Wan Yang

Milankovitch climatic signals in Lower Triassic (Olenekian) peritidal carbonate successions, Nanpanjiang Basin, South China

Abstract Meter-scale peritidal carbonate strata from an isolated platform in the Nanpanjiang Basin of South China were tested for the presence of periodic or quasi-periodic climatic signals. These signals provide information on factors controlling cyclic sedimentation in the equatorial eastern Tethys during the Early Triassic greenhouse period. Parasequences are composed of shallowing-upward successions of subtidal facies, including skeletal packstone and grainstone, calcimicrobial boundstone, and oolitic packstone and grainstone, intertidal laminated lutite and ribbon rock, and rare supratidal microbial laminite. Parasequence stacking patterns display three orders of cyclicity, suggesting hierarchical stratigraphic relationships. Gamma analysis of Kominz and Bond [Earth Planet. Sci. Lett. 48 (1990) 233–244] was used to estimate facies-dependent thickness–time conversion factors (i.e. γ s), and to construct γ -tuned and γ -untuned time series for two stratigraphic sections. Spectra of these time series indicate the presence of quasi-periodic signals, with prominent short-eccentricity (94.9–131.2 kyr), short-obliquity (35.8 kyr), and long-precessional index (21.2 kyr) peaks, and minor long-eccentricity (412.9 kyr), long-obliquity (45.3 kyr), short-precessional index (17.7 kyr), and constructional-tone (9.7 kyr) peaks when calibrated on the γ -tuned spectra. Thus, Milankovitch climatic forcing probably greatly influenced sedimentation. The calibration indicates that the subtidal facies has a sedimentation rate of 24.6–30.7 cm/kyr and the intertidal–supratidal facies has a rate of 2.7–6.0 cm/kyr. The estimated duration of deposition of the two sections is 1139–1423 kyr, and corresponds to a stratigraphic completeness of 32–92%. The completeness is much greater than that of icehouse stratigraphic records. We speculate that variations in carbonate productivity and environmental conditions driven by Milankovitch climatic forcing, combined with low-amplitude sea-level fluctuations, were likely major controls on cyclic sedimentation. Furthermore, evolutional spectra of the two sections show that dominant Milankovitch climatic forcings varied from short-eccentricity, obliquity, to long-precessional index during the course of sedimentation. Some spectral differences between the two sections suggest variations in depositional/recording mechanisms of Milankovitch climatic signals between platform interior and windward platform margin.

Cycle Symmetry and its Causes, Cisco Group (Virgilian and Wolfcampian), Texas

ABSTRACT 181 transgressive-regressive cycles composed of nonmarine and marine carbonate and siliciclastic rocks of the Cisco Group on the Eastern Shelf, Texas, display complex characteristics at both hemicycle and full-cycle scales. They are delineated on the basis of successive changes of depositional environments, stratal boundary relations, and stratigraphic position. Transgressive and regressive stratigraphic environment gradients are defined as the magnitude of environmental shift divided by thickness for each hemicycle. They indicate the rates of lateral environmental shifts during transgression and regression. Cycle symmetry index is defined as the ratio between transgressive and regressive stratigraphic environment gradients. It provides a measure of stratigraphic response to controlli g processes. Most Cisco cycles are asymmetrical with steep transgressive and shallow regressive stratigraphic environment gradients, indicating cyclic but asymmetrical variations of major allogenic processes over a cycle interval. These systematic variations are delineated from climate-sensitive rock types, stratal thickness, and sedimentary features. The relative dominance of processes controlling accommodation (i.e., glacio-eustasy and basement subsidence) versus those controlling sedimentation (i.e., climate, upland sediment yield, and sediment supply at the depositional site) determines cycle symmetry. A formulated relation between cycle symmetry index (i.e., stratigraphic response) and rates of these processes demonstrates that transgressive sedimentation is controlled by sediment supply, whe eas regressive sedimentation is controlled by accommodation. This relation is potentially useful in quantitatively predicting stratigraphic responses to specific allogenic processes in cyclic sedimentation. Five Cisco cycle types defined by the type of component lithofacies display the stratigraphic response mainly to noncyclic allogenic and autogenic processes. The cycle types have varying magnitude, thickness , and symmetry. They also vary in lateral extent and in abundance. A process-response model of cyclic sedimentation of the Cisco Group on the Eastern Shelf is established. It emphasizes the interplay between autogenic and allogenic processes at the sub-cycle scale. Understanding interactions among glacio-eustasy, climate, shelf subsidence, sediment supply, and depositional dynamics during various stages of transgression and regression is central to a clearer comprehension of the observed variations in cycle characteristics.

Sediments, facies tracts, and variations in sedimentation rates of holocene platform carbonate sediments and associated deposits, Northern Belize-Implications for "Representative" sedimentation rates

Full text is not available on SOAR. Publishers website: http://jsedres.geoscienceworld.org/cgi/content/abstract/74/4/498 DOI: 10.1306/012004740498

Estimation of Duration of Subaerial Exposure in Shallow-Marine Limestones--an Isotopic Approach

ABSTRACT Subaerial exposure surfaces in shallow-marine limestones may represent a significant period of nondeposition and/or erosion. The minimum duration of subaerial exposure is estimated through modeling the maximum whole-rock 13C depletion toward exposure surfaces in vadose meteorically altered limestones. The model simulates the water-rock interaction that caused the depletion, and calculates the volume of meteoric water needed to achieve the observed 13C value. Then, the duration of vadose meteoric diagenesis, which approximates exposure duration, is the volume of water divided by the infiltration rate of vadose water. Sensitivity tests indicate that estimated duration is moderately sensitive to many input parameters, particularly the isotope composition and concentration of total dissolved carbon in vadose water, sediment porosity, and infiltration rate of vadose water. The method is mostly effective in estimating exposure durations of less than 400 ky. The estimated durations of nine subaerial exposures in Pleistocene reef tracts of Barbados, West Indies agree well with available radiometric dates. Application of the method to subaerial exposures in Mississippian, Pennsylvanian, and Cretaceous shallow-marine limestones confines their durations to 21-54 ky and distinguishes exposures of different durations. The proposed method offers a potential tool and direction in estimating minimum duration of stratal hiatuses in meter-scale stratigraphic studies.