Stratigraphic Unconformities Research Articles

Tectonic Setting and Hydrocarbon Habitat of External Carpathian Basins in Romania

During the Alpine evolution of Romania, two distinct depositional areas evolved in the external zones of the Carpathians: the Paleogene flysch and Neogene Molasse basin of the eastern Carpathians, and the Paleogene and Neogene Molasse basin of the southern Carpathians. Both basins were compressionally deformed during the Neogene, giving rise to the development of a succession of nappes and thrust sheets. The internal elements of the external Carpathians corresponding to the Tarcau and marginal folds nappes and the external elements forming the sub-carpathian nappe and foredeep were thrusted over significant distances onto the European platform. Intense exploration of the external Carpathian thrustbelt has led to the discovery of more than 100 oil and gas pools. Reservoirs are provided by Oligocene, Burdigalian, Sarmatian, and Pliocene clastic rocks. A prolific hydrocarbon charge is derived from regionally distributed Oligocene oil source rocks. Traps are mainly of the structural type and involve faulted anticlines, [open quotes]scale folds,[close quotes] and compressional structures modified by salt; stratigraphic pinch-out and unconformity related traps play a secondary role. On the basis of selected examples, the development and distribution of hydrocarbon pools will be discussed in terms of thrust kinematics and the structure of different platform blocks. The philosophymore » of past exploration activities will be reviewed, and both success cases and failures will be discussed. Remaining oil and gas plays, aimed at shallow as well as at deep objectives, will be highlighted.« less

Geological transect across the Northwestern Himalaya in eastern Ladakh and Lahul (a model for the continental collision of India and Asia)

The detailed geological mapping and structural study of a complete transect across the northwestern Himalaya allow to describe the tectonic evolution of the north Indian continental margin during the Tethys ocean opening and the Himalayan Orogeny. The Late Paleozoic Tethys rifting is associated with several tectonomagmatic events. In Upper Lahul and SE Zanskar, this extensional phase is recorded by Lower Carboniferous synsedimentary transtensional faults, a Lower Permian stratigraphic unconformity, a Lower Permian granitic intrusion and middle Permian basaltic extrusions (Panjal Traps). In eastern Ladakh, a Permian listric normal fault is also related to this phase. The scarcity of synsedimentary faults and the gradual increase of the Permian syn-rift sediment thickness towards the NE suggest a flexural type margin. The collision of India and Asia is characterized by a succession of contrasting orogenic phases. South of the Suture Zone, the initiation of the SW vergent Nyimaling-Tsarap Nappe corresponds to an early phase of continental underthrusting. To the S, in Lahul, an opposite underthrusting within the Indian plate is recorded by the NE vergent Tandi Syncline. This structure is associated with the newly defined Shikar Beh Nappe, now partly eroded, which is responsible for the high grade (amphibolite facies) regional metamorphism of South Lahul. The main thrusting of the Nyimaling-Tsarap Nappe followed the formation of the Shikar Beh Nappe. The Nyimaling-Tsarap Nappe developed by ductile shear of the upper part of the subducted Indian continental margin and is responsible for the progressive regional metamorphism of SE Zanskar, reaching amphibolite facies below the frontal part of the nappe, near Sarchu. In Upper Lahul, the frontal parts of the Nyimaling-Tsarap and Shikar Beh nappes are separated by a zone of low grade metamorphic rocks (pumpellyite-actinolite facies to lower greenschist facies). At high structural level, the Nyimaling-Tsarap Nappe is characterized by imbricate structures, which grade into a large ductile shear zone with depth. The related crustal shortening is about 87 km. The root zone and the frontal part of this nappe have been subsequently affected by two zones of dextral transpression and underthrusting: the Nyimaling Shear Zone and the Sarchu Shear Zone. These shear zones are interpreted as consequences of the counterclockwise rotation of the continental underthrusting direction of India relative to Asia, which occurred some 45 and 36 Ma ago, according to plate tectonic models. Later, a phase of NE vergent `'backfolding'' developed on these two zones of dextral transpression, creating isoclinal folds in SE Zanskar and more open folds in the Nyimaling Dome and in the Indus Molasse sediments. During a late stage of the Himalayan Orogeny, the frontal part of the Nyimaling-Tsarap Nappe underwent an extension of about 15 km. This phase is represented by two types of structures, responsible for the tectonic unroofing of the amphibolite facies rocks of the Sarchu area: the Sarchu high angle Normal Fault, cutting a first set of low angle normal faults, which have been created by reactivation of older thrust planes related to the Nyimaling-Tsarap Nappe.

Late Quaternary stratigraphy of loess in the Channeled Scabland and Palouse regions of Washington state

Loess deposits of late Quaternary age in the Channeled Scabland and western Palouse regions of Washington state contain distinctive sequences of calcic buried soils, loess layers, tephra layers, and stratigraphic unconformities caused by cataclysmic flooding. Four new soil-stratigraphic units, the Sand Hills Coulee Soil, Washtucna Soil, Old Maid Coulee Soil, and the Devils Canyon Soil, are recognized across a 100-km transect. The pedostratigraphic correlations are strengthened by two tephra layers found at several sites that are correlative with the Mount St. Helens set S (radiocarbon ages about 13,000 yr B.P.), Mount St. Helens set C (radiocarbon ages about 36,000 yr B.P.), and by a third one that may be from an earlier (pre-set C) Mount St. Helens eruption. The set S tephra layer separates the Sand Hills Coulee and Washtucna Soils; the set C tephra layer separates the Old Maid Coulee and Devils Canyon Soils; and the pre-set C tephra layer underlies the Devils Canyon Soil. The well-known late Wisconsin episode of Scabland flooding and two older episodes of flooding are marked by regional stratigraphic unconformities that lie between pedostratigraphic units. One unconformity that underlies the Washtucna and Old Maid Coulee Soils was caused by a major episode of Scabland flooding that occurred before 35,000 yr B.P., probably during the early Wisconsin (80,000 to 65,000 yr B.P.). A possible second and older episode of flooding, recognized by an unconformity that underlies the Devils Canyon Soil, may be Illinoian (ca. 130,000 to 200,000 yr B.P.) in age.

CARBPLAT—A computer model to simulate the development of carbonate platforms

Research Article| March 01, 1992 CARBPLAT—A computer model to simulate the development of carbonate platforms Hemmo Bosscher; Hemmo Bosscher 1Faculty of Earth Sciences, Sedimentary Geology, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands Search for other works by this author on: GSW Google Scholar John Southam John Southam 2Rosenstiel School of Marine and Atmospheric Sciences, Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Virginia Key, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information Hemmo Bosscher 1Faculty of Earth Sciences, Sedimentary Geology, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands John Southam 2Rosenstiel School of Marine and Atmospheric Sciences, Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Virginia Key, Miami, Florida 33149 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (3): 235–238. https://doi.org/10.1130/0091-7613(1992)020<0235:CACMTS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Hemmo Bosscher, John Southam; CARBPLAT—A computer model to simulate the development of carbonate platforms. Geology 1992;; 20 (3): 235–238. doi: https://doi.org/10.1130/0091-7613(1992)020<0235:CACMTS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract This paper presents a computer model with two new main components that simulates the development of carbonate platforms. First, it uses a function for the decrease of carbonate production with depth that combines the photosynthetic growth of carbonate-producing organisms and the extinction of light with depth. Platform growth is simulated with a differential equation incorporating sea level. These processes therefore act continuously rather than in steps. Second, it attempts a more realistic approach to predicting carbonate-platform slopes by incorporating variation of slope angle with textural composition of the sediment. In this model, the angle at which slope sedimentation begins depends on the ratio between sediment produced on the platform interior (mud) and that produced on the platform margin (sand). The shape of the slope is represented by an exponential function that can be changed to accommodate the total amount of slope sediment. In the model, stratigraphic unconformities due to changing textural composition are produced on the fore-reef slopes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Burial Diagenesis in Two Montana Tertiary Basins

AbstractSamples from initially smectite-rich Tertiary continental volcaniclastic sediment from the Deer Lodge and Big Hole basins of southwestern Montana show a general decrease in illite/smectite (I/S) expandability with increasing burial depth. The mineralogical trends in cuttings from seven wells are interrupted by discontinuities in which I/S expandability abruptly decreases by 30 to 80%. These discontinuities coincide with stratigraphic unconformities in four wells in which the stratigraphy is known. Core samples show a wide range of I/S expandabilities over short stratigraphic intervals, possibly due to composition, porosity, and permeability variations. Sericite coexists with I/S in the deep core samples. A core sample from 7958 ft (2425 m) contains an R3-ordered I/S having a nearly ideal 3:1 illite: smectite ratio, similar to the mineral tarasovite. The structure of this I/S is dominated by stacks of four 2:1-layer fundamental illite particles and small proportions of thicker particles randomly interstratified among the four-layer particles.

Stratigraphic Sequences and Unconformities in Cretaceous Rocks of East-Central MT: ABSTRACT

Stratigraphic Sequences and Unconformities in Cretaceous Rocks of East-Central MT: ABSTRACT

Geology of Cedar Creek Anticline in Baker, Wibaux, and Glendive 30 ? 60-Min Quadrangles, Eastern Montana and Adjacent North Dakota: ABSTRACT

Geology of the asymmetrical, northwest-trending Cedar Creek anticline, a major oil- and gas-producing structure of the Williston basin, was mapped in the Wibaux and Glendive Quadrangles as part of the USGS-funded Coal Lands Mapping Program. Thinning of stratigraphic units, unconformities, and relationships between tectonic structures provide suggestive evidence for the timing of Late Cretaceous and Tertiary uplift in this area. The Colgate Member of the Fox Hills Formation (Maestrichtian) pinches out toward the axis of the anticline. The contact between the Hell Creek Formation (Maestrichtian) and the underlying Colgate Member is disconformable near the axis of the anticline and becomes conformable about 8 km (5 mi) east of it. This suggests Late Cretaceous uplift during or after deposition of the Colgate Member. The Hell Creek Formation also thins by up to 40 m (131.2 ft) toward the axis. Part of the Fort Union Formation is not present on the eastern limb of the anticline and in an area southwest of the anticline, suggesting that these areas were uplifted at the time of or just following deposition of this part of the section during the middle Paleocene. The present anticline developed following deposition of the Fort Union Formation in the latemore » Paleocene. Several associated faults and folds developed subparallel to the axial trend, including a pronounced synclinal flexure along much of the western limb of the anticline. Subsequently, northeast-trending faults with a component of right-lateral slip offset the northwest-trending linear features associated with the anticline.« less

The geometry and high‐temperature brittle deformation of the Skaergaard Intrusion

The Skaergaard magma chamber formed approximately 55 m.y. ago along the embryonic rift between North America and Europe as tholeiitic basalt magma flowed upward along fractures in basement gneiss and then infiltrated the stratigraphic unconformity at the base of a 7‐ to 9‐km‐thick section of continental basalts. The magma deflected and faulted the overburden as it formed a 4.5‐km‐thick, 189 km3, laccolithlike chamber with elliptical form (a= 6 km,b= 4 km) in map view. As the chamber grew, its feeder pipes were eroded into conical depressions; blocks of gneiss were rafted to the chamber top, and some blocks were fused and entrained in the main magma as “immiscible” fluids. Crystallization and cooling produced at least four distinct fracture events: (1) At 1050°–1000°C, residual magma accumulated in fractures in the Layered Series, forming gabbro pegmatites, (2) at 1050°–700°C, near‐vertical fractures were formed, providing channels for the main pulse of meteoric‐hydrothermal activity; these fractures developed near the margin of the magma chamber, then expanded outward into the permeable basalts and inward, following the gabbro‐magma interface. Ground waters derived from joints in the surrounding basalts flowed into the gabbro, were heated, lowered the18O/16O ratio of the intrusion, and filled its fractures with hornblende, clinopyroxene, biotite, and magnetite‐ilmenite, (3) at 800°–750°C, volatile‐rich granophyric melts derived from sloped blocks of basement gneiss expanded and crystallized as both sill‐like and dike‐like bodies in the gabbro, and (4) below 700°C, fractures continued to form and hydrothermal activity continued to cool the intrusion. The relative age, abundance, continuity, and mineralogy of the veins are consistent with parameters used in previous studies of this intrusion that predict the occurrence of a high‐temperature hydrothermal system and a time‐ and volume‐averaged permeability of 10−13cm2. Our new data indicate that the permeability of the layered gabbro decreased with time because the flow channels were sealed by high‐temperature mineral deposition. We thus conclude the following: (1) layered gabbros fracture in response to local stress at conditions just below their solidus temperature if the confining pressures are typical of the upper crust. This observation contravenes the conceptual viewpoint that the style of deformation at such elevated temperatures is only by plastic flow, and (2) because an extensive fracture network develops at these near‐solidus temperatures in layered gabbros, the bulk of the hydrothermal alteration of such bodies takes place at extremely high temperatures. This helps clarify the apparent paradox that extreme18O depletions are found in “fresh” layered gabbros.

Models of climate cycles recorded in Martian polar layered deposits

Layered strata observed in orbital images of polar sedimentary terrains on Mars have been hypothesized to contain information about recent variations in the Martian climate. Celestial mechanicians have determined that Mars has experienced large-amplitude quasi-periodic variations in obliquity and eccentricity: planetary meteorologists have inferred that the resulting changing patterns of solar heating must have had dramatic effects on Martian climate. Accordingly, it might be possible to link the known recent history of orbit/axial variations with the physical characteristics of the most recently deposited strate determined from orbital imagery. Our paper explores that possibility quantitatively for various models of the formation of the polar layered deposits and the layering developed within them. We modeled the formation of polar layered deposits during the past 10 my for two general classes of model. Uniform deposition rate (UDR) models assume that the major constituent of the layered deposits is deposited at a uniform rate and that layering is established by climate-induced variations in the concentration of a minor constituent. Climate-modulated deposition rate (CMDR) models assume that the deposition rate of the major constituent is controlled by climate and that layer boundaries correspond to intervals of nondeposition. In both models, abrupt changes in deposition regime occur when a function of the orbit/axial parameters crosses a threshold value. Detailed physical mechanisms for the control of sedimentation by orbit/axial variations were developed for two specific models of the CMDR type. In one model, the deposits are assumed to be primarily composed of dust; in the other, they are assumed to be predominantly water ice. We calculated the times at which abrupt changes in deposition regime occur and determined the relative thicknesses of successive layers which can be compared against layer thicknesses made from orbital imagery. In all these models, obliquity variations exert the dominant influence on the sequential variations in layer thickness, although eccentricity and precession index (longitude of perihelion) may take recognizable contributions as well. The layer patterns for the north and south polar deposits are identical for the dust model and only slightly different for the ice model, but this is at least partly a consequence of simplifying assumptions. The sequential variation in layer thickness is also very sensitive to those model parameters which control the fraction of time for which deposition occurs. When the climate threshold for deposition is low, deposition occurs a large fraction of the total time and characteristic patterns form with one thick layer and a number of much thinner ones. Such patterns would be easily recognized in photogeological measurements of layer thickness. When the threshold for deposition lies in the mid-range with respect to excursions of the climate function, deposition occurs approximately half of the time, and there is only slight modulation of layer thickness. When the threshold for deposition is high, deposition occurs only a small fraction of the total time, and groups of layers form which are separated form other groups by long periods of nondeposition. The stratigraphic unconformities that separate groupings of five to ten layers in the polar deposits may indicate that the Martian polar region has experienced such a depositional history. The feasibility of using these model results to verify the theory of climate control of polar stratigraphy on Mars and to gain additional insights into the significant processes is considered. The stratigraphy issues raised here are also explored in the context of recent work on the interpretation of stratigraphy of ocean margins. From this comparison there emerge some valuable insights into the stratigraphic effects of periods of nondeposition and the uncertaintiesin interpreting images of exposed polar strata on Mars.

Bacteria in Cream

Abstract We present a revised tectonostratigraphy of the Jan Mayen microcontinent (JMMC) and its southern extent, with the focus on its relationship to the Greenland–Iceland–Faroe Ridge area and the Faroe–Iceland Fracture Zone. The microcontinent9s Cenozoic evolution consists of six main phases corresponding to regional stratigraphic unconformities. Emplacement of Early Eocene plateau basalts at pre-break-up time (56–55 Ma), preceded the continental break-up (55 Ma) and the formation of seawards-dipping reflectors (SDRs) along the eastern and SE flanks of the JMMC. Simultaneously with SDR formation, orthogonal seafloor spreading initiated along the AEgir Ridge (Norway Basin) during the Early Eocene (C24n2r, 53.36 Ma to C22n, 49.3 Ma). Changes in plate motions at C21n (47.33 Ma) led to oblique seafloor spreading offset by transform faults and uplift along the microcontinent9s southern flank. At C13n (33.2 Ma), spreading rates along the AEgir Ridge started to decrease, first south and then in the north. This was probably complemented by intra-continental extension within the JMMC, as indicated by the opening of the Jan Mayen Basin – a series of small pull-apart basins along the microcontinent9s NW flank. JMMC was completely isolated when the mid-oceanic Kolbeinsey Ridge became fully established and the AEgir Ridge was abandoned between C7 and C6b (24–21.56 Ma).

Schichtlücken und Diskordanzen im Devon des Frankenwaldes

Schichtlücken und Diskordanzen im Devon des Frankenwaldes

Dynamic Relations and Terminology of Stratigraphic Conformity and Unconformity

Next article FreeDynamic Relations and Terminology of Stratigraphic Conformity and UnconformityW. O. CrosbyW. O. Crosby Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 20, Number 4May - Jun., 1912 Article DOIhttps://doi.org/10.1086/621968 Views: 160Total views on this site Citations: 2Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:S.I. Tomkeieff Unconformity—An Historical Study, Proceedings of the Geologists' Association 73, no.44 (Jan 1962): 383–IN7.https://doi.org/10.1016/S0016-7878(62)80031-5John E. Sanders SECTION OF GEOLOGY AND MINERALOGY: DISCONTINUITIES IN THE STRATIGRAPHIC RECORD*, Transactions of the New York Academy of Sciences 19, no.4 Series II4 Series II (Apr 2012): 287–297.https://doi.org/10.1111/j.2164-0947.1957.tb00532.x