Two-dimensional modeling of carbonate ramp sequences and component cycles

Two-dimensional stratigraphic models incorporating antecedent platform topography, rotational and regional driving subsidence, sediment and water loading using an elastic beam model, water-depth-dependent sedimentation rates and rock types, lag time of the flexural response, depositional lag time following initial platform flooding, and third- to fifth-order complex sea-level curves can be used to understand the development of cyclic carbonate platforms. Sea-level curves dominated by approximately 100-k.y. or 40-k.y. fluctuations developed a platform stratigraphy characterized by only a few cycles, whereas numerous cycles develop where the sea-level curve is dominated by 19-23 k.y. fluctuations. Low-amplitude sea-level curves in which the 100-k.y. fluctuation is greater than the 40-k.y. fluctuation, which in turn is greater than the 19-23-k.y. fluctuations, form a platform stratigraphy dominated by stacked cycles. Increased amplitude of the lower-frequency oscillations forms a shingled stacking pattern on the platform. Also, the increased amplitudes cause deposition of cycles with decreased thickness of tidal flat caps and longer duration of capping disconformities. Superimposing high-frequency sea-level fluctuations (20-100 k.y.) on longer-term 1-3-m.y. fluctuations generates synthetic platforms composed of stacked depositional sequences consisting of 1-10-m (3.3-33-ft) cycles. The model output illustrates how the systems tracts and their component cycles are related to the input sea-level curves. Erosion in the model decreases the thickness of tidal flat caps, increases the subtidal facies thicknesses of cycles because it increases accommodation, and bevels the highstand systems tract during long-term fall through erosion. The models show why picking boundaries between systems tracts is difficult when individual measured sections of cyclic platforms are used. Fischer plots were generated from the model output. The plots, when constructed for outer platform sections, are useful in estimating third-order sea-level fluctuations and in defining the positions of the systems tract boundaries

Two-dimensional modeling of carbonate ramp sequences and component cycles

Two-dimensional stratigraphic models incorporating antecedent platform topography, rotational and regional driving subsidence, sediment and water loading using an elastic beam model, water-depth-dependent sedimentation rates and rock types, lag time of the flexural response, depositional lag time following initial platform flooding, and third- to fifth-order complex sea-level curves can be used to understand the development of cyclic carbonate platforms. Sea-level curves dominated by approximately 100-k.y. or 40-k.y. fluctuations developed a platform stratigraphy characterized by only a few cycles, whereas numerous cycles develop where the sea-level curve is dominated by 19-23 k.y. fluctuations. Low-amplitude sea-level curves in which the 100-k.y. fluctuation is greater than the 40-k.y. fluctuation, which in turn is greater than the 19-23-k.y. fluctuations, form a platform stratigraphy dominated by stacked cycles. Increased amplitude of the lower-frequency oscillations forms a shingled stacking pattern on the platform. Also, the increased amplitudes cause deposition of cycles with decreased thickness of tidal flat caps and longer duration of capping disconformities. Superimposing high-frequency sea-level fluctuations (20-100 k.y.) on longer-term 1-3-m.y. fluctuations generates synthetic platforms composed of stacked depositional sequences consisting of 1-10-m (3.3-33-ft) cycles. The model output illustrates how the systems tracts and their component cycles are related to the input sea-level curves. Erosion in the model decreases the thickness of tidal flat caps, increases the subtidal facies thicknesses of cycles because it increases accommodation, and bevels the highstand systems tract during long-term fall through erosion. The models show why picking boundaries between systems tracts is difficult when individual measured sections of cyclic platforms are used. Fischer plots were generated from the model output. The plots, when constructed for outer platform sections, are useful in estimating third-order sea-level fluctuations and in defining the positions of the systems tract boundaries

Devonian Upper Ocean Redox Trends Across Laurussia: Testing Potential Influences of Marine Carbonate Lithology on Bulk Rock I/Ca Signals

The Devonian is characterized by major changes in ocean-atmosphere O2 concentrations, colonialization of continents by plants and animals, and widespread marine anoxic events associated with rapid d13C excursions and biotic crises. However, the long-term upper ocean redox trend for the Devonian is still not well understood. This study presents new I/Ca data from well-dated Lower Devonian through Upper Devonian limestone sections from the Great Basin (western Laurussia) and the Illinois Basin (central Laurussia). In addition, to better address potential influences of lithology and stratigraphy on I/Ca redox signals, I/Ca data are reported here as carbonate lithology-specific. Results indicate that lithologic changes do not exert a dominant control on bulk carbonate I/Ca trends, but the effects of some diagenetic overprints cannot be ruled out. For the Illinois Basin, low I/Ca values (more reducing) are recorded during the Pragian to Emsian and increased but fluctuating values are recorded during the Eifelian to Givetian. The Great Basin I/Ca trends suggest local upper oceans were more reducing in the Lochkovian, more oxic in the Pragian- Emsian, return to more reducing in the Eifelian, then to increasingly more oxic, but fluctuating in the Givetian-Frasnian. The local I/Ca variations at Great Basin likely share more similarity with global upper ocean condition (compared to the Illinois Basin) based on its position adjacent to the Panthalassic Ocean and its temporal co-variation with global environmental volatility trends. The overall reducing and variable redox conditions of local upper ocean (if not a diagenetic signal) during the Middle and Late Devonian of Great Basin coincide with evidence of increased global environmental volatility suggesting seawater redox may have been an important part of environmental instability at this time

Millennial-scale climate cycles modulated by Milankovitch forcing in the middle Cambrian (ca. 500 Ma) Marjum Formation, Utah, USA

peer reviewedMiddle Cambrian offshore deposits of the Marjum Formation, Utah, USA, are characterized by four scales of superimposed cyclicity defined by varying fine siliciclastic versus limestone abundances; these include limestone-marl couplets (rhythmites; 5–10 cm), which are bundled into parasequences (1–2 m) and small-scale (5–10 m) and large-scale (20–40 m) sequences. Time series analysis of SiO2 and lithologic rank stratigraphic series reveal cycles consistent with Milankovitch periods corresponding to Cambrian orbital eccentricity (20 m, 405 k.y.; 6 m, 110 k.y.), obliquity (1.8 m, 30 k.y.), climatic precession (1.15 m, 18 k.y.), and half-precession (0.64 m, 7 k.y.). Astronomical calibration of the lithologic rank series indicates that the main sub-Milankovitch cycle at 0.065 m represents ∼1 k.y. and corresponds to the basic rhythmite couplet. All scales of cyclicity are interpreted as the result of wet versus dry monsoonal climate oscillations controlling the abundance of fine siliciclastic sediment influx to the basin. A plausible millennial-scale climate driver is solar activity. These results describe one of the oldest known geological candidates for solar-influenced climate change modulated by Milankovitch forcing

Isotopic evidence for changes in the zinc cycle during Oceanic Anoxic Event 2 (Late Cretaceous)

Widespread deposition of organic-rich shales during the Late Cretaceous Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) occurred during a period of significant global paleo-environmental and geochemical change. It has been proposed that an increase in nutrient input to the ocean during OAE 2 was the key mechanism that generated and sustained high rates of organic-matter burial over time scales of 103–105 yr. Zinc is a bio-essential micronutrient and the proportion of Zn burial in oxic sediments relative to burial in organic-rich continental margin sediments is reflected in its seawater isotope composition. The first Zn-isotope records dating from the Cretaceous are presented here from three coeval carbonate successions: two from Europe (southern England and southern Italy) and one from southern Mexico. The new data show reproducible stratigraphic Zn-isotope patterns in spatially and lithologically diverse carbonate successions. Excursions to lower Zn-isotope values may be linked to the input of magmatic Zn, changes in the proportion of Zn burial into organic-rich sediments, and the liberation of previously buried Zn during an episode of widespread seafloor re-oxygenation during OAE 2 (the Plenus Cold Event)

Elrick_Maya_1986_Plate 1C.jpg

The Middle and Upper Devonian Guilmette Formation was deposited within the inner shelf region of the Cordilleran miogeosyncline. Twelve distinct lithofacies are recognized in the Guilmette Formation and represent deposition from various peritidal environments including quiet-water lagoons, organic buildups, a barrier bar/beach complex, and intertidal through supratidal flats. The relative sea-level curve determined from the vertical succession of lithofacies in the lower Guilmette Formation defines a period of relatively constant subtidal sedimentation punctuated by clusters of abrupt shallowing and deepening events; no well-defined shallowing-upward sequences are observed. Six distinct shallowing-upward cycles are recognized in the middle and upper Guilmette Formation; deepening events at the base of each cycle are abrupt, commonly local in extent, and progressively stronger in magnitude upsection, suggesting that crustal instabilities related to Anlter orogenic activity affected the inner shelf region prior to formation of the Pilot basin. Quartz-rich lithofacies (lithofacies 3, 8, arid 9) were deposited in intertidal and supratidal environments arid underwent very early diagenetic alteration, including silica and carbonate cementation, neomorphism, arid syndepositional dolomitization. Syndepositional dolomitization occurred as the result of surface-derived, downward-flowing fluids, possibly from hypersaline brines. Because of rapid subsidence arid sedimentation rates, subtidal. carbonate lithofacies (lithofacies 1, 2, 4, 5, 6, 7, 10, 11, and 12) were transported into moderate arid deep burial environments prior to significant alteration by shallow phreatic diagenetic processes. Diagenetic alteration inc1iii early marine cementation, neomorphism, mineral stabilization, calcite cementation, siicification, and late diagenetic dolomitization; late dolanitization affects less than 25% of the Guilmette Formation. The lack of any fresh or brackish water diagenetic textures, the limited and patchy distribution of late diagenetic dolomite, the lack of dolomite associated with the upper Guilmette Formation disconformity, and the association of dolomite with abundant stylolites preclude dolomitization by mixing-zone processes. The same criteria suggest that dolomitization occurred during moderate to deep burial from internally-derived Mg-ions arid fluids moving along stylolites. The major source for Mg-ions was supplied by interbedded syndepositional dolomite beds arid the underlying Simonson Dolomite. Late diagenetic dolanitization of the Guilmette Formation was indirectly paleogeographically controlled because inner shelf tidal flats are the most common environment for syndepositional dolomitization

Elrick_Maya_1986_Plate 2.jpg

The Middle and Upper Devonian Guilmette Formation was deposited within the inner shelf region of the Cordilleran miogeosyncline. Twelve distinct lithofacies are recognized in the Guilmette Formation and represent deposition from various peritidal environments including quiet-water lagoons, organic buildups, a barrier bar/beach complex, and intertidal through supratidal flats. The relative sea-level curve determined from the vertical succession of lithofacies in the lower Guilmette Formation defines a period of relatively constant subtidal sedimentation punctuated by clusters of abrupt shallowing and deepening events; no well-defined shallowing-upward sequences are observed. Six distinct shallowing-upward cycles are recognized in the middle and upper Guilmette Formation; deepening events at the base of each cycle are abrupt, commonly local in extent, and progressively stronger in magnitude upsection, suggesting that crustal instabilities related to Anlter orogenic activity affected the inner shelf region prior to formation of the Pilot basin. Quartz-rich lithofacies (lithofacies 3, 8, arid 9) were deposited in intertidal and supratidal environments arid underwent very early diagenetic alteration, including silica and carbonate cementation, neomorphism, arid syndepositional dolomitization. Syndepositional dolomitization occurred as the result of surface-derived, downward-flowing fluids, possibly from hypersaline brines. Because of rapid subsidence arid sedimentation rates, subtidal. carbonate lithofacies (lithofacies 1, 2, 4, 5, 6, 7, 10, 11, and 12) were transported into moderate arid deep burial environments prior to significant alteration by shallow phreatic diagenetic processes. Diagenetic alteration inc1iii early marine cementation, neomorphism, mineral stabilization, calcite cementation, siicification, and late diagenetic dolomitization; late dolanitization affects less than 25% of the Guilmette Formation. The lack of any fresh or brackish water diagenetic textures, the limited and patchy distribution of late diagenetic dolomite, the lack of dolomite associated with the upper Guilmette Formation disconformity, and the association of dolomite with abundant stylolites preclude dolomitization by mixing-zone processes. The same criteria suggest that dolomitization occurred during moderate to deep burial from internally-derived Mg-ions arid fluids moving along stylolites. The major source for Mg-ions was supplied by interbedded syndepositional dolomite beds arid the underlying Simonson Dolomite. Late diagenetic dolanitization of the Guilmette Formation was indirectly paleogeographically controlled because inner shelf tidal flats are the most common environment for syndepositional dolomitization

Elrick_Maya_1986.pdf

The Middle and Upper Devonian Guilmette Formation was deposited within the inner shelf region of the Cordilleran miogeosyncline. Twelve distinct lithofacies are recognized in the Guilmette Formation and represent deposition from various peritidal environments including quiet-water lagoons, organic buildups, a barrier bar/beach complex, and intertidal through supratidal flats. The relative sea-level curve determined from the vertical succession of lithofacies in the lower Guilmette Formation defines a period of relatively constant subtidal sedimentation punctuated by clusters of abrupt shallowing and deepening events; no well-defined shallowing-upward sequences are observed. Six distinct shallowing-upward cycles are recognized in the middle and upper Guilmette Formation; deepening events at the base of each cycle are abrupt, commonly local in extent, and progressively stronger in magnitude upsection, suggesting that crustal instabilities related to Anlter orogenic activity affected the inner shelf region prior to formation of the Pilot basin. Quartz-rich lithofacies (lithofacies 3, 8, arid 9) were deposited in intertidal and supratidal environments arid underwent very early diagenetic alteration, including silica and carbonate cementation, neomorphism, arid syndepositional dolomitization. Syndepositional dolomitization occurred as the result of surface-derived, downward-flowing fluids, possibly from hypersaline brines. Because of rapid subsidence arid sedimentation rates, subtidal. carbonate lithofacies (lithofacies 1, 2, 4, 5, 6, 7, 10, 11, and 12) were transported into moderate arid deep burial environments prior to significant alteration by shallow phreatic diagenetic processes. Diagenetic alteration inc1iii early marine cementation, neomorphism, mineral stabilization, calcite cementation, siicification, and late diagenetic dolomitization; late dolanitization affects less than 25% of the Guilmette Formation. The lack of any fresh or brackish water diagenetic textures, the limited and patchy distribution of late diagenetic dolomite, the lack of dolomite associated with the upper Guilmette Formation disconformity, and the association of dolomite with abundant stylolites preclude dolomitization by mixing-zone processes. The same criteria suggest that dolomitization occurred during moderate to deep burial from internally-derived Mg-ions arid fluids moving along stylolites. The major source for Mg-ions was supplied by interbedded syndepositional dolomite beds arid the underlying Simonson Dolomite. Late diagenetic dolanitization of the Guilmette Formation was indirectly paleogeographically controlled because inner shelf tidal flats are the most common environment for syndepositional dolomitization

Elrick_Maya_1986_Plate 1A.jpg

The Middle and Upper Devonian Guilmette Formation was deposited within the inner shelf region of the Cordilleran miogeosyncline. Twelve distinct lithofacies are recognized in the Guilmette Formation and represent deposition from various peritidal environments including quiet-water lagoons, organic buildups, a barrier bar/beach complex, and intertidal through supratidal flats. The relative sea-level curve determined from the vertical succession of lithofacies in the lower Guilmette Formation defines a period of relatively constant subtidal sedimentation punctuated by clusters of abrupt shallowing and deepening events; no well-defined shallowing-upward sequences are observed. Six distinct shallowing-upward cycles are recognized in the middle and upper Guilmette Formation; deepening events at the base of each cycle are abrupt, commonly local in extent, and progressively stronger in magnitude upsection, suggesting that crustal instabilities related to Anlter orogenic activity affected the inner shelf region prior to formation of the Pilot basin. Quartz-rich lithofacies (lithofacies 3, 8, arid 9) were deposited in intertidal and supratidal environments arid underwent very early diagenetic alteration, including silica and carbonate cementation, neomorphism, arid syndepositional dolomitization. Syndepositional dolomitization occurred as the result of surface-derived, downward-flowing fluids, possibly from hypersaline brines. Because of rapid subsidence arid sedimentation rates, subtidal. carbonate lithofacies (lithofacies 1, 2, 4, 5, 6, 7, 10, 11, and 12) were transported into moderate arid deep burial environments prior to significant alteration by shallow phreatic diagenetic processes. Diagenetic alteration inc1iii early marine cementation, neomorphism, mineral stabilization, calcite cementation, siicification, and late diagenetic dolomitization; late dolanitization affects less than 25% of the Guilmette Formation. The lack of any fresh or brackish water diagenetic textures, the limited and patchy distribution of late diagenetic dolomite, the lack of dolomite associated with the upper Guilmette Formation disconformity, and the association of dolomite with abundant stylolites preclude dolomitization by mixing-zone processes. The same criteria suggest that dolomitization occurred during moderate to deep burial from internally-derived Mg-ions arid fluids moving along stylolites. The major source for Mg-ions was supplied by interbedded syndepositional dolomite beds arid the underlying Simonson Dolomite. Late diagenetic dolanitization of the Guilmette Formation was indirectly paleogeographically controlled because inner shelf tidal flats are the most common environment for syndepositional dolomitization

Elrick_Maya_1986_Plate 1B.jpg

The Middle and Upper Devonian Guilmette Formation was deposited within the inner shelf region of the Cordilleran miogeosyncline. Twelve distinct lithofacies are recognized in the Guilmette Formation and represent deposition from various peritidal environments including quiet-water lagoons, organic buildups, a barrier bar/beach complex, and intertidal through supratidal flats. The relative sea-level curve determined from the vertical succession of lithofacies in the lower Guilmette Formation defines a period of relatively constant subtidal sedimentation punctuated by clusters of abrupt shallowing and deepening events; no well-defined shallowing-upward sequences are observed. Six distinct shallowing-upward cycles are recognized in the middle and upper Guilmette Formation; deepening events at the base of each cycle are abrupt, commonly local in extent, and progressively stronger in magnitude upsection, suggesting that crustal instabilities related to Anlter orogenic activity affected the inner shelf region prior to formation of the Pilot basin. Quartz-rich lithofacies (lithofacies 3, 8, arid 9) were deposited in intertidal and supratidal environments arid underwent very early diagenetic alteration, including silica and carbonate cementation, neomorphism, arid syndepositional dolomitization. Syndepositional dolomitization occurred as the result of surface-derived, downward-flowing fluids, possibly from hypersaline brines. Because of rapid subsidence arid sedimentation rates, subtidal. carbonate lithofacies (lithofacies 1, 2, 4, 5, 6, 7, 10, 11, and 12) were transported into moderate arid deep burial environments prior to significant alteration by shallow phreatic diagenetic processes. Diagenetic alteration inc1iii early marine cementation, neomorphism, mineral stabilization, calcite cementation, siicification, and late diagenetic dolomitization; late dolanitization affects less than 25% of the Guilmette Formation. The lack of any fresh or brackish water diagenetic textures, the limited and patchy distribution of late diagenetic dolomite, the lack of dolomite associated with the upper Guilmette Formation disconformity, and the association of dolomite with abundant stylolites preclude dolomitization by mixing-zone processes. The same criteria suggest that dolomitization occurred during moderate to deep burial from internally-derived Mg-ions arid fluids moving along stylolites. The major source for Mg-ions was supplied by interbedded syndepositional dolomite beds arid the underlying Simonson Dolomite. Late diagenetic dolanitization of the Guilmette Formation was indirectly paleogeographically controlled because inner shelf tidal flats are the most common environment for syndepositional dolomitization