Petrographical and geochemical evidences for paragenetic sequence interpretation of diagenesis in mixed siliciclastic–carbonate sediments: Mozduran Formation (Upper Jurassic), south of Agh-Darband, NE Iran

The Upper Jurassic Mozduran Formation with a thickness of 420 m at the type locality is the most important gas-bearing reservoir in NE Iran. It is mainly composed of limestone, dolostone with shale and gypsum interbeds that grade into coarser siliciclastics in the easternmost part of the basin. Eight stratigraphic sections were studied in detail in south of the Agh-Darband area. These analyses suggest that four carbonate facies associations and three siliciclastic lithofacies were deposited in shallow marine to shoreline environments, respectively. Cementation, compaction, dissolution, micritization, neomorphism, hematitization, dolomitization and fracturing are diagenetic processes that affected these sediments.Stable isotope variations of δ18O and δ13C in carbonate rocks show two different trends. High depletion of δ18O and low variation of δ13C probably reflect increasing temperatures during burial diagenesis, while the higher depletion in carbon isotope values with low variations in oxygen isotopes are related to fresh water flushing during meteoric diagenesis. Negative values of carbon isotopes may have also resulted from organic matter alteration during penetration of meteoric water. Fe and Mn enrichment with depletion of δ18O also supports the contention that alteration associated with higher depletion in carbon isotope values with low variations in oxygen isotopes took place during meteoric diagenesis. The presence of bright luminescence indicates redox conditions during precipitation of calcite cement

Ordered hierarchy versus scale invariance in sequence stratigraphy

200 x 10(6) years) are symmetrical transgressive- regressive cycles. However, the sequence record in the range of 1 x 10(4)-200 x 10(6) years, the principal domain of sequence stratigraphy, shows a rather irregular succession of sequences with variable symmetry and bounded by flooding surfaces or exposure surfaces. For these time scales, scale-invariant models are a good first approximation, particularly because the evidence for scale-invariance and randomness in the stratigraphic record is strong: Frequency spectra of sea-level change as well as rates of sedimentation and rates of accommodation change plotted against length of observation span show basic trends indistinguishable from random walk. These trends, combined with scale-invariant sequence models may be the most efficient tools for across-the-board predictions on sequences and for locating islands of order in the sequence record

Sistema Zacatón: Volcanically Controlled Hypogenic Karst, Tamaulipas, Mexico

Sistema Zacatón includes the second deepest underwater cave in the world. It is hypothesized to have formed by volcanogenic karstification, a process that relies on four components to initiate and develop deep subsurface voids: a carbonate matrix, a system of preferential flow paths (e.g., fractures), volcanic activity that increases groundwater acidity, and groundwater flux through the system. Results are compiled into a multiphase speleogenetic model, most phases of which are of Late Pleistocene age. Surface rocks consist of carbonate travertine with Pleistocene mammoth fossils found within the rock matrix. The rocks are interpreted as a hydrothermal travertine terrace formed as nearby volcanic activity peaked, and thus representing the end member of a carbonate mass transfer system originating deep in the subsurface. The modern karst system includes a dynamic array of deep, phreatic sinkholes (also called cenotes) propagated upward through the travertine and exposing hydrothermal water supersaturated with carbon dioxide to the atmosphere. In some cenotes, seals of a second stage of travertine formed as CO2 degassed, capping the sinkholes with hydrologic barriers. Volcanogenic karstification worldwide is not limited to Sistema Zacatón, although the localized nature, coupled with the extreme degree of karstification, makes it an ideal modern analog for classifying certain other karst systems as volcanogenic