability and subsequent pore system occlusion; and (3) decrease of porosity and permeability and creation of a barrier for pore fluids. Knowledge of the time span involved in meteoric diagenesis and the nature of the climatic regime helped to explain the origins and control mechanisms of the meteoric pore systems. The study provided evidence that a well-connected, large karst system, typical of a humid climate, is likely to be sealed subsequently by sediment and cement. Under arid climatic conditions, tight palaeosols developed at the unconformity and small karst pore systems developed which had the potential to remain open during basin evolution. Depending on the aforementioned parameters, carbonates affected by meteoric diagenesis may either become tight rocks or reservoirs.

Description: This paper describes an integrated field, petrographic and geochemical study of an Albian carbonate platform from the Basque-Cantabrian basin (northern Spain). It examines the distribution and evolution of porosity and the relationships between facies variations, sequences and variations in diagenesis. These platform carbonates were deposited on the footwall crests of active tilted blocks formed during continental rifting related to the Mesozoic opening of the Bay of Biscay and North Atlantic. The studied units document an early Albian aggradational steep-sloping platform and a late Albian, low-gradient expansive shelf separated by a hiatal unconformity spanning the middle-early late Albian (c. 5 Ma). The late Albian platform unit also exhibits a major internal unconformity. Platform geometries and facies architecture were mainly controlled by tectonics, hydrodynamic energy level and water depth. Petrographic, cathodoluminescence and geochemical analyses suggest that early meteoric diagenesis developed during sub-aerial exposure in strata below these two major unconformities. The platform carbonates have been affected during burial by a number of diagenetic processes that include four phases of dissolution, several fracture generations, and six cement sequences with development of at least 13 calcite and dolomite cement zones (Z0-Z12). A contrasting diagenetic response from the different platform environments illustrates the role of primary sediment composition and unconformity development in controlling porosity and cement distribution. Limestone stabilization and cementation were relatively early processes that were mostly completed within the first kilometre of burial depth beneath the depositional surface. Below this burial depth, fluid circulation was concentrated along restricted pathways (fractures and fault zones). Migration of hydrothermal-related fluids along fault zones created localized dolomite patches and large-scale porosity associated with cavities and collapse breccias, but did not significantly increase the small-scale porosity.

Description: Precipitation of dolomite cements in Jurassic carbonate platform sediments and slope breccias has been studied from well cores and outcrops of the central Southern Alps and central Apennines in Italy. In both areas, an initial, massive dolomite replacement was followed by multiphase precipitation of dolomite cements. The replacement occurred during burial, in a passive margin regime, in response to compaction-driven flow of formational fluids. This interpretation is based on results from fluid inclusion and stable isotope analyses which have been related to the thermal history. The dolomite cements precipitated when both areas were involved in collisional tectonics. In spite of the similar diagenetic evolution, the fluids causing dolomite cementation in each case were compositionally different. In the Alps a decrease in salinity was recorded from sea water to brackish fluids, whereas in the Apennines an increase in salinity from sea water up to 〉10% NaCl equivalent was recorded. The remarkable salinity differences in diagenetic fluids are considered to be related to the different sub-aerial relief of the two belts during dolomite precipitation. In the Alps, the dilution of fluids is related to the infiltration of meteoric waters from the mountain chain, that was widely emergent. In the Apennines, dolomite cements precipitated whilst the structural units were still widely submerged, preventing meteoric dilution of cementing fluids and promoting an increase in salinity through mixing with fluids rising from older evaporate-bearing layers. In both Alpine and Apennine cases, the same diagenetic trend is observed in thrust-fold belt and foreland basin units; in both structural systems the diagenetic events start precipitating dolomite cements in the inner part of the collision zone and then the diagenetic processes migrate towards the foreland basin along with the structural evolution of the area.

Description: A 380 m thick Aptian platform to basin transition has been studied along a 16 km long transect of excellent and continuous outcrops in NE Spain. The series has been dated using biostratigraphy (foraminifera and ammonites) and carbon-isotope stratigraphy, and has been subdivided at four scales of depositional sequences. The Aptian marine succession is subdivided into two-large scale sequences separated by a middle Aptian sub-aerial exposure surface. A characteristic trend of the floral-faunal fossil assemblages is present, which evolves from orbitolinid-ooid dominated ramps in Sequence I-1, to a coral-stromatoporoid-microbialite dominated platform in Sequence I-2, to a rudist-dominated platform top in Sequence II-1, and finally to a second episode of orbitolinid-ooid dominated ramp system in Sequence II-2. There was an influx of siliciclastic sediments at the base and at the top of this succession. The detailed carbon-isotope curve measured along the Miravete section and covering almost the complete Aptian succession, is compared with published Aptian curves recorded in both basinal and carbonate platform settings along the northern and southern NeoTethys margins. It shows that the Galve sub-basin curve represents all the major isotope excursions of the lower and upper Aptian, in a dominantly shallow-water succession.

Description: The Zagros Simply Folded Belt (ZSFB) is an active fold-and-thrust belt resulting from the still continuing continental collision between the Arabian plate and the Iranian plate, which probably started in the Oligocene. The present-day shortening (N25{degrees}) is well documented by focal mechanisms of earthquakes and global positioning system (GPS) surveys. We propose in this study a comparison of published palaeostress markers, including magnetic fabric, brittle deformation and calcite twinning data. In addition, we describe the magnetic fabric from Palaeocene carbonates (10 sites) and Mio-Pliocene clastic deposits (15 sites). The magnetic fabrics are intermediate, with magnetic foliation parallel to the bedding, and a magnetic lineation mostly at right angles to the shortening direction. This suggests that the magnetic fabric retains the record of an early layer-parallel shortening (LPS) that occurred prior to folding. The record of LPS allows the identification of originally oblique folds such as the Mand Fold, which have developed in front of the Kazerun Fault. The shape parameter of the magnetic fabric indicates a weak strain compatible with the development of detachment folds in the ZSFB. The palaeostress datasets, covering the Palaeocene to Pleistocene time interval, support several folding episodes accompanied by a counter-clockwise rotation of the stress field direction. The Palaeocene carbonates in the ZSFB record a N47 LPS during early to middle Miocene detachment folding in the High Zagros Belt (HZB). The Mio-Pliocene clastic deposits recorded a N38 LPS prior to and during detachment folding within the ZSFB at the end of the Miocene-Pliocene. Similarly, fault slip and calcite twin data from the ZSFB also support a counter-clockwise rotation from NE to N20 between the pre-folding stage and the late rejuvenation of folds. This counter-clockwise trend of palaeostress data agrees with fault slip data from the HZB. During the late stage of folding in the ZSFB, the Plio-Quaternary palaeostress trends are consistently parallel to the present-day shortening direction.

Description: We present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across the Central and Northern Zagros. The receiver function analysis of teleseismic earthquake records gives a high-resolution image of the Moho beneath the seismic transects. On both cross-sections, the crust has an average thickness of 42{+/-}2 km beneath the Zagros fold-and-thrust belt and the Central domain. The crust is thicker beneath the hanging wall of the Main Zagros Reverse Fault (MZRF), with a greater maximum Moho depth in the Central (69{+/-}2 km) than in the Northern Zagros (56{+/-}2 km). The thickening affects a narrower region (170 km) beneath the Sanandaj-Sirjan zone of the Central Zagros and a wider region (320 km) in the Northern Zagros. We propose that this thickening is related to overthrusting of the crust of the Arabian margin by the crust of Central Iran along the MZRF, which is considered as a major thrust fault cross-cutting the whole crust. The fault is imaged as a low-velocity layer in the receiver function data of the Northern Zagros profile. Moreover, the crustal-scale thrust model reconciles the imaged seismic Moho with the Bouguer anomaly data measured on the Central Zagros transect. At upper mantle depth, P-wave tomography confirms the previously observed strong contrast between the faster velocities of the Arabian margin and the lower velocities of the Iranian micro-blocks. Our higher-resolution tomography combined with surface-wave analysis locates the suture in the shallow mantle of the Sanandaj-Sirjan zone beneath the Central Zagros. The Arabian upper mantle has shield-like shear-wave velocities, whereas the lower velocities of the Iranian upper mantle are probably due to higher temperature. However, these velocities are not low enough and the low-velocity layer not thick enough to conclude that delamination of the lithospheric mantle lid has occurred beneath Iran. The lack of a high-velocity anomaly in the mantle beneath Central Iran suggests that the Neotethyan oceanic lithosphere is probably detached from the Arabian margin.

Description: SE Iran is the site of a rare case of young transition between subduction and collision. We have synthesized recent results in geodesy, tectonics, seismology and magnetism to help understand the structure and kinematics of the Zagros-Makran transition. Surface observations (tectonics, magnetism and geodesy) indicate a transpressive discontinuity consisting of several faults striking obliquely to the convergent plate motion, whereas deeper observations (seismology) support a smooth transition across the fault system. No lithospheric transform fault has been created, although the transition already behaves like a major boundary in terms of tectonic style, seismic structure, lithology and magnetism. The Zendan-Minab-Palami fault system consists of several faults that accommodate a transpressive tectonic regime. It is the surface expression of a southward propagation of the north-south-trending right-lateral strike-slip fault system of Jiroft-Sabzevaran. Within each system the numerous faults will coalesce into a single, lithospheric, wrench fault.

Description: The Mountain Frontal Flexure shows a single step along the front of the Pusht-e Kuh Arc with about 3 km of structural relief. This front has been interpreted as being formed by a basement monocline above a blind crustal-scale and low-angle thrust with a ramp-flat geometry (the ramp dips 12-15{degrees} towards the inner part of the orogen and cuts the entire crust). The Anaran anticline on top of the Mountain Frontal Flexure shows an irregular geometry in map view and consists of four segments with diverse directions of which the SE Anaran, the Central Anaran and the NW Dome are culminations. The North-South Anaran segment may form a linking zone developed during the rise and amplification of single culminations, the NW Dome and the Central Anaran, above the Mountain Frontal Flexure. The asymmetric Anaran anticline is characterized by the existence of multiple normal faults, some of them with significant dip-slip displacements of up to 1000 m. These faults limit grabens located along the crests of the anticline segments. Cross-cutting relationships show that the normal faults along the Central Anaran are older than along the North-South Anaran, reinforcing the temporal constraints on the later growth of this segment of the anticline. The geometry of the Anaran anticline is asymmetric with the subvertical forelimb very little exposed. This forelimb is cut above and below by a thrust system that seems to develop along the fold hinges. The lower thrust, with a ramp-flat geometry, carries the entire anticline towards the foreland on top of slightly deformed rocks in the footwall. The thrust flattens in the Gachsaran evaporitic level forming a typical triangular zone filled with evaporites, which produce a strong fold disharmony between the overburden (Passive Group) and the underlying rocks (Competent Group). The growth of the Anaran anticline lasted for about 6 Ma and was the consequence of detachment folding that was subsequently thrust, rotated and uplifted above the Mountain Frontal Flexure with coeval reactivation of earlier crestal layer-parallel extension normal faults to accommodate the large increase of structural relief between the foreland and the tectonic arc. Three main results from analogue modelling have been combined with field data to resolve the geometry of the Anaran anticline as well as its evolution: (1) a thickening of intermediate evaporites (Gachsaran Formation) is produced above the flat segment of the thrust carrying the anticline on top of foreland strata; (2) growth strata deposited in the adjacent syncline modify the geometry of the anticline by increasing the dip and the length of its forelimb; (3) coeval erosion to anticline growth, as well as thick growth strata deposition, increases fold amplification rather than foreland propagation of deformation. The proposed fold model may be applied to other anticlines on top of this major basement-related thrust, such as the Siah Kuh and Khaviz anticlines in the Pusht-e Kuh Arc and Dezful Embayment domains.