Chalk: all we need is a fracture log!

Chalk fracture logging is reviewed in the context of the broader geology needed to build conceptual ground models. Examples of drilling damaged core illustrate the many issues faced by core-loggers including identification of marker beds (marl seams, hardgrounds, flint bands, fossil shell beds) and the ‘interpretations’ necessary to complete a fracture log. Stratabound fractures impart a special style of fracturing to each Chalk formation. Lithology is a key factor in development of fracture style where marl seams control inclined conjugate fracture sets, development of listric growth faults and interbed slides. Lateral changes in lithology and thickness and consequent controls on fracture evolution are related to intra-Chalk tectonic episodes and tectonic setting with onshore interpretations supported by offshore seismic profiles. Strike-slip faults are illustrated in the Chalk cliffs of the Sussex coast. Fracture log reports should highlight special features such as shear zones and use annotated core photographs to illustrate issues requiring discussion.

Stratigraphy of habitation deposits and fire history of the early medieval town Dzhankent (Kazakhstan, Eastern Aral region)

<p>Anthropogenic soils and soil-sedimentary systems (cultural layer, occupation deposit) in settlement archaeological settings are highly valuable and underappreciated archives of past environments, land-use activities, and life cycles within past residential areas. This study is aimed to reconstruct fire history for the early medieval town of Dzhankent located in Eastern Aral region, Kazakstan as based on the C14 dated stratigraphy, morphology and micromorphology, data on charcoal morphology, C:N and C13 isotope records.  </p><p>Several sections of cultural layers were studied within excavated areas. Stratigraphic units were thoroughly C14 dated (58 dates). Most 14C dates are between the 7th and 10th centuries, and most of the dates have overlapping intervals of calibrated age although clear up-section trends from older to younger ages may be seen. This demonstrates the slow, progressive accumulation of occupation deposits. The analysed excavation sections are very well stratified. Stratigraphic units based on char-enriched marker beds could sometimes be traced for long distances. Char enriched layers contain enormous quantities of both grass and wood charcoals. Thin, about 1 m long lenses of ash and charcoals of poor and unified taxonomic variety are thought to be fireplaces. Extended thick char-enriched layers (about 10 meters long and 0,1 m thick), well stratified at macro-, and micro-levels, with sub-parallel oriented charcoals of highly variable taxonomic compositions considered to be traces of big fires. Three fire events were detected based on the stratigraphy, morphology, charcoal amounts, C, N and C13 isotope depth variability.  </p><p>Filed studies and were funded by DFG project 389351859. The analytical part was supported by RFBR 19-29-05238, and DFG 389351859.</p>

Re-assessing the Upper Permian Stratigraphic Succession of the Northern Sydney Basin, Australia, by CA-IDTIMS

High precision Chemical abrasion-isotope dilution thermal ionisation mass spectrometry (CA-IDTIMS) U-Pb zircon results from tuff marker beds that are interstratified within the Upper Permian deposits of the northern Sydney Basin add constraints on the timing of sediment deposition, and afford a better understanding of the regional stratigraphy. The results indicate a magmatic influence during the deposition of the sediments, with episodic events spanning at least from 255.65 ± 0.08 to 255.08 ± 0.09 Ma. The zircon data suggest that the studied sedimentary rocks and tuffs have accumulated simultaneously over a short time interval, which contrasts with current stratigraphic models that suggest a much greater period of deposition and stratigraphic thickness. Therefore, an updated stratigraphic correlation of the basin is suggested, which combines the presently defined Lambton, Adamstown, and Boolaroo sub-groups into a single Lambton sub-group. This updated correlation framework is stratigraphically and geochronologically constrained and provides a more precise exploration model for the northern Sydney Basin. This case study highlights the valuable contribution of the CA-IDTIMS method in intrabasinal correlations of sedimentary successions, when integrated with a robust sedimentological framework, to minimize the stratigraphic uncertainties.

Tectonic geomorphology and Plio-Quaternary structural evolution of the Tuzgölü fault zone, Turkey: Implications for deformation in the interior of the Central Anatolian Plateau

Situated within the interior of the Central Anatolian Plateau (Turkey), the 200-km-long Tuzgölü extensional fault zone offers first-order constraints on the timing and pattern of regional deformation and uplift. In this study, we analyze the morphometrics of catchments along the Tuzgölü range-front fault and the parallel, basinward Hamzalı fault using a variety of measured morphometric indicators coupled with regional geomorphic observations and longitudinal profile analysis. In addition, we use field and remote mapping to constrain the geometry of two key marker beds, the Pliocene Kızılkaya ignimbrite and Kışladaǧ limestone, in order to investigate deformation in the footwall of the Tuzgölü fault zone. The marker beds form a broad arch along the footwall of the fault, with greatest cumulative displacement along the central part of the fault zone, suggesting early Pliocene extensional reactivation of the Tuzgölü fault with a typical fault-displacement profile. However, a change in deformation pattern is marked by transient knickpoints along river channels; morphometric indicators sensitive to shorter (1−3 Ma) time scales, including river steepness, basin elongation, and mountain front sinuosity, indicate an overall southeastward increase in footwall uplift rate of the Tuzgölü fault zone, which could reflect block rotation or interaction with the Hasan Dag volcano. Basin asymmetry and basin-fault azimuth measurements indicate north-northwest tilting of footwall catchments, which may be linked to regional tilting across the Central Anatolian Plateau interior. Varying patterns of spatial and temporal deformation along the length of the Tuzgölü fault zone are likely due to the interference of crustal- and lithospheric-scale processes, such as rotation of crustal blocks, extrusion of the Anatolian microplate, crustal heating, gravitational collapse associated with plateau uplift, and mantle-driven vertical displacements.

РАДИОУГЛЕРОДНОЕ ДАТИРОВАНИЕ И ПОЖАРНАЯ ИСТОРИЯ РАННЕСРЕДНЕВЕКОВОГО ГОРОДА ДЖАНКЕНТ (Ю-В ПРИАРАЛЬЕ, КАЗАХСТАН)

A series of 63 14C dates were obtained from non-destructive core-drilling across the Dzhankent site (the early medie- val town located in Eastern Aral region), a second series – 58 dates from stratigraphic sections within excavated areas. Most of 14C dates are between the 7th and 10th centuries; clear up-section trends from older to younger ages may be seen. The analysed excavation sections are very well stratified. Stratigraphic units based on char-enriched marker beds could sometimes be traced for long distances. Extended thick char-enriched layers are considered to be traces of big fires, while small lenses of ash and charcoal are thought to be fireplaces.

Subsurface observations of deformation bands and their impact on hydrocarbon production within the Holstein Field, Gulf of Mexico, USA

The Holstein Field consists of poorly lithified turbidite sands deposited during the Pliocene Epoch. Dense arrays of cataclastic deformation bands have been observed in all cores from wells that penetrate the K2 reservoir sand, the highest density of which are located near the hinge of a monocline. The predominant set of deformation bands strikes parallel to the fold axis, and dips at both high and low angles with respect to bedding. Deformation band orientation and offset of marker beds indicate reverse shear and are consistent with a flexural slip origin during folding. Restorations suggest that the monocline and associated deformation bands formed early during the burial process with high pore pressure.Reservoir permeability estimates from well tests indicate a bulk permeability approximately one-third of the reservoir core permeability in regions with deformation bands, whereas other areas are unaffected. Bulk permeability estimated from the permeability of the reservoir and deformation band network is lower than the reservoir permeability alone, but exceeds the permeability observed in the well tests by a factor of 2. A reduction in permeability of oil relative to water for both the fault and host sand is required to match the well-test permeability with that measured from core.