Depositional sequences in shallow carbonate-dominated sedimentary systems: concepts for a high-resolution analysis

Over the past 25 yr, the science of stratigraphy has evolved to include time-correlative data from vastly disparate components of the Earth system. Not least of these is the global signal afforded by cyclostratigraphy, which has recorded the evolution of Earth’s astronomical (“Milankovitch”) forcing of insolation and the paleoclimate system. Fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies, geochemistry, mineralogy, rock magnetism, color, etc. In step with the documentation of astronomically forced paleoclimate from ever-older older geologic times, innovations in computational science have provided ever-longer high-accuracy astronomical model “targets” that can be used for time scale calibration. The Earth’s orbit is affected by motions of other planets, notably the orbital perihelia of Venus and Jupiter, which impose a dominant 405 k.y. eccentricity cycle on Earth’s orbital evolution. The large mass of Jupiter stabilizes this cycle over hundreds of millions of years. The cyclostratigraphic record of 405 k.y. cycles is therefore often used to correct chronologies affected by variable sedimentation. Earth’s shape and rotation rate are influenced by tidal dissipation and climate friction; these effects affect Earth’s precession rate through time. Thus, a record of Earth-Moon evolution is also embedded in cyclostratigraphy. The geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high-precision radioisotope dating, which today has the potential to define the ages of stratigraphic horizons with 2σ uncertainties at the scale of a precession cycle. Precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system. Cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca. 50 Ma limit of accurate astronomical solutions, and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion. Astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time. Finally, space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of Mars, with patterns suggestive of astronomical forcing analogous to Earth.

Cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences

Microbially catalyzed dolomite formation: From near-surface to burial

The Great Bank of Guizhou (GBG) is an exceptionally well exposed isolated Triassic platform in the Nanpanjiang Basin of South China. The platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth, evolution, and demise. The GBG formed near the southern margin of the Yangtze Platform during a deepening event that expanded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian. Initial accumulation of the GBG began in the latest Permian with small reef mounds and open-marine skeletal packstones. Following the end-Permian extinction, cyanobacterial boundstones grew over the bank top. During the Early Triassic the GBG developed a low-relief bank profile with mobile oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and gentle slopes dominated by pelagics, debris-flow deposits, and turbidites at the basin margin. In the Middle Triassic (Anisian) the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose. In the Middle Triassic (Ladinian) an erosional escarpment up to 1700 m high developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior, producing an atoll-like morphology. Later, a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening contributed to termination of the GBG before siliciclastic turbidites and shales were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundant muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture. The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries. The GBG's larger size increased mud production and protected it from extensive winnowing of mud, which in turn resulted in off-bank shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes which progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their angle-of-repose geometries.

Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China

As paleobiology continues to address an ever broader array of questions, it becomes increasingly important to interpret confidently the meaning of the pattern of fossil occurrences as found in outcrop. To this end, sequence stratigraphy is an important tool for paleobiologists because it predicts the distribution of unconformities, facies changes, and changes in sedimentation rate, all factors known from numerous previous studies to affect the quality of the fossil record. Computer simulations now make it possible not only to model sequence architecture within sedimentary basins, but also to model the occurrence of fossils within those basins. These models generate predictions regarding the stratigraphic distribution of first and last occurrences, changes in species abundance, changes in species morphology, and the distribution of gaps in fossil ranges. Although confirmation of some of these predictions has been found in field studies, the extent to which these predictions describe the fossil record in general is still unknown. If the predicted patterns of fossil occurrences are found to be widespread, it will suggest that a relatively simple model of fossil occurrences in outcrops could become a new tool for solving a wide array of paleobiologic and biostratigraphic problems. With such models, paleobiologists and biostratigraphers will be able to use model data to test the accuracy of newly developed methods of analysis.

The quality of the fossil record: a sequence stratigraphic perspective

The passive-margin succession of the Diablo Platform (Lower Ordovician, west Texas) is represented by the second-order Sauk C supersequence set, consisting of a basal transgressive clastic unit (The Bliss Sandstone) above the breakup unconformity, marking the second-order basal lowstand transgressive phase), overlain by 750 m of drift-related, shallow-marine platform carbonates (the El Paso Group) recording the second-order highstand. Due to late Paleozoic structuring of the Gondwanan passive margin, present exposures in Texas are in an updip shelf position and lack internal stratal geometries across depositional strike, so sequences and systems tracts are identified solely by the vertical stacking patterns of depositional subfacies and higher-frequency, fifth-order cycles. Accommodation plots (Fischer plots) of high-frequency cycles gauge systematic shifts in third-order accommodation of two complete third-order sequences, each about 2 m.y. long and 60-140 m thick, within the El Paso Group. This is expressed in the vertical succession of cycle types, systematic changes in cycle thickness, and variations in subfacies as revealed by histograms of subfacies types tied to Fischer plots. A complete El Paso shelf sequence contains a thin lowstand systems tract of quartz arenite, a thick transgressive systems tract dominated by upward-thickening, thrombolitic subtidal cycles, and a highstand systems tract marked by dolomitic, thinning-upward peritidal cycles containing quartz sand. The El Paso third-order depositional sequences have been correlated by biostrat graphy and cycle stacking pattern from the Franklin Mountains southeast to the Diablo Arch, northeast into the Ardmore Basin, and thence to the Appalachian Basin. Hence, the third-order sequences appear to be eustatic, based on biostratigraphic correlation of accommodation plots around the periphery of Gondwana. Time-series analyses suggest that a strict allocyclic, Milankovitch-driven, glacio-eustatic mechanism alone is insufficient to account for the higher-frequency fifth-order and lower-frequency third-order accommodation cycles. We used a two-dimensional forward computer simulation to compare allocyclic and autocyclic models for high-frequency cycles. The allocyclic simulation superimposes a deterministic, Milankovitch glacio-eustatic driver on a lower-frequency, third-order accommodation cycle and generates a two-dimensional synthetic cross section composed of high-frequency peritidal and subtidal cycles stacked into third-order sequences. The autocyclic simulation is generated by a Ginsburg autocyclic mechanism for high-frequency cyclicity triggered by third-order accommodation changes The autocyclic simulation quantitatively replicates an autocyclic mechanism for fifth-order cycle development linked to onshore sediment transport and seaward progradation of tidal flats. The two-dimensional internal facies architecture of a third-order sequence and the high-frequency cycle stacking patterns are simulated in both scenarios. Important differences in patterns of cycles and subfacies stacking in the simulations shed light on the interpretation of the Diablo platform cycles and sequences. As an alternative or a supplement to autocyclicity or Milankovitch-forced glacio-eustasy, we have studied stochastic models for driving high-frequency changes in relative sea level. These intriguing results maintain essentially all the features observed in the rock record. Our smoothed sto hastic simulations suggest that the "cycle problem" is much more complex than presently realized.

The Origin of High-Frequency Platform Carbonate Cycles and Third-Order Sequences (Lower Ordovician El Paso Gp, West Texas): Constraints from Outcrop Data and Stratigraphic Modeling

The Lower Ordovician passive margin succession of the Franklin Mountains is represented by the second-order Sauk C supersequence set consisting of the Bliss Sandstone and the overlying El Paso Group. The Bliss marks the second order basal lowstand-transgressive phase and the El Paso Group records the second-order highstand. The El Paso Group contains several third-order depositional sequences, which in this updip, shelfal position lack internal stratal geometries along dip. Thus, sequences and systems tracts are identified solely on the basis of the vertical stacking patterns of depositional subfacies and high frequency, fifth-order parasequences. 'Fischer plots' of high-frequency parasequences gauge systematic shifts in third-order accommodation of two complete third-order sequences within the Arenigian portion of the El Paso Group. This is expressed in the vertical succession of parasequence types, systematic changes in parasequence thicknesses, plus variations in subfacies as revealed by histograms of subfacies types tied to 'Fischer plots.' A complete El Paso shelfal sequence contains a thin lowstand (LST) of quartzarenite, a thick transgressive systems tract (TST) dominated by upward-thickening, thrombolitic subtidal parasequences, and a highstand systems tract (HST) marked by dolomitic, thinning-upward peritidal parasequences containing admixed quartz sand. The authors have investigated the mechanics of third-order sequence formation andmore » contrasted allocyclic models with autocyclic models for high-frequency parasequence formation through 1-D and 2-D forward modeling.« less

Third-order sequences and parasequence stacking patterns of Lower Ordovician platform carbonates of the El Paso Group, Franklin Mountains, west Texas

Third-order sequences and high frequency cycle stacking patterns in Lower Ordovician platform carbonates, El Paso Group (Texas); implications for carbonate sequence stratigraphy

P. A. Dunn

Papers

The Origin of High-Frequency Platform Carbonate Cycles and Third-Order Sequences (Lower Ordovician El Paso Gp, West Texas): Constraints from Outcrop Data and Stratigraphic Modeling

Third-order sequences and parasequence stacking patterns of Lower Ordovician platform carbonates of the El Paso Group, Franklin Mountains, west Texas

Third-order sequences and high frequency cycle stacking patterns in Lower Ordovician platform carbonates, El Paso Group (Texas); implications for carbonate sequence stratigraphy