Sedimentary Dikes Research Articles

Tracking quartz and zircon provenance in sedimentary rocks using Ti distributions: Unlocking the volcanic-plutonic connection in old igneous systems

Although the relationship between silicic volcanic and plutonic rocks has been extensively studied, it remains controversial whether most plutons have volcanic counterparts and how the volcanic-to-plutonic ratio has evolved through time. This stems primarily from the scarcity of geologic examples with both volcanic and plutonic counterparts, especially in ancient terranes where volcanic deposits have likely been eroded. Here, we introduce a novel approach to identify remnant volcanic crystals in proximal siliciclastic/detrital rock formations. As most erupted crystals stop their growth before reaching the solidus, in contrast to plutonic crystals, the distribution of Ti (a proxy for temperature of crystallisation and/or magma evolution) in (weathering-resistant) quartz and zircon can help in identifying volcanic crystals in sedimentary rocks derived from supracrustal sources that have been completely eroded. Laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of quartz and zircon from volcanic and plutonic units that are known to be spatially and temporally related (from Southern Rocky Mountain Volcanic Field, Colorado, and Taupo Volcanic Zone, New Zealand) reveal that volcanic crystals have generally distinct distributions of, and on average higher Ti contents than, plutonic crystals. To further develop and test this methodology, we analysed samples from the 1.075 Ga Pikes Peak granite (CO, USA) and from the associated series of intra-granite sedimentary dikes that form part of the Cryogenian Period Tava sandstone. Blue band cathodoluminescence (CL) images can be used as a proxy for Ti-in-quartz. Our CL-screening method provides a statistically representative distribution of Ti contents in quartz crystals from individual thin sections. The Tava sandstone contains a higher concentration of high-Ti (and hence high-temperature) quartz and zircon grains than the average Pikes Peak batholith, interpreted as grains eroded from a no longer exposed, volcanic counterpart of the Pikes Peak magmatic system. Our study illustrates a means to test whether magmatic reservoirs that are now represented by crystallised ancient plutons once fed volcanic eruptions, and identifies a strategy for reconstructing volcanic-plutonic relationships, even when parts of the geological association have been eroded.

STRATIGRAFIA DEL GIURASSICO DEI DINTORNI DI FOZA (ALTOPIANO DI ASIAGO)

The Middle and Upper Jurassic sequence near Foza (Asiago plateau; Venetian Alps) is characterized by important hiatuses. The relevant deposits are furthermore often found as sedimentary dikes, filling up fractures in the underlying carbonatic platform of the Calcari Grigi, Middle Liassic in age in the uppermost levels. The normal Middle and Upper Jurassic sequence includes, aver the Calcari Grigi the following formations: Lower Rosso Ammonitico Veronese and "Lumachella a Posidonia alpina", both Middle Ju­rassic in age; "Scisti ad Aptici", Upper Rosso Ammonitico Veronese and the basal part of the "Biancone", Upper Jurassic in age. In the Foza area the stratigraphic sequence changes quite rapidly in very short distances, with hiatuses involving one or more of the above mentioned formations. The presence of these hiatuses and of sedimentary dikes is in agreement with the location of the studied area near the edge of the "Trento plateau" during the Jurassic. A fossil assemblage was collected from a sedimentary dike near Lazzaretti of Foza. It consists of Brachiopods, Belemnites, Bivalves and Crinoid remains. Brachiopods are prevailing both in specimens and species number; among them 6 species, even if generally poorly preserved have been determined and described.

Soft-sediment deformation structures in alkaline lake deposits of Lower Permian Fengcheng Formation, Junggar Basin, NW China: Implications for syn-sedimentary tectonic activity

Soft-sediment deformation structures (SSDS) are an important tool for interpreting paleoseismicity and depositional environments in ancient basin successions. The northwestern margin of Junggar Basin is a key region for understanding the paleogeography and basin attribute of the Junggar Basin during the early Permian. In the Lower Permian (Kungurian) Fengcheng Formation, nine lithofacies are grouped into fan delta, fluvial delta, and shallow, deep and alkaline lacustrine facies associations. Vertical transitions of the sedimentary environments show that the lake basin experienced expansion during deposition of the Lower (P1f1) and Middle (P1f2) members and the shrinkage during the Upper Member (P1f3), which is consistent with the paleogeographic distribution of lithofacies in different members. Conventional cores show a wide variety of SSDS, including ductile (folds, convolute lamination, and load structures), hybrid ductile-brittle, and brittle deformation structures (microfaults, breccias, boudinage and sedimentary dikes). SSDS mainly occurred in a still and deep lacustrine environment. These deformation structures were not formed in any specific lacustrine environment, as there is no difference with the undeformed sedimentary units of the same sedimentary facies. Thus, the triggering mechanism is interpreted as intermittent earthquakes. Lake expansion under arid condition and SSDS intervals with vertical repetition and possible lateral continuity indicate that most seismicity was during the Middle Member (P1f2), corresponding with the main activity on basin-bounding normal faults. The enhanced faulting at this time generated accommodation, creating lacustrine conditions in the basin depocenter. It is thus clear that the early Permian Mahu sag represents an active rift basin with bounding faults, and is not a foreland basin.

Alpine halite-mudstone-polyhalite tectonite: Sedimentology and early diagenesis of evaporites in an ancient rift setting (Haselgebirge Formation, eastern Alps)

The Permian Haselgebirge Formation of the eastern Alps is among the most highly deformed evaporites worldwide. In underground salt mines, it is composed of a melange of halite, anhydrite, and mudstone, with a subordinate amount of magmatic rocks. The only potassium mineral is polyhalite [K2Ca2Mg(SO4)(4)center dot 2H(2)O]. Its extreme tectonic deformation provides a challenge for reconstructing the tectono-sedimentary environment. In this context, we were able to compile a set of characteristic sedimentary and early diagenetic features of the non-halite components. In particular, mudstone and anhydrite pseudomorphs after gypsum preserve synsedimentary structures. While mudstone represents the overall background sedimentation, sulfate precipitated mainly at the brine surface under pelagic conditions. The large amount of mudstone (50%) was probably trapped as mud in a quiet lagoonal basin. Its magnesium-dominated mineral composition is in accordance with coeval halite precipitation from seawater. Earthquake-induced deformation structures formed in the semiductile mud sediment as mostly ramified, centimeter-scale sedimentary dikes, which originated intrastratally by liquefaction. Permian volcanism is documented by a basaltic body from the Hallstatt salt mine, which exhibits a sedimentary contact with the mudstone. Its immobile trace-element composition favors an intraplate origin consistent with a rift stage. During subsequent burial, authigenic minerals such as quartz and feldspar started to grow. The origin of polyhalite relates to the mobilization of complex fluids and roughly coincides with the final, late Middle Triassic opening of the Meliata Ocean. All evidence from syn-sedimentary deformation and early diagenesis confirms the deposition of the evaporites in a late thinning stage of an ancient rift.

Sedimentary record of seismic events in the Eocene Green River Formation and its implications for regional tectonics on lake evolution (Bridger Basin, Wyoming)

Outcrops and cores from the top of the lacustrine Tipton Member and the base of the Wilkins Peak Member (~51.5Ma) of the Eocene Green River Formation, Bridger Basin in southwestern Wyoming yield a wide variety of sedimentary deformation features many of which are laterally extensive for more than 50km. They include various types of folds, load structures, pinch-and-swell structures, microfaults, breccias and sedimentary dikes. In most cases deformation is represented by hybrid brittle–ductile structures exhibiting lateral variation in deformation style. These occur in low-energy, profundal organic-rich carbonate mudstones (oil shales), trona beds, tuffs, and profundal to sublittoral silty carbonate deposited in paleolake Gosiute. The deformation is not specific to the depositional environment because sedimentary units stratigraphically higher with similar facies show no deformation.The studied interval lacks any evidence for possible trigger mechanisms intrinsic to the depositional environment, such as strong wave action, rapid sediment loading, evaporite dissolution and collapse, or desiccation, so ‘endogenic’ causes are ruled out. Thus, the deformation features are interpreted as seismites, and change in deformation style and inferred increase in intensity towards the south suggest that the earthquakes were sourced from the nearby Uinta Fault System. The 22 levels exhibiting seismites recognized in cores indicate earthquakes with minimum magnitudes between 6 and 7, minimum epicentral intensity (MCS) of ~9, and varying recurrence intervals in the seismic history of the Uinta Fault System, with a mean apparent recurrence period of ~8.1k.y. using average sedimentation rates and dated tuffs; in detail, however, there are two noticeably active periods followed by relative quiescence. The stratigraphic position of these deformed intervals also marks the transition between two distinct stages in lake evolution, from the balanced-filled Tipton Member to the overlying, underfilled Wilkins Peak Member. Thus, these seismites are evidence for regional-scale changes in lacustrine sedimentation of Eocene Lake Gosiute in response to syndepositional tectonic activity. Analysis of synsedimentary deformation features is, therefore, a promising yet under-utilized tool to trace the tectonic evolution of lacustrine deposits of the Green River Formation and other tectonically active marine and non-marine basins.

Core evidence of paleoseismic events in Paleogene deposits of the Shulu Sag in the Bohai Bay Basin, east China, and their petroleum geologic significance

The Shulu Sag, located in the southwestern corner of the Jizhong Depression, Bohai Bay Basin of east China, is a NE–SW trending, elongate Cenozoic half-graben basin. The lowermost part of the third member of the Shahejie Formation in this basin is characterized by continental rudstone and calcilutite to calcisiltite facies. Based on core observation and regional geologic analysis, seismites are recognized in these lacustrine deposits, which include soft-sediment deformation structures (sedimentary dikes, hydraulic shattering, diapir structures, convolute lamination, load-flame structures, ball-and-pillow structures, loop bedding, and subsidence structures), synsedimentary faults, and seismoturbidites. In addition, mixed-source rudstones, consisting of the Paleozoic carbonate clasts and in situ calcilutite clasts in the lowermost submember of Shahejie 3, appear in the seismites, suggesting an earthquake origin. A complete representative vertical sequence in the lowermost part of the third member found in well ST1H located in the central part of the Shulu Sag shows, from the base to the top: underlying undeformed layers, synsedimentary faults, liquefied carbonate rocks, allogenetic seismoturbidites, and overlying undeformed layers. Seismites are widely distributed around this well and there are multiple sets of stacked seismites separated by undeformed sediment. The nearby NW-trending Taijiazhuang fault whose fault growth index is from 1.1 to 1.8 and the NNE-trending Xinhe fault with a fault growth index of 1.3–1.9 may be the source of the instability to create the seismites. These deformed sedimentary layers are favorable for the accumulation of oil and gas; for example, sedimentary dikes can cut through many layers and serve as conduits for fluid migration. Sedimentary faults and fractures induced by earthquakes can act as oil and gas migration channels or store petroleum products as well. Seismoturbidites and mixed-source rudstones are excellent reservoirs due to their abundant primary or dissolved pores.

Eocene Paleoseismic Record of the Green River Formation, Fossil Basin, Wyoming, U.S.A.: Implications of Synsedimentary Deformation Structures In Lacustrine Carbonate Mudstones

Abstract Laterally extensive intervals containing sedimentary deformation features are identified in outcrops of lacustrine deposits of the Eocene (∼ 52–51 Ma) Green River Formation in Fossil Basin, southwestern Wyoming. Fossil Basin is the smallest of the paleo-lakes in the Green River system and is located in the Sevier Fold and Thrust Belt, which was tectonically active during deposition. Deformation structures include convolute lamination, load structures, hybrid brittle–ductile features, sedimentary dikes, microfaults, fluid-escape features such as cusps and pipes, oil-shale breccias, and mass-transport deposits. In most cases these structures are hosted by finely laminated, variably organic-rich carbonate mudstones that accumulated in a low-energy profundal lacustrine environment. Deformed intervals range from a few centimeters to 5 m in thickness and are bound above and below by undeformed strata of the same facies, which demonstrates the early post-depositional timing of deformation. The distribution of these features indicates a primary control exerted by sediment rheology, resulting from variation in grain size, clay and organic content, and degree of lithification, which in turn were governed by the depositional environment and early diagenesis. Based on their morphological attributes, lateral extent, and recurrence at different stratigraphic levels, along with their sedimentary environment and the tectonic setting of the basin, the deformation structures are interpreted to be “seismites,” i.e., the result of stresses induced by syndepositional earthquakes, and other potential trigger agents are rejected. Moreover, stratigraphic relationships indicate that these small-scale deformation features as well as the mass-transport deposits, formed on a virtually flat lake floor, ruling out instability caused by the gravitational regime. Syndepositional deformation in Fossil Basin points to tectonic activity along the nearby Absaroka and/or Hogsback thrusts and related fault systems. Seismites point to possible tectonic forcing as a mechanism for changes in bathymetry and sedimentation in the Green River Formation and, consequently, demonstrate their potential for elucidating the tectonic history of other lacustrine and marine basins.

Endogenous and nonimpact origin of the Arkenu circular structures (al‐Kufrah basin—SE Libya)

Abstract– The twin Arkenu circular structures (ACS), located in the al‐Kufrah basin in southeastern Libya, were previously considered as double impact craters (the “Arkenu craters”). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica‐impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported “shatter cones” are wind‐erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a “paired” intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite–hematite horizons (typical of iron oxide copper‐gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic‐phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW–SSE and NE–SW, that crosscut the structural axis of the basin.

The Latemar: A Middle Triassic polygonal fault-block platform controlled by synsedimentary tectonics

Detailed field mapping of a Middle Triassic carbonate buildup, the Latemar in the western Dolomites, northern Italy, has been carried out. The Latemar is an isolated carbonate buildup that nucleates on a fault-bounded structural high (horst) cut into the underlying late Anisian carbonate bank of the Contrin Fm. This study demonstrates that extensional synsedimentary tectonics is the main factor controlling its geometry and provides an age for this tectonic phase. In an early phase, slopes were mostly composed of well bedded, clinostratified grainstones and rudstones. In a later stage, the deposition of grainstones was accompanied by the emplacement of clinostratified megabreccias. The upper portion of slopes is a microbial boundstone with abundant Tubiphytes and patches or lenses of grainstone. Boundstones may occasionally expand into the platform interior and downward to the base of the slope. The depositional profile was that of a mounded platform. The buildup is dissected by a dense framework of high angle fractures and faults, and by magmatic and sedimentary dikes, exhibiting two principal directions trending NNW–SSE and ENE–WSW. Faults trending WNW–ESE were also observed. Magmatic dikes are related to the emplacement of the nearby Predazzo intrusion and are thus upper Ladinian. Kinematic indicators of strike-slip activity were observed on fault planes trending NNE–SSW and NNW–SSE, that can be attributed to Cenozoic Alpine tectonics. Faults, magmatic dikes and sedimentary dikes show systematic cross-cutting relationships, with strike-slip faults cutting magmatic dikes, and magmatic dikes cutting sedimentary (neptunian) dikes. ENE–WSW and WNW–ESE faults are cut by all other structures, and record the oldest tectonic activity in the region. Structural analysis attributes this tectonic phase to an extensional stress field, with a direction of maximum extension oriented ca. N–S. Several lines of evidence, including sealed faults and growth wedge geometries allow us to recognize this old tectonic phase as syndepositional, and to date it to the latest Anisian ( avisianum and crassus ammonoid biochronozones). The horse-shoe shape of the platform and its margins, rectilinear for long tracts, are explained by the existence of a network of extensional faults, whose activity lasted for part of the growth of the platform. The Latemar is thus a polygonal, fault-block platform controlled by synsedimentary tectonics. The resulting sedimentary architecture is that of a growth wedge, deposited probably on the uplifted side of a submerged half graben. Continuous reactivation of faults at platform margins locally determined anomalous facies transitions, with well layered platform interior directly in contact with the clinostratified slopes.

Gerollführende sedimentare Gänge der Eisenach-Formation (Oberrotliegend): modifizierte artesische Injektionen am Fuß alluvialer Fächer?

Gerollführende sedimentare Gänge der Eisenach-Formation (Oberrotliegend): modifizierte artesische Injektionen am Fuß alluvialer Fächer?

First data on age of radiolarian assemblages from sedimentary dikes in suevites of the Kara impact structure

First data on age of radiolarian assemblages from sedimentary dikes in suevites of the Kara impact structure

Skeletal-Carbonate Neptunian Dikes of the Capitan Reef: Permian, Guadalupe Mountains, Texas, U.S.A.

Sedimentary dikes in the Permian (Guadalupian) Capitan Reef of West Texas and New Mexico are composed of siliceous sand, skeletal carbonate, or cement. Skeletal-carbonate neptunian dikes appear to be a relatively uncommon type; only four are known by the authors, but they are important in providing information about platform growth, sediment sources, and dispersal on the platform, and the biota that were present on the platform. The dikes occur within massive poorly sorted skeletal grainstones typical of the reefal part of the Capitan Formation (the Capitan-massive) that is correlative with upper Guadalupian Yates B strata on the platform. They formed during later Yates B, Yates C, and Tansill deposition on the platform, following closely a major period of progradation of the platform margin. The dikes are approximately vertical, and parallel to and within about 100 m of the platform margin. Fissure formation was a probable consequence of flexing and fracturing of lithified shallow-water Capitan-massive over compacting unconsolidated slope and basinal sediment. At least three stages of opening and filling are recorded in the largest and most complex dike, with each sequential fissure opening forming on the basinward side of the previous cemented filling. In two of the dikes, centimeter-thick deposits of vertically laminated peloidal grainstone, which formed in situ within microbial films on the fissure wall, preceded filling by skeletal sediment. Skeletal fossils within two of the dikes are well preserved and represent organisms that either were living within the fissures or were living on the adjacent sea floor and were washed into the fissures soon after death. Other skeletal grains in these dikes, and all of those in the other two dikes, are abraded, strongly micritized, and encrusted with microbial micrite, indicative of having lain on the sea floor for a considerable time before being deposited in the fissures. Fossils in the dikes are characteristic of the reefal and adjacent outer-shelf facies tracts, but they are a biased record of the biota of that depositional area in that they consist largely of vagile or only weakly attached members of the dweller and baffler guilds living on the reef surface and of organisms living within the fissures; members of the reefal framework communities are largely absent. The absence in the dikes of fossils diagnostic of more distant facies tracts indicates that there was little across-platform movement of carbonate sediment on the platform. Deposition of terrigenous sediment is correlated with a sea-level low-stand, indicating that land-derived sediment moved across the platform otherwise infrequently.

Hydraulic fracturing during the formation and deformation of a basin: A factor in the dewatering of low-permeability sediments

The geological expression of hydraulic fracturing is varied and is controlled primarily by the magnitude of the differential stress and the intrinsic properties of the rock. The orientation and type of fractures that develop within a basin are determined by the state of stress, which in turn is controlled by the geological boundary conditions. During the early stages of burial and diagenesis the formation of hydraulic fractures is thought to be an important factor in the movement of fluids through and out of low-permeability, semilithified sediments. Unfortunately, these fractures are not generally preserved and are presumed to heal once the fluid pressure is relieved. The low-permeability Mercia Mudstones of the Bristol Channel Basin, southwest England, however, contain bodies of sand that, during the opening of the basin, were injected along some of the hydraulic fractures in the mudstones, preserving them as sedimentary dikes and sills. Field observations indicate that fluid pressures within the Mercia Mudstones were also very high during basin inversion and that hydraulic fracturing provided a transient permeability that relieved this excess pressure. The fractures are not visible in most of the mudstones but have been preserved within evaporite-rich horizons as a network of satin spar veins. Thus, the chance preservation of the sedimentary dikes and satin spar veins shows that at different times during the evolution of the basin, fluids migrated through low-permeability units along transient networks of hydraulic fractures. In addition, the orientation and spatial organization of these fractures reflect the boundary conditions operating at various stages in the basin history.

Anatomy of an Anomaly: The Devonian Catastrophic Alamo Impact Breccia of Southern Nevada

The Alamo Breccia is a carbonate rock breccia of Late Devonian age in southern Nevada. It is an anomalous sedimentary unit because it has the properties of a massive debris-flow and turbidity-current deposit that would be expected to occur in deep water, but is intercalated over much of its area with typical shallow-water carbonate-platform beds. The Breccia was created by the catastrophic detachment and flow, over a nearly horizontal surface, of previously deposited platform carbonates. It crops out in 14 or more mountain ranges that cover an area of ∼10,000 km2, conservatively averages ∼50 m in thickness, and contains a volume of 500+ km3. Along the base it contains trains of individual detached blocks as much as 500 m long and 90 m high. Clasts generally grade upward to gravel-, sand-, or mud-sized particles at the top. The Breccia was generated by forces unleashed during the impact of an extraterrestrial object with Earth. The impact produced shocked quartz grains, unique ejecta spherules, and an iridium anomaly–which are present within the Breccia but absent from confining beds. Internally the Breccia is segmented vertically into as many as five sequentially thinner graded units created by successive tsunamis. In one range, peculiar deformed dolostone, shocked quartz sandstone, and sedimentary dikes and sills occur under the Breccia and deep-water limestones rest over it, indicating a near-crater location. Surrounding detached megablocks and tsunamites suggest an annular crater trough. The Breccia formed within the span of a few hours or days, and falls entirely within a single early Frasnian conodont zone, at ∼367 Ma. The welldocumented middle Late Devonian (Frasnian/Famennian) extinctions are ∼3 Ma later. An impact scenario explains the known features of the Alamo Breccia: impact occurred on the Late Devonian outer platform or slope; seismic shock delaminated the upper ∼50 to 100 m of the platform, loosening carbonate-platform bedrock and creating trains of large blocks that may rest in an annular trough; successive tsunamis reworked the loosened material, which was augmented by unknown proportions of ejecta containing shocked quartz from the crater, carbonate spherules from the vapor cloud, and iridium from the projectile. Mass flows west of the platform likely represent tsunami backwash, shock-induced failure along the platform margin, and slumps from offshore topographic highs.

THE MIDDLE-UPPER TRIASSIC OF THE SAN DONATO UNIT AUCT. (NORTHERN CALABRIA): STRATIGRAPHY, PALEOGEOGRAPHY AND TECTONIC IMPLICATIONS

In this paper are presented stratigraphic and sedimentological data on the carbonate successions occurring in the Cozzo del Pellegrino massif (Cosenza), generally considered as pertaining to the metamorphic San Donato Unit. The successions start with thick phyllites and intercalated carbonate lenses containing Anisian-lower Ladinian algae. To the phyllites follows the informally defined calcari formation, locally occurring also as its lateral equivalent. The calcari formation consists of two members, the first of which (Piano del Minatore Mbr.)consists of black, often marly limestones, showing nodular and bioturbated textures, with a scarce and banal fauna represented by ostracods, gastropods and bivalves. Algal mounds, with porostromata and crinoids are locally intercalated in these facies. In the upper part of the formation the black limestones pass laterally to a reef complex (Monte Caramolo Mbr.) of Ladino-Carnian age, consisting mainly of boundstones with sponges and biogenic crusts, as well as of fore-reef breccias. These lithotypes have been interpreted as deposited on a carbonate ramp evolving to a restricted, poorly oxygenated lagoonal area, bordered by bioconstructed margins. In the lower Carnian a carbonate-marly horizon, containing traces of evaporites, whose thickness is decreasing toward the east, allows a lithostratigraphic correlation betweenmost of the studied successions. The calcari formation is followed by the Scifarello formation, mainly outcropping in the eastern parts of the studied area. It consists generally of tidal dolomites with some tempestites deposited on an shallow open shelf, followed by dolomites and laminated, often marly, dolomitic limestones, deposited in a subtidal, restricted environment. On the basis of foraminifers and bivalves data, the upper part of the Scifarello formation has been ascribed to the upper Carnian-Norian. Moreover, a strong tectonic activity of Lower Norian age is evidenced by the presence of mass-flows and sedimentary dikes in the more easterly areas of the massif. The general paleoenvironmental evolution, in the period spanning between Anisian and Early Norian, can be envisaged firstly in a carbonate sedimentation on a wide shelf, grading toward the east to deeper, possibly basinal areas, whose location was controlled by the Ladinian and/or Norian synsedimentary tectonics. This shelf was bordered on the west by a peri-continental area, with silicoclastic to evaporitic deposits, better represented in the Cetraro area. Finally, the Norian dolomites pertaining to the Verbicaro Unit, usually considered as have been thrusted onto rhe Triassic carbonates of the San Donato Unit, have been often observed to occur in normal stratigraphical superposition over the latter. Therefore, also considering the ambiguous evidence presented in the literature on the Jurassic-Miocenic evolution of the San Donato Unit AUCT, we interpret the Norian Verbicaro dolomites as being the most natural evolution of the Anisian-Lower Norian San Donato lithotypes. It follows that for a really valid paleogeographic reconstruction a complete redefinition of the tectonic units in the whole area is needed.

Geologic Structures That Affect Coal-Mine Roof Stability

The US Bureau of Mines has investigated several geologic structures that plague underground coal mining. These structures have caused numerous roof falls and, consequently, many miners have been hurt or fatally injured. Hazardous geologic anomalies that can be encountered in the eastern and western US coal fields include slips, hill seams, slickensides, sedimentary dikes, kettlebottoms, and paleochannels. Slips occur throughout the United States and are high-angled fractures in the roof rock. Typically, the rock adjacent to slips is highly slickensided and striated. Hill seams are weathered fracture zones and usually are found in areas where the overburden is shallow. Due to their nature and regional mining conditions, hill seams usually are found only in the southern Appalachian coal basin. Slickensides are lower angled planes of weakness found in coal-mine roof rock throughout the country. Slickensides usually are found in clusters and the adjacent rock is highly polished and striated. Sedimentary dikes (also clastic dikes and clay veins) are the remnants of ancient fissures that have been filled through sedimentary processes, Sedimentary dikes normally are found in coal basins that were slowly subsided. Kettlebottoms are the fossilized remains of trees that once grew in and above the swamp forest and generallymore » are found in coal basins that were rapidly subsided. Paleochannels are the lithified remains of rivers and frequently are found in all coal basins except the Black Warrior.« less

Sulfur isotope studies of the Stillwater Complex and associated rocks, Montana

The Stillwater Complex in south-central Montana is a late Archean layered, ultramafic to mafic intrusion emplaced into middle to late Archean metagraywacke, metashale, and ironformation. Sulfide minerals are concentrated near the base of the intrusion in some chromitite layers, podiform pegmatoids, discontinuous discordant pods or pipes, and numerous thin, stratiform layers. This investigation focuses on the isotopic composition of sulfur in metamorphosed sedimentary rocks, Stillwater-associated sills and dikes, sulfide accumulations near the base of the complex, and platinum-group element-enriched sulfide mineralization that constitutes the J-M reef and the Picket Pin deposit.The delta 34 S values for 233 samples analyzed in this study have a mean of 0.4 per mil, a standard deviation of 1.7, and a maximum and minimum of-3.8 and 7.8 per mil, respectively. Despite the very narrow range of values, analysis of variance calculations confirms that the data comprise three statistically distinct groups. These groups are (1) metagraywackes and metashales (mean = 2.8ppm); (2) iron-formation, Stillwater-associated sills and dikes, and basal sulfide accumulations in the complex (mean = -0.7ppm); and (3) disseminated sulfide minerals that occur above the basal accumulations in the Stillwater Complex (mean -- 1.2ppm). The isotopic composition of sulfur in the basal sulfide accumulations indicates that assimilation of substantial amounts of sulfur from iron-formation may have occurred. The low tenor of the basal sulfide ores is also consistent with assimilation of sulfur into a limited volume of magma near the base of the intrusion.

Clastic Dykes and Sills from Numidian Flysch (Sicily and Tunisia): Sandy Injection Related to a High-Density Turbidity Deposit: ABSTRACT

The Numidian flysch is a thick (approximately 2,000-4,000 m) turbiditic formation (Oligocene to lower Miocene) of the southern part of the Alpine chain in western Europe. In Sicily (Geraci-Siculo) and Tunisia (Tabarka, Cap Negro, Ras El Korane), the Numidian basin paleocurrent pattern is southward. Numerous sedimentary sills and dikes cut as well as shales, argillaceous slumps, and intercalated sandstones. These sandy injections fossilize a complex network of horizontal, oblique, and vertical synsedimentary joints controlled by stratification planes, general paleoslope, differential compaction, and regional tectonic setting. The sills may exceed 5 m in thickness and 100 m in lateral extent; the dikes are less than 0.9 m wide, locally 1.2 m. Depth of penetration below the mother sand is 30 M to 150 m. The dikes are often folded in ptygmatic fashion, and the sills and dikes show pillars; these occurrences allow quantification of a postinjection compaction ratio ranging from 1.2 to 2.8. This late compaction of the surrounding mud together with the presence of burrows on both flanks of the dikes and through the sills, and the connection with channeling sandstones bodies, indicates the early occurrence of this injection. Sand was injected downward from high-density turbidity currents filling erosive, deep,more » steep-walled channels or gullies.« less

Early Rifting Deposition: Examples from Carbonate Sequences of Sardinia (Cambrian) and Tuscany (Triassic-Jurassic), Italy: An Analogous Tectono-Sedimentary and Climatic Context: ABSTRACT

Lower Cambrian Ceroide Limestone (Sardinia) and Lower Jurassic Massiccio Limestone (Tuscany) belong to sequences deposited in analogous tectono-sedimentary context: the former linked to the Caledonian Sardic Phase, the latter to the Alpine Orogeny. Both units consist of massive pure limestone characterized by marginal and lagoonal sequences repeatedly interfingering in the same geological structure. This distribution indicates a morphology of the platforms composed of banks (marginal facies) and shallow basins (lagoonal facies) comparable with a Bahamian complex. Dolomitization affects patchily the massive limestone bodies, and karstic features, breccias, and sedimentary dikes occur at their upper boundary. Both units overlie early dolomite and evaporites (sabkha facies) containing siliciclastic intercalations in their lower and/or upper part and are unconformably covered by open-shelf red (hematitic), nodular limestone Ammonitico Rosso facies). The sedimentary evolution of the two sequences appears to have been controlled by synsedimentary tectonics whose major effects are the end of the terrigenous input, the bank-and-basin morphology of the platform, the irregular distribution of the dolomitization, and the nodular fabric of the overlying facies. The end of the Bahamian-type system is marked by the karstification of the emerged blocks and is followed by their differential sinking and burial under red-nodular facies. From amore » geodynamic viewpoint, sequences composed of Bahamian-like platform carbonates followed by Ammonitico Rosso facies imply deposition along continental margins subjected to block-faulting during an extensional regime connected with the beginning of continental rifting. Moreover, the variation from sabkha to Bahamian conditions suggests the drifting of the continent from arid to humid, tropical areas.« less

Role of Neptunian Dikes in Structural Evolution of Reefs: ABSTRACT

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