Stratigraphic distribution of the Codell Sandstone in the Denver Basin using wireline logs and core

Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Formation across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin. Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) The NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) A stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath. North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2 to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5 to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from a basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.

The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event

The Paleoproterozoic Francevillian succession of Gabon has figured prominently in concepts about Earth’s early oxygenation and genesis of a large positive excursion in carbon-isotope values, the Lomagundi-Jatuli event (LJE). Here we present a detailed study of a 139-m-long core of Francevillian rocks marked by carbonate δ13C (δ13Ccarb) values of 5‰–9‰ that decline upsection to near 0‰, a trend inferred by many workers as a fingerprint of the LJE and its termination. However, we show that the shift in δ13Ccarb values coincides with a facies change: shallow-marine facies are marked by the strongly positive values, whereas deeper-marine facies (below storm wave base) are at ∼0‰. The most circumspect interpretation of such facies dependence of δ13Ccarb is that shallow-marine settings record the isotope effects of local physical and biochemical processes driving the ambient dissolved inorganic carbon (DIC) pool to heavier values, and the lighter values (∼0‰) in deeper-water facies track the DIC of the open-marine realm where δ13C was largely unaffected by fractionations occurring in shallow-water settings. Further, a transgressing redoxcline created conditions for precipitation of Mn-bearing minerals and chemotrophic microbial biota, including methane cycling communities evident by organic δ13C (δ13Corg) values of −47‰ and Δδcarb-org values as high as 46‰. Thus, the Francevillian C-isotope profile reflects basin-specific conditions and is not a priori an indicator of global C-cycle disturbances nor of the termination of the LJE.

UNCONFINED GROUNDWATER FLOW PATTERN AND FACIES CHANGES AT WAY HUWI VILLAGE, SOUTH LAMPUNG

Way Huwi Village is located in South Lampung, near the Institut Teknologi Sumatera (ITERA). The purposes of this research is to know the unconfined groundwater flow pattern and groundwater facies changes. We measured the depth of water table at nine dig wells, analyzed piper diagram for groundwater facies identification. Then, we integrated groundwater flow patterns and groundwater facies from each well to analyze groundwater facies change pattern in research area. The result indicated that the unconfined groundwater flows from SW to NE of research area, following higher (SW) to lower elevation (NE). There are six patterns of unconfined groundwater facies changes: from Facies Na-Cl to Facies Na-HCO3-Cl, Facies Na-HCO3-Cl to Facies Ca-Mg-HCO3, Facies Na-HCO3-Cl to Facies Na-Cl, Facies Na-HCO3-Cl to Facies Na-SO4-Cl, Facies Ca-Mg-HCO3 to Facies Na-SO4-Cl, and Facies Ca-Mg-HCO3 to Facies Na-HCO3-Cl. ABSTRAK - Pola aliran airtanah tidak tertekan dan perubahan fasiesnya di Desa Way Huwi, Lampung Selatan. Desa Way Huwi terletak di Lampung Selatan, di dekat Institut Teknologi Sumatera (ITERA). Tujuan dari penelitian ini adalah untuk mengetahui perubahan pola aliran airtanah dan fasies airtanah yang terjadi. Kami mengukur kedalaman muka airtanah pada sembilan sumur gali, menganalisis Diagram Piper untuk mengetahui fasies airtanah. Kemudian kami mengintegrasikan pola aliran airtanah dan fasies airtanah setiap sumur untuk mengetahui pola perubahan fasies air tanah. Hasil analisa menunjukkan bahwa airtanah tidak tertekan mengalir dari Barat Daya ke Timur Laut mengikuti ketinggian yang lebih tinggi (SW) ke ketinggian yang lebih rendah (NE). Ada enam pola perubahan fasies airtanah tidak tertekan: dari Facies Na-Cl ke Facies Na-HCO3-Cl, Facies Na-HCO3-Cl ke Facies Ca-Mg-HCO3, Facies Na-HCO3-Cl ke Facies Na-Cl, Facies Na -HCO3-Cl ke Facies Na-SO4-Cl, Facies Ca-Mg-HCO3 ke Facies Na-SO4-Cl, dan Facies Ca-Mg-HCO3 ke Facies Na-HCO3-Cl

Testing of Permian – Lower Triassic stratigraphic data in a half-graben/tilt-block system: evidence for the initial rifting phase in Antalya Nappes

Testing of Middle Permian – Lower Triassic stratigraphic data from the Antalya Nappes in a half-graben/tilt-block system has revealed the presence of episodic rifting events separated by periods of tectonic quiescence. Following a period of uplift during the Permian (Late Artinskian to Roadian), the basement rocks have been activated by displacement faults and several depocenters in half-graben-like asymmetrical basins began to be filled with Roadian to Wordian continental clastic deposits intercalated with coal and marine rocks. The Early Capitanian time was a period of tectonic quiescence. The second event occurred in Middle to Late Capitanian times and produced basaltic volcanic rocks intercalated in the shallow marine fossiliferous carbonate successions. Following the Lopingian (Wuchiapingian and Changhsingian) and Permian–Triassic boundary interval representing a long tectonic quiescence, the last rifting episode started with an abrupt facies change in the late Griesbachian. Variegated shales, limestones, volcanics, talus breccia, and debris flow deposits were laid down in a half-graben/tilt-block system. As normal faulting has become active, the deposition continued on the subsiding hanging wall side. The stratigraphic gap increased in magnitude as the erosional truncation has incised deeply the footwall side. This initial rifting phase in the Antalya Nappes is prior to the onset of a stronger and more continuous rifting event that occurred in the Anisian–Carnian interval including a variety of deepwater clastic and carbonate deposits, radiolarites containing sometimes blocks and clasts derived from the basin margins, and volcanic rocks carrying intraoceanic setting character.

Case Studies of Rock Bursts in Tectonic Areas with Facies Change

Although tectonic areas with facies change (i.e., variation of coal seam thickness, coal seam dip angle, or coal quality) are one of the three major geological structures that induce rock bursts, case studies of rock bursts in these tectonic structures are rare. The main objective of this study is to illustrate this issue and provide case studies that may inspire future research. Based on several typical cases of rock bursts induced by tectonic areas with facies change, the conditions conducive for these bursts are introduced and investigated in detail. Subsequently, numerical simulation is performed, showing that stress concentration exists in regions with variable coal seam thicknesses or dip angle. When stoping or tunnelling approaches this region, the peak stress increases rapidly. Thus, the burst occurs via a mechanism involving the superposition of high in-situ stress from tectonic areas with facies change and abutment pressure from stoping or tunnelling, leading to high stress concentration. Strategies for mitigating rock bursts are also provided. Rock bursts induced by tectonic areas with facies change can be mitigated by avoiding regions of high tectonic stress concentration and reducing mining induced stress.

Caracterización litológica y paleontológica del Cretáceo inferior en Cajamarca: Las formaciones Santa y Carhuáz

Caracterización litológica y paleontológica del Cretáceo inferior en Cajamarca: Las formaciones Santa y Carhuáz Alejandro Lagos M.&, Zenón Quispe M. Universidad Nacional de Cajamarca DOI: https://doi.org/10.33017/RevECIPeru2012.0009/ RESUMEN En este estudio se analizan tanto los fósiles y las rocas que pertenecen a las formaciones (fms.) Santa y Carhuáz de edad cretáceo inferior, hallados en las localidades de San Marcos y Cajamarca. Estos análisis han permitido determinar su edad y las condiciones de depositación. La determinación de la posición media pesada relativa (PMPR) de las areniscas de la fm. Carhuaz determinó su caracterización depositacional regional. Cabe indicar que, tanto la fm. Santa como la Carhuaz, en contacto con intrusivos miocénicos se comporta muchas veces como buenas rocas receptoras para eventos de mineralización, como es el caso del yacimiento tipo Pórfido de Cu- Mo El Galeno, ubicado en el Departamento de Cajamarca. La fm. Santa, nombre dado por [1] de edad cretácica inferior, está constituida por una alternancia de arcillitas gris oscuras y limolitas calcáreas de carácter marino- transgresivo, descansa en concordancia sobre las areniscas litoral- playero- continental de la fm. Chimu. De acuerdo con [2] esta unidad muestra regionalmente un cambio de facies notables, mientras que en el norte del país la proporción de calizas es mayor, en Cajamarca es más lutacea. En las localidades de Namora y Matara ubicados en el Cuadrángulo de San Marcos, se identificaron Gasterópodos, que según [3] son (Cassiope) neumayri (NAGAO); paraglauconia strombiformis (SCHLOTHEIM); Corbícula sp. que indican una edad Valanginiano medio - Hauteriviano y un ambiente marino somero de aguas salobres muy cercano a la costa. Esta misma fauna fósil ha sido reportada también en las zonas de Cruz Blanca, Shaullo Chico y carretera de Huambocancha (Cuadrángulo de Cajamarca). La fm. Carhuaz, nombre dado también por [1] de edad cretácica inferior, consta de una alternancia de limolitas y areniscas de grano fino en estratos comúnmente delgados. En Namora y Matara se ha identificado una flora fósil que de acuerdo con [3] son: Weichselia peruviana ZEILLER (improntas de ramas y hojas): (Cassiope) neumayri (NAGAO), Pelcurdia sp. que indica una edad Hauteriviano medio a superior y una depositación en un ambiente continental y muy cercano a una zona transicional, con predominancia de climas cálidos y húmedos. Aplicando el PMPR, metodología propuesta por [4], a las areniscas de la fm. Carhuáz en una columna sedimentología elaborada en la zona de Cruz Blanca (localidad de Cajamarca) se determinó que las areniscas se ubican a 46% de la parte superior de toda la columna, indicando que esta ubicación es cercana a la parte central de toda la columna. El PMPR en la zona de Namora (Cuadrángulo San Marcos), está a 55%. Estos resultados indican que las facies ubicadas en Cajamarca son menos continentales que las ubicadas en San Marcos. Parece ser que el abundante contenido de materia orgánica, el carácter depositacional cálido y de aguas salobres de ambas formaciones han favorecido las reacciones químicas entre los fluidos hidrotermales y estas. En cambio estas reacciones químicas se producen de igual manera tanto dentro de un carácter ambiental transgresivo como regresivo dentro de la formación Carhuaz. Descriptores: caracterización, formación, fósil. ABSTRACT In this study, we analyzed the fossils and rocks of the formations Santa and Carhuaz of Cretaceous age found in the towns of San Marcos and Cajamarca determining its age and condition of their deposition. The determination of the relative heavy middle position (PMPR) of the sandstones of the fm. Carhuaz determined regional depositional characterization. It should be noted that both the fm. Santa as Carhuaz Miocene intrusive contact often behave as good host rocks for mineralization events, such as reservoir type Cu-Mo Porphyry El Galeno, located in the Department of Cajamarca. The fm. Santa, a name given by [1] of Cretaceous age, consists of an alternation of dark gray claystones and calcareous siltstones of marine transgressive character, lies in keeping on-shore beach sands of the fmcontinental. Chimú. According to [2] this machine exhibits regionally significant facies change, while in the northern limestone proportion is higher, more lutacea Cajamarca. In the towns of Matara- Namora and located in the San Marcos Quadrangle, gastropods were identified, which according to [3] are (Cassiope) neumayri (GAAS); paraglauconia strombiformis (Schlotheim), Corbicula sp. Valanginian age indicate a half - Hauterivian shallow marine environment and brackish waters close to shore. This same fossil fauna has been reported also in the areas of Cruz Blanca, Shaullo Chico y Huambocancha (Quadrangle Cajamarca). The fm. Carhuaz, also the name given by [1] of Cretaceous age, consists of alternating sandstones, siltstones and fine-grained strata commonly thin. In Matara Namora and has identified a fossil flora according to [3] are: Weichselia peruviana Zeiller (imprints of branches and leaves, Pelcurdia sp. indicating an age Valanginian - Hauterivian and a deposition in a continental environment and close to a transitional zone, with predominantly hot and humid climates. Applying the PMPR, methodology proposed by [4], the sandstones of the Fm. Sedimentology Carhuaz made in a column in the White Cross (Cajamarca) determined that the sandstones are located 46% of the top of the whole column, indicating that this location is close to the central part of the whole column. The area PMPR Namora (Quad San Marcos) is 55%. These results indicate that the facies located in Cajamarca are less continental those located in San Marcos. It seems that the rich content of organic matter, depositional character warm and salty waters of both bands have favored chemical reactions between hydrothermal fluids and such. Instead these chemical reactions occur equally well in a transgressive and regressive environmental nature within the formation Carhuáz. Keywords: characterization, formation, fossil.

ONSHORE-OFFSHORE FACIES CHANGE OF NGRAYONG SANDSTONE IN MADURA AREAINDONESIA

Ngrayong sandstone composes a siliciclastic reservoir that produces oil for more than a century in North East Java Basin of Indonesia. Clean sand facies makes the best reservoir in western part of the basin, onshore East Java Island. Meanwhile, sand quality decreases eastward to Madura Island. In Madura, rock gradually changes to be more calcareous and shaly, due to the change of depositional environment. In offshore of Madura, the depositional environment is still questionable. This study is aimed to reveal differences between onshore and offshore facies of Ngrayong sandstone in Madura as their distribution is not well known and oil potential of Madura Strait is not well identified. Study methods consisted of geological field work, well correlation, and petrophysical analysis from several well data. The results show that Ngrayong sandstone was deposited during Middle Miocene, composed by interbedding of thickly bedded sandstone and alternating thinly bedded sandstoneshale. The succession is commonly intercalated by mudstone and thinly bedded limestone. At Madura Island, Ngrayong sandstone overlies the Early-Middle Miocene Tawun Formation, which both represent the Megasequence (MS) 3 interval. Due to regional subsidence and transgression during Late Miocene, Ngrayong sandstone and other equivalent rocks were overlain by monotonous mudstone and calcareous sandstone of Wonocolo Formation. The Ngrayong sandstone is evenly distributed in whole surface area of Madura Island and it spreads further 25-50 Km to the south and 100-125 Km to eastern part of offshore Madura Strait. Sandstone distribution is roughly depleted from the gross thickness hundreds of meters in northern part to only few centimeters in southeast part of study area. The facies changes to be more calcareous to the east, while the southward facies is shaly due to a deeper depositional environment. Despite of facies changing into shales or carbonate facies, Ngrayong sandstone potential in offshore Madura Strait needs to be considered as upside potential due to its distribution is wider than initial estimation, and its petrology and petrophysic data support it as a clastic reservoir.