Abstract A 160-m-long section measured in the lagoonal facies of the Middle Triassic Latemar platform (Dolomites, Italy) reveals a set of frequency components that we interpret as a strong Milankovitch signal. In this interpretation, all principal frequencies associated with the theoretical Middle Triassic precession index, P1 = 1/(21.7 ky), P2 = 1/(17.6 ky), and its modulations, E1 = 1/(400 ky), E2 = 1/(95 ky),and E3 = 1/(125 ky), were detected in a time-frequency evaluation of the cycles. A weak obliquity signal is also present in part ofthe section.Thus, the Latemar cycles appear to have recorded the clearest orbital forcing signal yet found in a carbonate platform. This astronomical calibration indicates that the section was deposited in ca. 3.1 My and therefore that the entire Latemar cyclic succession (~470 m) took at least 9 My to form. However, the calibration also leads to serious conflicts with other interpreted geological data: U/Pb radiometric ages of zircons collected from tuffites within theLatemar lagoon and in coeval basinal sediments point to a timescale that is five times shorter than this astronomically calibrated estimate; similar discrepancies arise when the average duration of Triassic ammonoid biozones or the sedimentation rates of coeval basinal series are considered. Nonetheless,all of the methods that have been used to estimate the time of formation of the Latemar platform continue to have shortcomings, and the contradictions among these different geologicalcalibrations remain unresolved.

Abstract The Middle Jurassic Vajont Limestone of the Venetian Alps, Italy, is pre-dominantly composed of resedimented ooids that were deposited in slope and basin settings. The Vajont Limestone has been partly replaced by massive dolomite that can be mapped at both regional and local scales. Dolomite bodies that are present within or are associated with the Vajont Limestone include: (1) a large-scale wedge, ~ 25 km long, 10-15 km wide, and > 400-500 m thick (50-94 km3), located on the hanging wall of the Alpine-aged, thrust- based Mt. Grappa-Visentin anticline. This dolomite body is located within the axis of the anticline and crosscuts the Stratigraphie section where subver-tical to vertical faults penetrate the crest of the anticline; (2) Isolated, rootless plume-shaped bodies, 100-200 m wide and > 300 m high (> 2 × 10~ 2 km3), which penetrate a footwall syncline within an Alpine-aged thrust sheet. These dolomite “plumes” possess extensively brecciated cores and exhibit sharp to gradational transitions with surrounding Lower to Middle Jurassic basinal limestone; (3) Isolated dolomite “towers” that have partly replaced Cretaceous-age synsedimentary fault breccia. These bodies are found in overlying basinal strata (i.e., the Fonzaso Formation, the Ammonitico Rosso, and the Biancone Formation), but emanate from the underlying dolomitized Vajont; and (4) Small-scale wedge-shaped dolomite bodies on the scale of meters found along small faults and fractures. The connection between these dolomite bodies and Alpine-aged faults and fractures clearly indicates that dolomitization was a late burial process. It is proposed that during the Alpine deformation event, convection-driven fluids derived from Late Tertiary seawater were circulated through subaque-ous Alpine-aged faults and fractures and paleosynsedimentary breccias, thus creating the multitude of dolomite bodies now found in the Vajont and other Mesozoic basinal sediments. Paleogeographic, tectonic, and hydrologic sys-tems, similar to the one proposed for dolomitization of the Vajont, appear to be active in modern subaqueous thrust zones of the Caribbean and Northwest Pacific Coast. Potential reservoir attributes of Vajont dolomite bodies include their large size and medium to coarsely crystalline replacement fabric that is character-ized by significant amounts of partial moldic, intercrystalline, and vug pore space. Visual estimates of porosity within dolomitized grainstone and pack-stone range up to 10% to 15%, with inferred permeabilities of 1-100 md. Permeability of Vajont dolomite replacement fabrics is enhanced through recrystallization and the formation of touching-vug networks (inferred per-meabilities >100 md). >Results of this study indicate that (1) massive replacement dolomitization in thermotectonic (i.e., burial) settings may be much more important than previously thought, and (2) significant reservoirs may be hosted in otherwise tight basinal limestones as the result of late-stage burial dolomitization. Consequently, the geometries of the Vajont dolomite bodies may provide analogs for reservoir characterization and new exploration plays in the subsurface. Exploration methods for analogous dolomite reservoirs in the subsurface may include the mapping of dolomitization fronts using core and log data and seismic reflection identification of crosscutting dolomite bodies. The focus of such efforts should be placed on anticlinal and synclinal struc-tures within buried fold and thrust belts, and along zones of deep-seated tec-tonic fractures and faults within intracratonic basins.

Abstract The intensive studies in the 1960's and 1970's of modern shallow marine carbonate environments in the Persian Gulf (e.g., Shearman, 1963, 1966; Kinsman, 1966; Butler, 1970; Kendall and Skipwith, 1969; Purser, 1973), Florida and the Bahamas (e.g., Shinn and others, 1969; Hardie, 1977a; Enos and Perkins, 1979), and Western Australia (e.g., Logan and others, 1970, 1974b), led to spectacular advances in our understanding of the deposition and early diagenesis of carbonate rocks. These studies were part of a major revolution in sedimentology that saw a radical change from an approach based heavily on grain textures to one based on sedimentary structures and early diagenetic features. In this new approach, paleo-environments of sedimentary deposits are diagnosed from the vertical and lateral distribution of elemental rock units (subfacies and facies), characterized principally by their assemblages of sedimentary structures and early diagenetic features in combination with other properties such as sedimentary textures and biota, using analogs established from observations of processes and their sedimentary records in modern depositional environments (the "comparative sedimentology" method of Ginsburg, 1974). Modern shallow marine carbonate environments carry a particularly rich inventory of primary sedimentary structures and early diagenetic features, such as current and wave bedforms, trough and tabular cross-stratification, "herringbone" cross-stratification, flat lamination, wavy and crinkled lamination, thin bedding, stromatolites, thrombolites, mudcracks, sheet cracks, prism cracks, flat pebble gravels, fenestrae, burrows and roots (and their casts and molds), evaporite minerals (and their casts and molds), early cements, hardgrounds, caliche crusts, tepee structures, dissolution cavities, and so on. And most significantly, the stratigraphic record back at least into the Proterozoic is replete with carbonate deposits that delicately preserve these primary and early diagenetic sedimentary features (see, for example, Ginsburg, 1975; Wilson, 1975; Hardie and Shinn, 1986; Grotzinger, 1989), demonstrating the existence through much of geologic time of environments and environmental processes analogous to those of modern shallow marine carbonate platforms and shelves.

Abstract In compiling this atlas, we were faced with an enormous array of features to describe and illustrate. Many of the features of shallow marine carbonates are common to the deposits of other depositional environments. We have chosen to present only those features that are pivotal to the recognition of those carbonate rocks that were deposited in shallow marine paleoenvironments. A significant exception to this is made in the case of metazoan reefs. The properties of these reefs, modern and ancient, are too extensive to be included in this atlas and the reader is referred to the excellent summaries by James (1983), James and MacIntyre (1985), and James and Bourque(1992). Stratification or layering is the most characteristic and fundamental of all sedimentary structures. In outcrops, the physical resemblance of stratification to the pages of a book conjures up the idea that sedimentary rocks are natural history records, records that can be deciphered to yield a glimpse of the chronological succession of events that befell the surface environments of the earth. Stratification by its very nature reveals sedimentation as an episodic process in which periods of non-deposition are punctuated by bursts of sediment influx. The periods between sedimentation events may be as short as seconds (as occurs on the avalanche slopes of actively migrating ripples), may be months or years long (as occurs on the storm-built supratidal flats of the Bahamas), or they may occupy many millennia (as happened on the Florida platform during the Pleistocene glacio-eustatic sea level oscillations).