A new geological map and review of the Middle Devonian rocks of Westray and Papa Westray, Orkney, Scotland

The Middle Devonian lacustrine sediments of Orkney, off the northeast Scottish mainland, are composed largely of the Lower and Upper Stromness Formations and overlying Rousay Formation. These three formations have been subdivided and defined by vertebrate biostratigraphic biozones with recent division of the Rousay Formation into three further units based on characteristic fish fossils. The division of the Rousay Formation has enabled a map to be constructed of the solid geology of the island of Westray, Orkney, based on fish identification, detailed logging of sedimentary cycles throughout the Rousay succession, parameters of divisional boundaries, and a survey of faults marking sinistral transtensional movement parallel to the Great Glen Fault. Post-Carboniferous shortening and basin inversion led to uplift, folding and reactivation of normal faults as reverse faults, to form a positive strike-slip flower structure in Westray. A suite of Permian igneous dykes intruded across Orkney include three minor offshoots in Westray. The resulting map is the first to make use of biostratigraphic units within the Rousay Flagstone which are now regarded as Members.

Reservoir quality and petrophysical evaluation of the Paleocene paralic sandstones in the Ranchería sub-basin, Colombia

The Ranchería sub-basin is an unexplored basin which gives a valuable opportunity to find new perspectives about the hydrocarbon potential in Colombia. The sandy facies of the Paleocene Cerrejón Formation accomplish with the characteristics within the petroleum system, to be considered as a potential reservoir rock. This work aims to evaluate the reservoir quality of the Cerrejón Formation using three cored wells, through the integration of well logs, core sedimentological descriptions, porosity and permeability lab data, and thin-sections analysis. Using GR log patterns and sedimentary cycles, four facies associations (A,B,C,D) were defined associated to an environment of a deltaic plain complex. The well correlation was performed using the sedimentary cycles methodology. Five petrophysical rock-types (mudstone (M), coal (P), sandy mudstone (sM), muddy sandstone (mS), and sandstone (S)) were identified, according to textural and lithological features from the core descriptions, adjusted with well logs analysis and NTG ratio. A supervised neural network model of electrofacies was generated with the GR, RHOB, NPHI, and DT logs combination, using as training curves the wells B and C. The blind test validation was executed in the well C with an 82% of correlation. Petrophysical evaluation was performed calculating shale volume, effective porosity, permeability and water saturation. Reservoir rock properties are fair to moderate with porosities between 0.3-22.9 %, and permeability values that range from 0.02mD to 47mD. The interest intervals average water saturations vary between 0.3 and 0.5. Four hydraulic flow units were estimated with the Winland’s R35 method for the assessment of reservoir quality. There is not a clear relationship between F.A. and HFUs for the Cerrejón Formation, however, the F.A. B comprises the HFUs with the best petrophysical properties. The reservoir quality of Cerrejón Formation is controlled mainly for diagenetic and compositional processes than related to primary deposition. The diagenetic controls in reservoir properties are the calcite cementation, quartz overgrowth, pseudomatrix presence, and feldspar and calcite dissolution. This integrated study provides a good approximation to the understanding of the Cerrejón Formation and the evidence of a moderate reservoir quality.

Stratigraphy and Sedimentology of Upper Cretaceous to Upper Palaeocene Succession in Zimam Formation Along Wadi Tar al Kabir, NW Libya

The Upper Cretaceous to Upper Palaeocene rocks of the Zimam Formation along the southwestern escarpment of the Hun Graben of NW Libya have been stratigraphically investigated from two stratigraphical sections in wadi Tar al Kabir. The field investigations led to the recognition of three members, from the oldest to the youngest, the Lower Tar Member, the Upper Tar Member and the Had Member. Eight sedimentary facies were distinguished at outcrop-scale and several microfacies were recognized and the outcome indicates that the depositions of the Zimam Formation are corresponding to two transgressive-regressive sedimentary cycles. The first cycle is attributed to the Lower Tar Member in which small planktonic foraminifera is quite common in the Campanian whereas the larger benthic foraminifera, namely, Omphalocyclus macroporus and Siderolites calcitrapoides are abundant in the Maastrichtian. The last occurrence of the latter two taxa, however, was used to delineate the contact between the Maastrichtian and Danian stages in the studied sequence. Up-sequence the sediments of the Upper Tar Member along with the overlying Had Member correspond to the second transgressive-regressive sedimentary cycle. Herein, the Upper Tar Member is enriched by small benthic foraminifera; Neoeponides duwi and Cibicides cf. libycus, and has been ascribed to the Danian (Lower Palaeocene). The reaming sediments of Zimam Formation, however, are belonging to the overlying Had Member and is tentatively ascribed to the Selandian (Upper Palaeocene) based on the last occurrence of the Danian fauna and the total range of the codiacean algae Ovulites morelleti.

Decoupled lithostratigraphy, orbitally-driven climate, and tectonics for a middle Pleistocene stratigraphic section in the Northern Apennines, Italy

<p>Cyclical patterns in the lithology of terrestrial Pleistocene sedimentary deposits are traditionally interpreted as the result of exogenic interglacial-glacial cycles, with deposition accommodated by constant basin subsidence. Recent challenges to this model propose that autogenic surface processes inherent to hillslope, fluvial, and marine systems can both obscure exogenic signals in the sedimentological record and encode their own quasi-periodic signal that mimics exogenic cyclicity. We used rock-magnetic cyclostratigraphy to test the canonical climate-driven sedimentation model for terrestrial Pleistocene sedimentary cycles against competing tectonic- and autogenic process-modulated sedimentation models with a continuous 60 m exposure of middle Pleistocene fluvial sedimentary cycles located at the edge of the actively subsiding Po foreland basin in the Northern Apennines of Italy. We correlated magnetic susceptibility, sampled at 40 cm intervals, to orbital cyclicity to generate a high-resultion age model anchored by terrestrial cosmogenic nuclide (TCN) burial ages, optically stimulated luminescence (OSL), and magnetostratigraphy. Two new <sup>26</sup>Al-<sup>10</sup>Be burial ages are 160±320 ka and 680±310 ka (2σSE); the age of a third buried sample is consistent with continuous exposure and thus recent burial. We mapped the age model into section lithostratigraphy and then compared to the global benthic δ<sup>18</sup>O stack to determine whether sedimentary cyclicity coincides with glacial-interglacial cycles. In addition, we calculated paleo-erosion rates based on the <sup>10</sup>Be concentration of six samples distributed through the age model and find that they range from 244±23 to 444±52 m/Ma, which bracket the modern TCN-determined erosion rate of the Enza River of 351±40 m/Ma. Results show no clear correlation between lithostratigraphy, glacial-interglacial climate cycles, or paleo-erosion rates, indicating that the stratigraphy is probably not driven by exogenic climate forcing. Rather, based on the decoupling of lithology and paleo-erosion rates and the little variation in paleo-erosion and modern erosion rates (<20%), the cyclicity is best explained by periodic autogenic delta processes in a system where accommodation space in the depositional basin is limited. These findings exemplify the complex interplay of tectonics, climate, and autogenic processes in the generation, transport, and deposition of sediments. Results of this study contribute to the ongoing debate over whether signals generated by large scale, exogenic forcing can survive transport to be preserved in the sedimentary record and help define the temporal and spatial scales at which these processes operate.</p>

Sequence Stratigraphy of South Central Coastal Quaternary Sandy Formation and Stratigraphic Significance

The South Central Coastal Quaternary sandy formation has been studied by many authors according to different objectives, mainly for establishing quaternary geological maps ata different scale. There are 5 sedimentary cycles established based on the absolute age of the sand, that was analyzed by the method of thermoluminescent dating of quartz (TL), and by the comparison between 5 global glacial /interglacialcycles and 5 sedimentary cycles of the Red River Delta. The Southern Central coastal sandy cycle corresponds to 5 sequences. Sequence 1 has the age of early Pleistocene (Q11); Sequence 2 has middle Pleistocene age, early part (Q12a); Sequence 3 has Pleistocene age in the middle of the late part (Q12b); Sequence 4 has age of early Late Pleistocene part (Q13a) and Sequence 5 has age of late Pleistocene - Holocene (Q13b-Q2). Each sequence has 3 systems which tract corresponding to 3 lithofacies complex: (1) Lowstand systems tractcorresponding to alluvial gravelly sand facies complex (arLST) or marine-wind red sand facies complex (mvLST); (2) Transgressive systems tract corresponding to the white sand and spotted white sandy barrier bar facies complex (mtTST); Highstand systems tract corresponding to the marine-wind red sand facies complex (mvHST). Keywords: Sequence startigraphy, sedimentary cycle, systems tract, lithofacies complex. References: [1] Nguyễn Đức Tâm, Đỗ Tuyết, Thuyết minh bản đồ địa chất Đệ Tứ Việt Nam tỷ lệ 1/500000. Viện Khoa học Địa chất và Khoáng sản Việt Nam, 1994.[2] Ngô Quang Toàn (Chủ biên), Vỏ phong hóa và trầm tích Đệ tứ Việt Nam, Cục Địa chất và Khoáng sản Việt Nam, Hà Nội, 2001.[3] Nguyễn Văn Cường (chủ biên), Báo cáo thuyết minh đo vẽ địa chất và tìm kiếm khoáng sản nhóm tờ Hàm Tân-Côn Đảo tỷ lệ 1/50.000, Tổng cục Địa chất và Khoáng sản Việt Nam, 2001.[4] Hoàng Phương (chủ biên), Báo cáo kết quả đo vẽ bản đồ địa chất và tìm kiếm khoáng sản nhóm tờ Phan Thiết tỷ lệ 1/50.000, Tổng cục Địa chất và Khoáng sản Việt Nam, 1997.[5] Lê Đức An, Đinh Ngọc Lựu, Những phát hiện mới về tectit và ý nghĩa của chúng trong việc nghiên cứu địa chất-địa mạo lãnh thổ phía nam Việt Nam. Tập san Bản đồ địa chất, số 36 (1978) 37- 40. [6] V.Mu. Colin, G.J. Brian, T. Nghi, M.P. David, V.V. Vinh, T.N. Tinh, C.N. Gerald, Thermoluminescene ages for a reworked coastal barier, southeastern Vietnam: preminary report, Journal of Asian Earth Sciences 20 (2002) 535-548.[7] Trần Nghi, Nguyễn Địch Dỹ, Đinh Văn Thuận, Vũ Văn Vĩnh, Ma Công Cọ, Trịnh Nguyên Tính. Môi trường và cơ chế thành tạo cát đỏ Phan Thiết. Tạp Chí Địa chất, A/245 (1998) 10-12.[8] Trần Nghi, Địa chất trầm tích Việt Nam, Nhà Xuất bản Đại học Quốc gia Hà Nội, 2018.[9] Trần Nghi, Tiến hóa các thành tạo cát ven biển Miền Trung trong mối tương tác với sự thay đổi mực nước biển trong Đệ Tứ, Tuyển tập các công trình nghiên cứu Địa chất và Địa vật lí biển, Tập II (1996) 130-138.

THE COMBINATION OF HORIZONTAL AND VERTICAL INVERSION OF CYCLITES IN SEDIMENTARY BASINS: PROBLEMS AND SOLUTIONS

The model of horizontal inversion of sedimentary cycles proposed by the author is supplemented by the provision on vertical inversion arising in large regional transgressions and regressions. The combination of these types of transformation of sedimentary cycles allows to explain the violation of the natural alternation of sand and clay strata at the level of regional horizons, adopted in the official stratigraphic schemes. It allows occurrence of sandy horizons on sandy, and clay – on clay within transition zones. The possibility of revision on this basis of the volume, stratigraphic position and correlation of some suites and horizons on the stratigraphic chart of the Jurassic of Western Siberia is shown.