Middle Jurassic Units Research Articles

3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and implications for petrophysical properties of potential Mesozoic shale host rocks for nuclear waste storage

AbstractJurassic sedimentary sequences suitable for nuclear waste storage in northern Germany consist of organic-lean claystone and were uplifted to < 100 m depth in the Hils Syncline area (southern Lower Saxony Basin). This Hils Syncline, showcasing a northwestward increase in thermal maturity, facilitates the study of shale petrophysical properties influenced by burial history. This study introduces a 3D-thermally calibrated numerical model of the Hils Syncline area to analyze its geodynamic evolution and maturity variations. It provides new vitrinite reflectance and sonic velocity data for modeling calibration and erosion estimation. The Hils Syncline area has undergone continuous subsidence, interrupted by a Cretaceous uplift documented by an erosional unconformity. During the latest Early Cretaceous, Jurassic rocks underwent maximum burial reaching up to several thousand meters depth and temperatures up to 160 °C in the northwest. The Late Cretaceous inversion caused stronger erosion towards the northwest removing up to 3300 m of sediment compared to about 1300 m in the south, according to vitrinite reflectance-based estimations. Numerical modeling results along the study area indicate decreasing porosity and permeability northwestward with increasing thermal maturity. Porosity and vertical permeability decreased to 5–14% and 2.8 × 10–23 to 1.5 × 10–19 m2 [1 mD = 10−15 m2], respectively, while vertical thermal conductivity increased to 1.30–2.12 (W/m/K). These trends of porosity/permeability and thermal conductivity with burial align with sonic velocity and published experimental porosity data, except for the thermally most mature region (Haddessen). This anomaly is tentatively attributed here to localized overpressure generation in the Posidonia Shale during maximum burial, affecting both the underlying Pliensbachian and overlying Doggerian units. Graphical abstract 3D numerical model of the Hils Syncline and surrounding area revealing that a northwestward increase in maximum burial resulted in higher temperatures and varying maturity levels. While most locations align well with calibration data (i.e. measured vitrinite reflectance and porosity), discrepancies arise in the Haddessen/Bensen area. The mismatch between porosity, vitrinite reflectance, and sonic velocity response indicates local overpressure in the northernmost region mainly during the Cretaceous. It was likely caused by gas generation in the Posidonia Shale affecting nearby Lower and Middle Jurassic units.

Clustering has a meaning: optimization of angular similarity to detect 3D geometric anomalies in geological terrains

Abstract. The geological potential of sparse subsurface data is not being fully exploited since the available workflows are not specifically designed to detect and interpret 3D geometric anomalies hidden in the data. We develop a new unsupervised machine learning framework to cluster and analyze the spatial distribution of orientations sampled throughout a geological interface. Our method employs Delaunay triangulation and clustering with the squared Euclidean distance to cluster local unit orientations, which results in minimization of the within-cluster cosine distance. We performed the clustering on two representations of the triangles: normal and dip vectors. The classes resulting from clustering were attached to a geometric center of a triangle (irregular version). We also developed a regular version of spatial clustering which allows the question to be answered as to whether points from a grid structure can be affected by anomalies. To illustrate the usefulness of the combination between cosine distance as a dissimilarity metric and two cartographic versions, we analyzed subsurface data documenting two horizons: (1) the bottom Jurassic surface from the Central European Basin System (CEBS) and (2) an interface between Middle Jurassic units within the Kraków–Silesian Homocline (KSH), which is a part of the CEBS. The empirical results suggest that clustering normal vectors may result in near-collinear cluster centers and boundaries between clusters of similar trend, thus pointing to axis of a potential megacylinder. Clustering dip vectors, on the other hand, resulted in near-co-circular cluster centers, thus pointing to a potential megacone. We also show that the linear arrangements of the anomalies and their topological relationships and internal structure can provide insights regarding the internal structure of the singularity, e.g., whether it may be due to drilling a nonvertical fault plane or due to a wider deformation zone composed of many smaller faults.

Provenance of the Middle Jurassic-Cretaceous sedimentary rocks of the Arequipa Basin (South Peru) and implications for the geodynamic evolution of the Central Andes

The provenance of Middle Jurassic-Cretaceous sedimentary rocks deposited in the southern Peruvian Arequipa Basin has been investigated based on the measurement of 41 Sr-Sm-Nd isotopic compositions of sedimentary rocks, 993 U-Pb zircons ages, and 202 Lu-Hf zircon isotope data. The Middle Jurassic units are dominated by Brazilian/Pampean and Grenville/Sunsas and have juvenile to evolved Hf isotopic compositions of Permian zircons. These are consistent with an Eastern Cordillera (EC) provenance. In contrast, the Late Jurassic Labra Formation presents the first appearance of Triassic-Jurassic zircons and an increase in Famatinian zircons thus suggesting a mixed provenance from the EC and/or Altiplano and the Coastal Cordillera (CC). The Gramadal Formation shows older U-Pb zircon ages, which reflect a major contribution from Precambrian sources either from the Amazon craton or the EC thus implying a change in provenance. Another change in provenance is documented for the Hualhuani Formation with sources located both in the CC and EC. Finally, the Santonian Chilcane Formation is characterized by a large contribution of Cretaceous to Famatinian zircons indicating that it was mainly sourced from the CC. Our provenance dataset associated with the depositional setting of the Middle Jurassic to Early Cretaceous formations and the cumulative curves of zircon age distribution, indicate that the Arequipa Basin was not a back-arc setting but rather a rift-like setting. Contrary to the older formations, the Chilcane Formation is dominated by contributions from the CC in a retroarc foreland basin setting. This statement further implies that the Andean basin located eastward of the volcanic arc had evolved directly from a rift to a retroarc foreland basin and implies that the control of Andean subduction on these basins might be much younger than previously thought.

Middle Jurassic sandstone deposition in the Wandel Sea Basin: evidence from cardioceratid and kosmoceratid ammonites in the Mågensfjeld Formation in Kilen, North Greenland

Age assessments from both palynostratigraphy and macrofossil biostratigraphy of the sandstone-dominated Mågensfjeld Formation, Wandel Sea Basin, North Greenland were hitherto hampered by post-burial thermal degradation of dinoflagellate cysts and a lack of well-preserved macrofossils. The formation was previously assigned to the Upper Cretaceous based on erroneous fossil identifications. Finds of cardioceratid and kosmoceratid ammonites during recent field work now provide the first age control of the unit, demonstrating it to be of late Bajocian – late Bathonian and perhaps Callovian (Middle Jurassic) age. This makes it among the oldest Jurassic units, perhaps even Mesozoic units, recorded in Kilen, North Greenland and eastern North Greenland. Previously, the complex structural and tectonic evolution of the area was poorly understood, and the structural relation of the Mågensfjeld Formation to the surrounding Mesozoic units was a puzzle. The new age assessment simplifies the structural situation in the area significantly. Further, the inference of a large reverse fault previously required to explain the proximity of the Mågensfjeld Formation to neighbouring Jurassic units is now unnecessary. The data show that the Wandel Sea Basin was influenced by the Middle Jurassic transgression and had sufficient accommodation space for marine deposition earlier than previously thought. The unit serves as a key datapoint and analogue for possible Middle Jurassic units in adjacent offshore basins.

Jarosite and Alunite in Ancient Terrestrial Sedimentary Rocks: Reinterpreting Martian Depositional and Diagenetic Environmental Conditions.

Members of the alunite group are precipitated at low pH (<1 to ~4) in oxidizing environments, are unstable in circumneutral conditions, and are widespread on Mars. At Mollies Nipple in Kane County, Utah, USA, jarosite and alunite are abundant as diagenetic cements in Jurassic sandstones. This research characterizes the jarosite and alunite cements with the goal of determining their origin, and tests the hypothesis that jarosite and alunite may be more stable than the current understanding indicates is possible. Previous studies have placed the jarosite- and alunite-bearing caprock at Mollies Nipple in the Navajo Sandstone, but the presence of water-lain deposits, volcanic ash, volcanic clasts, and peloids show that it is one of the overlying Middle Jurassic units that records sea level transgressions and regressions. A paragenetic timing, established from petrographic methods, shows that much of the cement was precipitated early in a marginal marine to coastal dune depositional environment with a fluctuating groundwater table that drove ferrolysis and evolved the groundwater to a low pH. Microbial interaction was likely a large contributor to the evolution of this acidity. Jarosite and alunite are clearly more stable in natural environments than is predicted by laboratory experiments, and therefore, the Martian environments that have been interpreted as largely acidic and/or dry over geologic time may have been more habitable than previously thought.

Litoestratigrafía del Grupo Tecocoyunca (Jurásico Medio) en el área del río Ñumí (cercanías de Tlaxiaco), Oaxaca y consideraciones sobre la distribución regional de su biota

La litoestratigrafía jurásica de la Región Mixteca es relativamente bien conocida, sin embargo, el esquema en que se basa incluye descripciones formacionales un tanto deficientes (e.g. vaguedad, caracterización por contenido fósil, delimitación cartográfica insuficiente). Con el propósito de contribuir a subsanar esta deficiencia, realizamos un estudio detallado del área Ñumí, cercanías de Tlaxiaco, donde las unidades mesojurásicas integrantes del Grupo Tecocoyunca despliegan sus atributos, permitiendo así suplementar las descripciones formacionales. Se encontró que el Grupo Tecocoyunca localmente incluye en la parte inferior a las Formaciones Asociadas Zorrillo/Taberna (Bajociano Temprano-Tardío inicial), constituidas por ~287 m de limolitas carbonosas, lodolitas y subarcosas, porta pelecípodos y plantas terrestres, así como dos zonas de carbón; se les interpreta como parte de un complejo deltaico. Estas formaciones subyacen en concordancia a la Formación Simón (Batoniano Medio-Tardío), integrada por ~270 m de subarcosas y limolitas dispuestas en estratos delgados y gruesos; se le considera un depósito transicional. Esta unidad subyace en concordancia a la Formación Otatera (Batoniano tardío), consiste en ~170 m de coquinas de pelecípodos con intercalaciones de estratos calcáreos de intraespatita; se le interpreta como un depósito nerítico somero, con un componente subordinado de playa. Esta unidad subyace concordantemente a la Formación Yucuñuti (Calloviano Medio), constituida por ~118 m de areniscas finas, coquinas, limolitas y biomicritas que portan pelecípodos y gasterópodos; se le interpreta como un depósito transicional a nerítico somero. A esta unidad le sobreyace en discordancia la Caliza con “Cidaris” del Oxfordiano, que no forma parte de este grupo. La biota del Grupo Tecocoyunca incluye una típica paleoflora mesojurásica, cuyos taxa se encuentran ampliamente distribuidos en la región centro-meridional de México y septentrional de Centroamérica; así como una paleofauna integrada por moluscos mesojurásicos comunes en la Región Mixteca. La biota en conjunto denota un clima regional cálido y húmedo. Finalmente, se considera que la descripción detallada de las formaciones que constituyen al Grupo Tecocoyunca, es de facto un avance en la redefinición de las unidades mesojurásicas de la Región Mixteca.

Palynofacies and TOC analysis of marine and non-marine sediments across the Middle–Upper Jurassic boundary in the central-northern Lusitanian Basin (Portugal)

The Middle-Upper Jurassic transition is a geodynamic benchmark in the evolutionary history of several peri-Atlantic basins. Contrary to the vast Tethyan and peri-Tethyan areas, in the Lusitanian Basin (Portugal), this interval corresponds to a major basin-wide disconfor- mity preceded by a complex forced regression that induced sharp facies variations across the depositional systems. The Middle Jurassic units below are fully marine, whereas the Upper Jurassic sediments (Lower?-Middle Oxfordian), correspond to freshwater/brackish lacustrine, grading into punctuated brackish-restricted lagoonal and shallow-mar- ine paleoenvironments. This study presents total organic carbon and palynofacies data of 34 samples (total analyzed thickness about 85 m) collected from three key sections encompassing the Middle-Upper Jurassic transition in the central-northern sector of the Lusitanian Basin. The pal- ynofacies of the analyzed part of Middle Jurassic units (Cabo Mondego Formation) are characteristic of marine environments at their base, evidencing upwards a regres- sive trend of the depositional systems. Total organic carbon content is generally low, reaching up to 1.94 wt%. The Upper Jurassic Cabacos Formation presents kerogen assemblages mostly of continental origin, although punc- tuated by minor intervals of marine influence. Total organic carbon content is more variable, reaching up to 30.56 wt%. Intraclasts of re-deposited fragments of microbial mats were found incorporated in the kerogen assemblages, which point to highly dynamic erosional and depositional processes. Diversity of Botryococcus sp. occurrences was confirmed as an indicator of the degree of paleoenvironmental stability. The vertical distribution and comparison of the kerogen assemblages of the different sections indicate major changes of these parameters among relatively close settings and along narrow vertical intervals, attesting to the high sedimentary dynamics observed in the Lusitanian Basin.

The Olivos Olistostrome: Remnant of a Late Permian Oceanic Basin along the Southwestern Margin of Laurentia, Chihuahua, Mexico

We report on the finding of an olistostrome containing Late Permian basalts (257 ± 7 Ma K-Ar, whole-rock) with MORB affinity embedded within deep-water Jurassic sediments (Upper Bajocian—Middle Callovian) in south-central Chihuahua, northern Mexico, at the southwestern margin of the North American craton (Laurentia). The largest Paleozoic outcrops in northern Mexico occur in the Chihuahua, Caborca, and Cortez terranes; the Los Olivos area is a small outlier with no discernible relation with either of those outcrops. Study of the Sierra de Olivos, in south-central Chihuahua, offers key elements to decipher the Late Paleozoic and Mesozoic evolution of the southern North American craton. Elements of oceanic crust in the olistostrome of Los Olivos are massive and pillowed basalts. Their chemistry shows that they are K-poor (K2O ~0.1%) and TiO2 rich (~0.8%); they are extremely depleted in elements such as Rb, Sr, and Zr, and trace-element compositions place them in the N-MORB and IAT fields of tectonic discrimination diagrams. The basalts, together with associated granitic tectonic blocks exhibiting a well-developed mylonitic texture, lie at the southern margin of Laurentia, and south of the inferred trace of the Mojave-Sonora megashear. They also lie along one of the inferred traces of the Ouachita suture. The Permian basaltic slabs were tectonically emplaced within the Middle Jurassic sedimentary sequence, but we infer that they were deposited as gravity slides. Their contacts with less competent rocks were reactivated, facilitating tectonic displacement during the Hidalgoan orogeny. The deep-marine Middle Jurassic sequence is informally assigned to the Pelayo Formation, which is covered by deep-water Cretaceous sedimentary facies of the Mezcalera Formation and by Tertiary volcanic rocks of the Sierra Madre Occidental magmatic arc. If the radiometric age determination of the oceanic basalts correctly dates the emplacement of these lavas, our study provides evidence on the existence of a Late Paleozoic—Mesozoic oceanic plate, subducted collision of either Gondwanic or Pacific elements with Laurentia, or during accretion of the Guerrero terrane (e.g., the Mezcalera plate). Characteristics of the markedly epiclastic Middle Jurassic units that host the olistostrome supports the assumption of an active margin setting for Jurassic deposition at the southern extreme of the Laurentia craton.