Clay-rich Units Research Articles

Impact of structural geometry of tilted fault blocks on hydrocarbon entrapment and deposition of syn-rift clastic reservoirs: Belayim Marine field (Gulf of Suez rift)

Belayim Marine oilfield is representative of the Gulf of Suez fields with hydrocarbon production from pre-rift and syn-rift reservoirs. Detailed subsurface mapping using a complete 3D seismic and borehole dataset throws light on the trapping mechanisms of tilted fault blocks in rift basins where the strike of the tilted block is parallel to the bounding fault. For such one-way tilted fault traps, linkage of rift-parallel and rift-oblique faults at the updip sides of the tilted blocks is necessary for trap integrity of laterally continuous reservoirs like the Nubia Sandstone. For laterally discontinuous reservoirs like tidal sand bars aligned normal or oblique to the block-bounding faults, a combination trap can be laterally closed by lateral change in facies of the reservoir unit along fault strike. Fault sealing of the proved hydrocarbon traps is mainly through juxtaposition. Shale smearing is valid for thick shale units with reasonable fault displacement and shale gouge is a proven sealing mechanism especially in clay-rich units. The ductile nature of syn-rift sediments leads to fault-propagation folding above the main-bounding faults of tilted fault blocks where folding is the trapping mechanism of these syn-rift reservoirs.The structural geometry of tilted fault blocks also controls the distribution of syn-rift reservoirs as the linkage of rift-parallel and rift-oblique faults controls the entry of main valleys transporting syn-rift clastics into the basin. Longitudinal depressions sculptured at the crests of tilted fault blocks represent another location for deposition of syn-rift reservoirs.

Carbon dioxide diffuse emission at the Tolhuaca hydrothermal system (Chile) controlled by tectonics and topography

The Tolhuaca hydrothermal system is one of the few attested geothermal resources in Chile. While recent investigations provided some insights into the depth and temperature of the geothermal reservoirs and the chemical and mineralogical evolution of the hydrothermal system, little is still known about the CO2 degassing of the system and the local and shallow control of fluid pathways. Here, we document the soil CO2 degassing and soil temperature distributions in the southern part of the Tolhuaca hydrothermal system and at one of its northern fumaroles, and provide a first estimate of its total CO2 release. The surveyed area is responsible for a total CO2 emission of up to 30 t d−1. Hydrothermal CO2 emissions (~ 4–27 t d−1) are mostly restricted to the thermal manifestations or generally distributed along NNW trending lineaments, sharing the same orientation as the volcanic vents and thermal springs and fumaroles. Hydrothermal CO2 fluxes, fumaroles and thermal springs are generally encountered in topographic lows, in close vicinity of streams and often in clay-rich pyroclastic units, highlighting a relation between landscape evolution and the activity of the hydrothermal system. We suggest that glacial unloading and incision of the stream inside the clay-rich units have likely enhanced locally the permeability, creating a preferential pathway for the migration of deeper fluid to the surface. As several hydrothermal systems in the Andes are found on the flank of volcanoes hosting glaciers, we propose that they could have had a similar development to that of the Tolhuaca hydrothermal system.

Core surprise: What's inside a plate boundary?

Despite the fact that 90% of global seismicity occurs at plate boundary faults, our understanding of their internal structure is lacking. It’s not easy to see inside a plate boundary fault – typically composed of a high-strain fault core surrounded by a fractured damage zone – and when we can, it often requires expensive drilling projects that yield limited information on the internal structure of the whole fault. Understanding the internal structure of large faults is crucial, because their chemical and mechanical properties control how and where earthquakes rupture, nucleate and propagate. This in turn limits the size of the earthquake or the amount of radiated seismic energy, and consequently the severity of surface damage. The 1999 magnitude 7.7 earthquake along the Chelungpu plate boundary fault, for example – the second deadliest earthquake in Taiwan’s recorded history – saw significant variations in slip and ground motion at different locations along the fault which resulted in large local variations in casualties and damage. Subsequent field investigations related these variations to changes in the fault’s structure (i.e., clay width, geometry), which in turn controlled how the fault moved.

Detailed internal structure and along-strike variability of the core of a plate boundary fault: the Highland Boundary fault, Scotland

The Highland Boundary fault near Stonehaven, NE Scotland, provides a rare opportunity to study the internal fault structure of a well-exposed, along-strike section of an ancient plate boundary fault. As in many plate boundaries, serpentinite juxtaposes quartzo-feldspathic crustal rocks of distinct terranes. We report, for the first time, the complex internal structure of the Highland Boundary fault core, comprised of four structurally and chemically distinct clay-rich units that remain unmixed. Despite the evidence for internal strain, relatively intact clasts of wall rock and microfossils are preserved within the clay. The fault core clay minerology is consistent with a shallow, low-temperature authigenesis derived from shear-enhanced chemical reactions between wall rocks of contrasting chemistry during sinistral strike-slip. The observed structure is comparable to those of other major weak-cored plate boundaries (e.g. the San Andreas fault). Through detailed mapping, we demonstrate that the internal structure of a plate boundary fault core can vary in thickness and composition along-strike over centimetre- to metre-length scales. Earthquake rupture mechanics critically depend on the physical properties of fault rock assemblages. Therefore, models that investigate rupture propagation at active plate boundaries should incorporate, or else assess tolerance and sensitivity to, variable fault core thickness and composition. Supplementary material : Further X-ray diffraction (XRD) analytical data are available at https://doi.org/10.6084/m9.figshare.c.4663640

Geochemistry of Sandstone Type Uranium Deposit in Tarat Formation from Tim-Mersoi Basin in Northern Niger (West Africa): Implication on Provenance, Paleo-Redox and Tectonic Setting

The study aimed at investigating the geochemistry of a sandstone-type uranium deposit in Tarat formation for provenance, paleo-redox, tectonic setting in order to propose a geological model of uranium. X-Ray Fluorescence (XRF) and Induced Coupled Plasma Mass Spectrometry (ICP-MS) analyses were used to determine the sandstone composition. Major, trace and rare earth element compositions of the sandstones have been investigated to determine their provenance, tectonic setting and the influence of weathering conditions. Field studies of Tarat formation revealed that a major accident N30° that can be interpreted as an extension of the Austriche accident ensures the controls of Tarat sedimentation and mineralization deposits and the presence oxidized zone and reduced zone indicate the direction of circulation of the mineralized fluids from West to East. The uranium mineralization is disseminated as pitchblende that gives a yellow color to the sandstone while, molybdenum mineralization gives a blue color to the sandstone. Results of the Geochemical analysis indicate that the sandstone-type uranium deposit of Taratis classified into Protoquarzite (i.e. lithearenite and sublitharenite), arkose, subarkose, wacke and quartz arenite and varied in their maturity. Their Rare Earth Elements (REE) pattern displays high light REE over heavy REE, flat HREE and a significant negative Eu and Ce anomalies and in general correlated well with the NASC and PAAS average composition. The source area may have contained quartzose sedimentary rocks. The geochemical data support deposition in reducing environment of arid to humid climatic conditions rich in organic matter affected by passive margin (PM) tectonic setting and the sediments are derived from felsic and mafic source rocks. Trace and REE data as well as the high values of the weathering indices: Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) suggest intermediate to extreme weathering conditions in the source area or during transportation. The studied sandstones are inferred as highly mature sediments evidenced from their high SiO2/Al2O3 > 1 and the presence of the ultra-stable clay minerals, smectite, kaolinite, chlorite and illite. Also the Th/U < 3 ratio suggests high uranium enrichment of Tarat sandstones over crust average composition. From the factor analysis of the analyzed sandstone, there is no direct relationship between organic carbon and uranium, indicating the absence of organo-uraniferous composite and a major source derived from felsic to the mafic rock of Air Massif. The mineralized fluids of circulating from west to east are usually capped and underlain by impermeable mudstone or clay-rich units and uranium mineralization occurs in the association U-Mo-Pb or U-Zn-Ni-Pb structurally controlled by the major fault in Azaoua lineament fault NS of Arlit in the West, the N70° Tin Adrar “faisceau” in the North, the N30° Mouron accident in the Southeast, N30 Autriche accident in the West and precipitated in areas of suitable reducing environment of arid to humid climatic conditions rich in organic matter affected by passive margin (PM) tectonic setting. The negative correlation of U-Th and U-Na in our samples indicates intense remobilization of uranium in Tarat formation.

Gas hydrate reservoirs and gas migration mechanisms in the Terrebonne Basin, Gulf of Mexico

The interactions of microbial methane generation in fine-grained clay-rich sediments, methane migration, and gas hydrate accumulation in coarse-grained, sand-rich sediments are not yet fully understood. The Terrebonne Basin in the northern Gulf of Mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds.Using 3D seismic and well log data, we have identified several previously unidentified hydrate bearing units in the Terrebonne Basin. Two units are >100 m-thick fine-grained clay-rich units where gas hydrate occurs in near-vertical fractures. In some locations, these fine-grained units lack fracture features, and they contain 1–4-m thick hydrate bearing-sands. In addition, several other thin sand units were identified that contain gas hydrate, including one sand that was intersected by a well at the location of a discontinuous bottom-simulating reflector. Using correlation of well log data to seismic data, we have mapped and described these new units in detail across the extent of the available data, allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and/or hydrate.We present several potential source-reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands. We observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units. Based on regional mapping showing the patchy lateral extent of the thin sand layers, we propose that diffusive methane migration or short-migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands, as discontinuous sands would not be conducive to long-range migration of methane from deeper reservoirs.

Hydrogeological monitoring and modelling in the S. Lorenzo road tunnel area (Passo della Morte, Udine) for the design of countermeasure works

In the eastern Italian Alps, a large deep-seated rock block slide affects the S. Lorenzo road tunnel (46°23'49 N, 12°42'51 E) along the National Road 52 Carnica, near to Passo della Morte (Udine Province); the tunnel was excavated through a fractured carbonatic rock mass overthrusted on clay-rich units. Previous studies of the unstable slope outlined the relationship between slope displacements and groundwater flow in the fractured rock mass. The conceptual geological model of the area, together with field observations, allowed specific investigation and monitoring activities to be defined in order to investigate this relationship. In particular, during the last years, six weirs were built and pressure transducers were installed in order to assess the total water discharge from the area. Moreover, 14 piezometers were drilled outside and inside the tunnel to monitor the hydraulic head. After 15 years of monitoring the groundwater system can be described in detail. In particular, results show a double flow system. The first is almost constant for all the year, while the second is influenced by rainfall events and is hosted in faults, fractures and joints in the western part of the tunnel, in the area of the larger and deep-seated rock slide. A three-dimensional groundwater flow model was built and calibrated along the total outflow from the road tunnel in order to better understand the interactions between groundwater flow and road infrastructure. In particular, the effects of countermeasures works, like the extension of the drainage adit in the carbonatic rock mass, was tested.

History of the clay-rich unit at Mawrth Vallis, Mars: High-resolution mapping of a candidate landing site

The Mawrth Vallis region is covered by some of the largest phyllosilicate-rich outcrops on Mars, making it a unique window into the past history of Mars in terms of water alteration, potential habitability, and the search for past life. A landing ellipse had been proposed for the Curiosity rover. This area has been extensively observed by the High Resolution Imaging Science Experiment and the Compact Reconnaissance Imaging Spectrometer for Mars, offering the possibility to produce geologic, structural, and topographic maps at very high resolution. These observations provide an unprecedented detailed context of the rocks at Mawrth Vallis, in terms of deposition, alteration, erosion, and mechanical constraints. Our analyses demonstrate the presence of a variety of alteration environments on the surface and readily accessible to a rover, the presence of flowing water at the surface postdating the formation of the clay-rich units, and evidence for probable circulation of fluids in the rocks at different depths. These rocks undergo continuous erosion, creating fresh outcrops where potential biomarkers may have been preserved. The diversity of aqueous environments over geological time coupled to excellent preservation properties make the area a very strong candidate for future robotic investigation on Mars, like the NASA Mars 2020 mission.

Geology of the Wilkes land sub-basin and stability of the East Antarctic Ice Sheet: Insights from rock magnetism at IODP Site U1361

IODP Expedition 318 drilled Site U1361 on the continental rise offshore of Adélie Land and the Wilkes subglacial basin. The objective was to reconstruct the stability of the East Antarctic Ice Sheet (EAIS) during Neogene warm periods, such as the late Miocene and the early Pliocene. The sedimentary record tells a complex story of compaction, and erosion (thus hiatuses). Teasing out the paleoenvironmental implications is essential for understanding the evolution of the EAIS. Anisotropy of magnetic susceptibility (AMS) is sensitive to differential compaction and other rock magnetic parameters like isothermal remanence and anhysteretic remanence are very sensitive to changes in the terrestrial source region. In general, highly anisotropic layers correspond with laminated clay-rich units, while more isotropic layers are bioturbated and have less clay. Layers enriched in diatoms are associated with the latter, which also have higher Ba/Al ratios consistent with higher productivity. Higher anisotropy layers have lower porosity and moisture contents and have fine grained magnetic mineralogy dominated by maghemite, the more oxidized form of iron oxide, while the lower anisotropy layers have magnetic mineralogies dominated by magnetite. The different magnetic mineralogies support the suggestion based on isotopic signatures by Cook et al. (2013) of different source regions during low productivity (cooler) and high productivity (warmer) times. These two facies were tied to the coastal outcrops of the Lower Paleozoic granitic terranes and the Ferrar Large Igneous Province in the more inland Wilkes Subglacial Basin respectively. Here we present evidence for a third geological unit, one eroded at the boundaries between the high and low clay zone with a “hard” (mostly hematite) dominated magnetic mineralogy. This unit likely outcrops in the Wilkes subglacial basin and could be hydrothermally altered Beacon sandstone similar to that detected by Craw and Findlay (1984) in Taylor Valley or the equivalent to the Elatina Formation in the Adelaide Geosyncline in Southern Australia (Schmidt and Williams, 2013). Correlation of the “hard” events with global oxygen isotope stacks of Zachos et al. (2001) and Lisiecki and Raymo (2005) suggest that the source region was eroded during times with higher global ice volume.

Natural fractures in a United Kingdom shale reservoir analog, Cleveland Basin, northeast England

Faults and fractures within the well-exposed Lower Jurassic Cleveland Ironstone and Whitby Mudstone Formations may provide insights into the tectonic history of gas-prospective, Mississippian shale in northern England. Subvertical opening mode fractures occur throughout the Cleveland Basin. Bed-parallel fractures, some of which contain blocky calcite fills, occur preferentially within well-bedded, clay-rich mudstones of the Cleveland Ironstone and Whitby Mudstone Formations at Jet Wyke and Port Mulgrave. Subvertical fractures display abutting or curving-parallel relationships with under- and overlying bed-parallel fractures. Together, these observations suggest that bed-parallel fractures, at times, acted as free surfaces. Some bed-parallel fractures curve toward and branch from calcite-filled fault slip surfaces, indicating that bed-parallel fracturing and normal faulting were synchronous, occurring within a regional stress field with vertical maximum principal stress. This apparent paradox can be explained by normal compaction, followed by cementation and coupling between pore pressure and minimum horizontal stress driven by poroelastic deformation or incipient slip along critically stressed normal faults, causing elevation of horizontal stress in excess of the vertical stress within clay-rich units. Propagation of bed-parallel fractures was enhanced by dilatational strains adjacent to normal fault planes. Bed-parallel fractures have not been observed within more -rich units at the top of the Whitby Mudstone Formation at Whitby East Cliff, or within well-bedded, clay-rich shale at Saltwick Nab. This observation is consistent with the lack of normal faulting at Saltwick Nab, and the Whitby Mudstone Formation having been drained by structural and/or stratigraphical juxtaposition against permeable Middle Jurassic sandstones at both these localities.

Normal faults, layering and elastic properties of rocks

We study mesoscale normal faults cutting alternating limestone and clay-rich layers in several localities in the South-Eastern Mesozoic sedimentary basin (France). The displacement gradients, defined as the displacement variation per unit length along fault profile, and the mean fault dips are correlated to the structural and petrophysical properties of the host rock, including the carbonate content, the stiffness, the layering pattern and the maximum burial depth. Analysis of the fault dips indicates that the faults propagate (downward or upward) from an initial fracture from one unit to another rather than through connections of fractures that nucleated in different units. The fault dips are consistent with shear, mixed mode and tensile failures in limestone units. They are consistent with shear failure or are abnormally low in the clay-rich units. Among the studied attributes, in limestone units the failure mode is related to the contrast of the Young's modulus between the limestone and the clay-rich layers. A high contrast promotes tensile failure, whereas a low contrast promotes mixed mode or shear failures. In clay-rich layers, the dip is related to the layering pattern and abnormally low dips are promoted in thin clay-rich layers surrounded by thick limestone units. Specific displacement gradients characterize each lithology of the layered section. It ranges from 0.06 to 0.2 in the clay-rich units and it increases with Young's modulus. The analyses of the fault dips and the displacement gradients have implications in term of local stress. Both analyses converge and they can be related to a variation in stress controlled by the variation in stiffness through the layering.

Mineralogy and grain-size distribution of clay-rich rock units of the Algarve Basin (South Portugal)

AbstractA detailed survey of the most clay-rich rock units of the Meso-Cenozoic geological section exposed in the Algarve Basin (South Portugal) was performed and data were analysed for the grain-size distribution and mineralogy (whole rock and clay fraction), aimed at a compositional study of the clay-rich sediments and their depositional environment. Granulometry was obtained using wet sieving and laser diffraction by attenuation of X-rays, and the mineralogical study was carried out by X-ray diffraction.Most clay-rich rock units of the Algarve are classified as silty clays and clayey silts, and only a minority is coarser. The mineralogical study enabled us to define two main types of clays: (1) noncalcareous clays, consisting largely of quartz and clay minerals, with goethite as the typical Fe-rich phase (sediments of Carboniferous, Neogene and Quaternary age and Cretaceous siliciclastic clays); and (2) calcareous clays, which can be calcite-rich clays (Middle and Upper Jurassic) or dolomiterich clays (Triassic and Lower Jurassic), the latter typically containing hematite as an accessory phase. Plagioclase, K-feldspar, and Ti-oxides are often accessory phases, whereas ankerite, anhydrite, gypsum and opal are rare.In the clay fraction illite generally predominates, resulting probably from weathering of preexisting rocks, as well as the less frequent Fe-chlorite, pointing to incipient chemical alteration under semi-arid climatic conditions. Kaolinite occurs in diverse proportions, being especially abundant in Cretaceous and Cenozoic units; it is mainly related to chemical weathering in continental environments under humid conditions. As the Atlantic Ocean opened during Triassic and the continental environment evolved to a shallow-marine environment with evaporitic conditions, smectite became more expressive, being sometimes accompanied by other Mg-rich phases (chlorite, sepiolite, corrensite and palygorskite). Especially during the Cenozoic the proportion of different phases in the clay mineral association of the sediments reflects the control of tectonic movements and fluctuations in sea level during their deposition.

Airborne geophysical mapping of aquifer water quality and structural controls in the Lower Okavango Delta, Botswana

Groundwater exploration in the Lower Okavango Delta to the west of Maun in Northern Botswana has focused on the search for shallow, fresh-water aquifers in Kalahari sediments. The target aquifers comprised thick packages of medium grained sands characterised by medium porosity and permeability, allowing for annual flushing and freshwater recharge during seasonal floods. Elsewhere, a combination of poor drainage, evaporation and evapotranspiration results in the widespread distribution of saline groundwater. The northeasterly-striking Tamalakane and Kunyere fault bounding the southern edge of the Okavango graben are still active, and in the recent past movements along these features has controlled the distribution of drainage systems across the Lower Delta. As a consequence, target freshwater palaeochannels may now be distant from present river channels, but still undergo recharge during annual Delta floods. The area is covered by Kalahari beds of variable thickness which overly Karoo sediments above a Damaran basement. Airborne geophysical mapping conducted by the Botswana Government at a scale akin to that of mineral exploration has played a major role in the groundwater exploration programme, as well as adding significantly to a structural understanding of the area. Airborne magnetic/DTM surveys have directly mapped the southern Thamalakane and Kunyere Fault zones as magnetic linears along fault-truncated magnetic lithologies of the Damaran basement, and as DTM scarp features at surface. Magnetic modelling of fault and dyke anomalies over the 65 km wide Okavango Dyke Swarm has quantified the expected step-wise northerly increase in thickness of the Kalahari sediments (from 10 m to 400 m). Basin or graben-like structures within the inter-fault area point to secondary fault linkages between the two major bounding fault zones. Airborne and follow-up time-domain EM (TDEM) surveys were used as hydrogeological mapping tools to locate and delineate potential freshwater-sand aquifers on the basis of their relatively high formation resistivities. Such aquifers appear as sinuous zones of moderate conductivity in an otherwise high-conductivity background environment reflecting the widespread distribution of saline groundwater away from permanent river channels and, to a possible lesser extent, clay-rich units interbedded with the sands. Airborne survey data was used to set the surface lateral limits of potential freshwater aquifers, and follow-up ground EM sounding surveys to determine the maximum thickness of such systems from mapping of the fresh-saline water interface. Over the better-developed aquifers this interface is found at depths of 60 m to 80 m. Elsewhere it may be within 10m of surface. Findings are exemplified by reference to follow-up work and borehole results over the Upper Thamalakane Aquifer which is proximal to the permanent river channel, and for which an isopach model was developed from extensive EM sounding work.

Trilobite Taphonomy and Temporal Resolution in the Mt. Orab Shale Bed (Upper Ordovician, Ohio, U.S.A.)

Clay-rich units, locally termed ‘‘butter shales,’’ contain the best-preserved trilobites in the richly fossiliferous Cincinnatian Series and likely provide the highest temporal resolution available within these rocks. Sedimentological and taphonomic evidence indicates that the 0.46-m-thick Mt. Orab ‘‘butter shale’’ bed of the Arnheim Formation is composed of a series of stacked event beds, each representing rapid deposition from a flow bearing fine-grained sediment, most likely associated with distal storm processes below storm-wave base. It contains sedimentary structures similar to those of distal mud turbidites, and comprises a total of at least seven, and possibly many more, alternating silt and clay couplets. These clay and silt layers are interpreted to represent the products of different energetic regimes in a series of discrete depositional events accumulated within a common depositional regime. Trilobites within individual clay beds represent census assemblages of animals alive at the same time, and evidence from sedimentology, taphonomy, and stratigraphic architecture are consistent with accumulation of the whole bed within a period from 10 1 to 10 3 years. Silt layers of the Mt. Orab events beds are interpreted to represent parautochthonous assemblages, while clay layers, although displaying reorientation of specimens, are interpreted as autochthonous assemblages. Both layers are deposited in a shallower-water environment than the comparable ‘‘granulosa’’ trilobite cluster of the Kope Formation, which represents an autochthonous assemblage with in-situ burial of trilobites.

Preferential pathways in an Eocene clay: hydrogeological and hydrogeochemical evidence

Abstract This study was initiated to test the extent to which the cation exchange capacity of marine clay-rich units, through which fresh water has been recharged, controls the composition of pore waters in an aquifer. The semi-confined Ledo-Paniselian (Eocene) aquifer in Flanders is recharged in elevated regions, where it is covered by Bartonian Clay. These sediments were initially laid down in marine conditions. The effects of recharging fresh CaHCO 3 water by downward flow through the overlying clay within the aquifer were modelled using PHREEQM. In the aquifer, progressively fresh waters are found in an upstream direction, which indicates a reduced influence of the initial marine conditions. Different stages of cation exchange produce a chromatographic sequence of groundwater types. Hydrogeological and hydrogeochemical evidence suggests the existence of preferential, faster flow pathways for recharge groundwater flow through the Bartonian Clay overlying the aquifer. These conclusions are indicated by differences between the observed and predicted features of the aquifer. It is necessary to invoke hydraulic conductivity values of 10 −9 m s −1 rather than 10 −10 m s −1 (measured values) for the Bartonian Clay so that the predicted head distributions in the aquifer match the observed heads. Furthermore, the measured exchangeable cations in clay collected from cores in the recharge area show less freshening than is predicted from the model to produce either the observed groundwater type within the aquifer in the recharge area or the overall distribution of groundwater types in the aquifer. Given the discrepancy between the observed and calculated state of freshening it is assumed that the analysed flow line in the core must be part of a slower flow pathway.

Abstract: Environmental Geology of Urban Growth Areas within the Edwards Aquifer and Balcones Fault Zone, South-Central Texas

Abstract: Environmental Geology of Urban Growth Areas within the Edwards Aquifer and Balcones Fault Zone, South-Central Texas

Rare Earth Elements in the manganese deposit of Úrkút (Bakony Mountains, Hungary)

First results are presented about the abundance and distribution of rare earth elements (REE) in the Úrkút manganese ore deposit. Geochemical analyses of samples of the manganese carbonate sequence reveal that the average REE content decreases from the base to the top. A positive Ce anomaly and the enrichment of REE in clay-rich units indicate that, in addition to adsorption of REE from sea water, a terrigenous REE source also displaying a positive Ce anomaly is involved. A Ce La ratio of 2.5 to 3.2 also indicates a terrigenous source for much of the REE in the samples studied. In the primary enrichment of REE, the clayey fractions played an important role. Oxidative processes have led to further enrichment of REE where primary carbonate ores have been altered to oxides. The clays in the ore deposit have no economic importance because their Mn content is very low. However, their distribution and enrichment in REE provides information about the conditions of formation of the deposit. Among the mining fields Csárdahegy differs in geochemical character from that of the field Shaft III and Kislöd.

Deformation by Soft-Sediment Extension in the Coastal Belt, Franciscan Complex

Coastal belt rocks are a thick flyschlike sequence of interbedded sandstone and mudstone. Between Laytonville, California, and the coast, beds strike northwest and dip moderately northeast. Pull-apart features and discontinuous bedding are common throughout the sequence. A major unit of highly disturbed bedding averaging 2 km thick can be traced for more than 35 km along strike. The unit is a composite of several thinner units of disrupted bedding. It is characterized by pinch-and-swell, pull-apart sandstone beds, boudins with rounded terminations, sheared mudstone, argille scagliose , intricate discontinuous folds, and exotic blocks of volcanic rock and limestone. Axes of sigmoidal folds in sandstone boudins and in thin clay-rich units generally parallel regional strike but show no preferred sense of rotation. At one place, undisturbed strata lie on this unit with sharp contact; however, most of its contacts are gradational. Pebbly mudstone and a large exotic block of chert were found in other smaller disrupted units. Several features indicate that extensional deformation occurred while sediments were unconsolidated. Pebbly mud-stone and large exotic blocks suggest that downslope mass movements were important. Deformation here probably reflects a combination of submarine sliding and intrastratal shearing; these conditions might be expected on trench slopes during active subduction.