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Susan M. Agar - One of the best experts on this subject based on the ideXlab platform.
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triassic latemar cycle tops Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
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Triassic Latemar cycle tops — Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
Nicolas Christ - One of the best experts on this subject based on the ideXlab platform.
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triassic latemar cycle tops Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
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Triassic Latemar cycle tops — Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
Maria Mutti - One of the best experts on this subject based on the ideXlab platform.
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triassic latemar cycle tops Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
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Triassic Latemar cycle tops — Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
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Characterization of a deeply buried paleokarst terrain in the Loppa High using core data and multiattribute seismic facies classification
AAPG Bulletin, 2012Co-Authors: Jhosnella Sayago, Maria Mutti, Matteo Di Lucia, Axum Cotti, Andrea Sitta, Kjetil Broberg, Artur Przybylo, Raffaele Buonaguro, Olesya ZiminaAbstract:The recognition of paleokarst in subsurface carbonate reservoirs is not straightforward because conventional seismic interpretation alone is generally not sufficient to discriminate karstified areas from their surroundings. In the Loppa High (Norwegian Barents Sea), a protracted episode of Subaerial Exposure occurring between the late Paleozoic and mid-Triassic—Late Permian to Anisian—resulted in a significant overprinting of the previously deposited carbonate units. Here, we map the extension of the karstified areas using an integrated approach consisting of (1) a core study of critical paleokarst intervals, (2) a three-dimensional (3-D) seismic stratigraphic analysis, and (3) a 3-D multiattribute seismic facies (SF) classification. A core retrieved in the flat-topped Loppa High revealed breccia deposits at least 50 m (164 ft) thick, which probably resulted from cave collapses following the burial of the karst terrain. The SF classification was tested on a 3-D cube to (1) discriminate the respective SF related to the breccia deposits compared with other SF and (2) to estimate their spatial extent. Seismic-facies analysis suggests that breccias occupied the topmost area of the structural high, extending up to 12 km (7 mi) in width, 46 km (29 mi) in length, and tens of meters in thickness. The inference of such a large amount of breccia suggests that a significant part of this terrain was derived from the amalgamation of successive cave-development events—including periods of Subaerial Exposure and subsequent burial and collapse—resulting in a coalesced collapsed paleocave system. Previous observations from the Loppa High revealed the presence of karst plains associated with sinkholes, caves, and other dissolution phenomena associated with the breccia facies, further suggesting that a large volume of carbonate rocks in this area was affected by Subaerial Exposure and karstification. Our integrated approach and proposed karstification model could be applied to similar sedimentary basins that accommodate deeply buried carbonate successions affected by protracted episodes of Subaerial Exposure, where only few wells as well as 3-D seismic data are available.
Arthur H. Saller - One of the best experts on this subject based on the ideXlab platform.
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Carbon Isotope Excursions and Crinoid Dissolution at Exposure Surfaces in Carbonates, West Texas, U.S.A.
Journal of Sedimentary Research, 2006Co-Authors: J.a.d. Dickson, Arthur H. SallerAbstract:Abstract Carbonates from the Horseshoe Atoll, Reinecke Field, West Texas, U.S.A., show four Subaerial Exposure events by the presence of rhizoliths, alveolar septal fabric, and thin calcretes that subdivide the Pennsylvanian succession into five sequences. Vertical profiles of bulk rock δ13C analyses show values that are generally ~ +1‰ (PDB), but negative shifts of up to 2‰ occur beneath Exposure surfaces. The bulk-rock δ13C values ~ +1‰ could be explained as a mixture of 63% marine carbonate (δ13C = +4‰) and 37% pedogenic carbonate (δ13C = −4‰) while a negative bulk-rock excursion of δ13C = ~ −1‰ can be explained as a mixture of 37% marine and 63% pedogenic calcite. Distinctive negative shifts in the δ13C profiles do not occur under some Exposure surfaces that contain soil-generated features, and one negative shift occurs that apparently is unrelated to an Exposure surface. Bulk-rock δ13C values are difficult to interpret because the volume of their various components is unknown. Furthermore, δ13C profiles may miss Subaerial Exposure surfaces. Certain styles of crinoid alteration may indicate paleo-Subaerial Exposure surfaces. Fossil crinoids can be preserved in a variety of ways, but crinoids with dissolution holes that subsequently either become filled with calcite cement or were crushed during burial are restricted to 2 m below Exposure surfaces. Dissolution of Mg calcite crinoids is due to their relatively high Mg composition, 12 mole % MgCO3, combined with their location beneath soil-capped surfaces from which meteoric waters greatly undersaturated with respect to calcite emerged. Late-stage dissolution removed finely crystalline calcite, but crinoids by this time had stabilized to coarsely crystalline calcite and dolomite and thus were unaffected. Evaluation of other stratigraphic systems is needed to determine whether the use of "holey" crinoids to identify Subaerial Exposure surfaces has broad application.
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Evolution and Distribution of Porosity Associated with Subaerial Exposure in Upper Paleozoic Platform Limestones, West Texas
AAPG Bulletin, 1999Co-Authors: Arthur H. Saller, J.a.d. Dickson, Fumiaki MatsudaAbstract:Middle Pennsylvanian-Lower Permian limestones in the subsurface of west Texas were studied to determine how Subaerial Exposure and freshwater diagenesis (karstification) affected porosity distribution in meter-scale cycles. Approximately 87 depositional cycles are present in the gross reservoir interval (depths of 2600-3000 m), and each cycle is interpreted to represent a glacio-eustatic sea level fluctuation. Using recent radiometric age dating, average cycle duration is estimated at 160,000 yr per cycle. Reservoir-grade porosity (>4%) occurs in 5-25% of the gross reservoir section. Porosity is stratified, occurring in 1-6-m-thick intervals in the upper part of cycles that were Subaerially exposed; however, many cycles that were Subaerially exposed now lack porosity. Diagenesis and porosity development have distinct patterns related to duration of Subaerial Exposure. Four stages of porosity development are identified. (1) Very brief or no Subaerial Exposure (estimated at less than 5000 yr) caused little or no diagenetic change. (2) Brief to moderate Subaerial Exposure (estimated at 5000-50,000 yr) resulted in most primary pores being filled with calcite cement, and dissolution creating fine matrix pores (molds and intercrystalline pores). (3) Moderately long Subaerial Exposure (estimated at 50,000-130,000 yr) resulted in cements filling primary pores and some fine secondary pores, and dissolution creating small conduit pores (vugs, fractures, fissures). (4) Prolonged Subaerial Exposure (estimated at greater than 130,000 yr) resulted in most primary and secondary matrix pores being filled with calcite cement, but dissolution enlarged conduit pores (vugs, fractures, fissures). Present subsurface porosity in this field preferentially occurs in thick grainstones, phylloid algal boundstones, and a few wackestone/packstones in cycles subjected to brief Subaerial Exposure (stages 2 and 3). Matrix porosity (molds, intercrystalline pores) is dominant because most conduit pores formed during prolonged Subaerial Exposure were filled with either shale during subsequent transgressions or burial cements derived from pressure solution associated with the shales. The distribution of porosity in the Southwest Andrews area indicates that duration of Subaerial Exposure and supply of clastics are major factors determining ultimate porosity in limestones subjected to Subaerial Exposure and karstification.
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Unconformities and Porosity in Carbonate Strata
1995Co-Authors: David A. Budd, Arthur H. Saller, Paul M. HarrisAbstract:The southwest Andrews area on the eastern side of the Central Basin platform (west Texas) contains cyclic Pennsylvanian and Lower Permian shelfal limestones. Limestones were deposited in shallow marine environments during numerous highstands of sea level, but most cycles are bounded by Subaerial Exposure surfaces. Reservoir porosity is developed in only 10-45% of those depositional cycles in any given well. The purposes of this paper are to determine: (1) features useful for identifying Subaerial Exposure surfaces, (2) factors that affect stable-isotope profiles around Subaerial Exposure surfaces, and (3) circumstances critical to porosity preservation in Subaerially exposed limestones. 1. Features commonly present at or below Subaerial Exposure surfaces include an abrupt change in depositional lithology, caliche crusts, micritic rhizoliths precipitated around roots, soil-related fractures, breccias, and mottling associated with plant roots. Rhizoliths, caliche crusts, and breccias have developed best in wackestones and packstones. Mottling associated with plant roots is distinct in grainstones and was caused by heterogeneous dissolution and cementation. 2. The stable isotope signature most characteristic of Subaerial Exposure is abrupt decreases in d13C of the carbonate immediately below Subaerial Exposure surfaces. This signature is displayed best in cycles with: (a) wackestones/packstones at the top, (b) moderate duration of Subaerial Exposure, (c) limited overprinting by later meteoric diagenesis, (d) little erosion during the subsequent transgression, and (e) negligible effects of late cements on the isotopic composition of the bulk rock.
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Cycle Stratigraphy and Porosity in Pennsylvanian and Lower Permian Shelf Limestones, Eastern Central Basin Platform, Texas
AAPG Bulletin, 1994Co-Authors: Arthur H. Saller, J.a.d. Dickson, Stacie A. BoydAbstract:Pennsylvanian and Lower Permian shelfal limestones were studied in core and wireline logs on the eastern side of the Central Basin platform in west Texas. Sixty-three (63) cycles were delineated in the study interval, which includes 200250 m of Canyon (Missourian), Cisco (Virgilian), and Wolfcamp strata. Four general lithofacies are present: fossiliferous wackestones and packstones, grainstones, phylloid algal boundstones, and shales. These lithologies typically occur in 118-m-thick cycles bounded by Subaerial Exposure surfaces. Grainstones in the upper part of some cycles indicate a shallowing of environments prior to Subaerial Exposure. Many cycles have Subaerial Exposure surfaces developed on subtidal fossiliferous wackestones or packstones suggesting rapid falls in se level. Long-term transgressive intervals (transgressive systems tracts or TST) are dominated by thick (>4 m) cycles, whereas long-term regressive intervals (highstand systems tracts or HST) are dominated by thinner cycles. Stable carbon isotope data suggest that thick cycles in TSTs were subjected to short periods of Subaerial Exposure, whereas thin cycles in the HSTs were subjected to much longer Subaerial Exposure. Porosity is quite variable beneath Subaerial Exposure surfaces. At any given well, reservoir-grade porosity (>4%) is present below only 1045% of the identified Subaerial Exposure surfaces. Where pres-ent, reservoir-grade porosity occurs in the upper part of cycles, 0.35 m below Subaerial Exposure surfaces. Porosity in thick cycles of the TST is relatively widespread and not facies-selective. Porosity is less abundant and facies-selective in the HST, with porosity occurring only in relatively thick (1.54 m) grainstones, which are concentrated near the shelf margin. Thin (
Arndt Peterhänsel - One of the best experts on this subject based on the ideXlab platform.
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triassic latemar cycle tops Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
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Triassic Latemar cycle tops — Subaerial Exposure of platform carbonates under tropical arid climate
Sedimentary Geology, 2012Co-Authors: Nicolas Christ, Adrian Immenhauser, Frédéric Amour, Maria Mutti, Rosalind Preston, Fiona F Whitaker, Arndt Peterhänsel, Sven O. Egenhoff, Paul A. Dunn, Susan M. AgarAbstract:The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for Subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing Exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by Exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived Exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to Subaerial Exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent Subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for Subaerial Exposure under strongly arid climates. As the Subaerial Exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.
