Abstract The Middle Archean Moodies Group (ca. 3.22 Ga), Barberton Greenstone Belt, South Africa, exposes one of the world’s oldest ecosystems. It includes kerogen-rich laminae and thin chert bands interbedded with coarse-grained and gravelly sandstones. The strata record a medium-energy, tidal coastal environment. Analyses of the microscopic structure and chemical composition of the chert bands through petrographic microscopy, Raman microspectroscopy, laser-induced breakdown spectroscopy (LIBS) analyses, C isotopes, and scanning electron microscope (SEM) photography of macerated material, supported by textural observations of hand samples, suggest that these laminae represent variably compressed and early-silicified microbial mats. Internal wavy laminations, amorphous carbon composition, and negative δ 13 C values strongly imply a biogenic origin. Complete HF maceration of chert bands revealed polygonal cell structures in a formerly extracellular polymeric substance matrix. The tuft- and dome-micromorphology of the laminations resembles that of recent photosynthetic filament-dominated microbial mats. Facies interpretations indicate that microbial mats extensively colonized subtidal to intertidal Archean siliciclastic coastlines.

Abstract: Continental ichnofaunas display a progressive increase in bioturbation depth and intensity through the Phanerozoic. Ichnologic data from Cenozoic fluvial deposits of the Chaco Basin, Subandean zone of Bolivia, indicate widespread colonization of deep infaunal ecospace by the Miocene. Trace fossils are described from the Tariquia Formation, which records deposition in anastomosed fluvial systems. Although the Tariquia ichnofauna is of low diversity and does not display significant compositional variations throughout the succession, ichnofabric analysis reveals some degree of variability linked to different taphonomic pathways that helps to understand depositional dynamics and environmental conditions during accumulation of this fluvial unit. Intense and deep bioturbation occurs in medium- to very fine-grained crevasse sandstone and overbank mudstone. Less pervasive bioturbation is recorded in deposits of abandoned main channels. The Tariquia ichnofauna is dominated by Taenidium barretti, representing an example of the Scoyenia ichnofacies. Overbank deposits are totally bioturbated (BI = 6), showing complete destruction of the primary sedimentary fabric. Main-channel and crevasse-splay sandstones display an upward increase in degree of bioturbation. The top of the channel and crevasse-splay sandstone represents colonization surfaces that allow direct measurements of maximum burrowing depth. Taenidium barretti extends up to 2.2 m into the crevasse sand sheets. Depth and intensity of bioturbation of the main-channel and crevasse sands seem to be a function of time between depositional events. Main-channel and crevasse sandstones underlying thick packages of bioturbated overbank mudstones are intensely bioturbated, recording prolonged periods of low-energy sediment fallout between crevassing events. Conversely, the lowest degree of bioturbation is found in amalgamated channel sandstone units underlying thin intervals of overbank mudstones, reflecting high-frequency depositional episodes.

Abstract Liuhua 11-1 field, located in the Pearl River Mouth Basin offshore south China, consists of diagenetically altered Miocene limestone comprising a shallow-water carbonate bank. This bank forms the topmost and youngest interval of a larger, extensively karsted, buried carbonate platform. A three-dimensional (3-D) seismic survey of Liuhua field yielded a very high-resolution data set (>200 Hz), allowing a spatial resolution less than 5 m. This data set was subsequently used to produce a reservoir model that closely linked petrophysical, log, and seismic data. The carbonate stratigraphy suggests several subaerial exposure events that significantly modify primary stratification of the carbonate bank through diagenesis. These include freshwater leaching, burial compaction, cementation, and late diagenetic flushing of the bank. The combined diagenetic changes had three principal effects: (1) exacerbation of primary facies-dependent differences in porosity through a series of dissolution-reprecipitation steps; (2) widespread incipient carbonate collapse at or below the scale of seismic resolution; and (3) formation of numerous regionally occurring karst sinkholes of as much as 400 m diameter shortly before final drowning of the platform. Incipient collapse of the friable carbonate framework is expressed seismically by a reduction in amplitude. Carbonate dissolution appears to be ongoing because sagging continues to affect all strata overlying the reservoir to the seafloor. Subsurface dissolution may be a result of either flushing of the carbonate platform by cold, undersaturated marine waters or may be a result of active biodegradation of the hydrocarbons along the oil-water contact and the concomitant release of acids.

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Full article available in PDF version.

Four major Paleogene sedimentary belts—the Peralta, El Mamey, and Tavera Belts of Hispaniola, and the Cerrillos Belt of Puerto Rico—provide important constraints on the Eocene-Miocene tectonic evolution of the northern Caribbean. The southeastern half of the Peralta Belt consists of two basinal sequences, the lower to upper Eocene Peralta Group, and the middle(?) Eocene to lower Miocene Rio Ocoa Group, which were deposited along the southwest margin of the Greater Antilles island arc during, and after, the last stages of arc magmatism. Peralta Group strata are cut by thick, stratally disrupted thrust fault zones, and probably record development of an Eocene accretionary prism composed of offscraped trench deposits. Ocoa Group strata lack these stratally disrupted fault zones, and we interpret these rocks as fore-arc basin deposits. The Peralta underthrusting event was probably amagmatic and may have been related either to transpression associated with the earliest stages of the presently active left-lateral, strike-slip regime of the northern Caribbean; or to “back-arc” compression associated with the last stages of the arc-Bahamas collision along the northeast margin of the arc; or to a combination of both. The tectonic evolution of the northwestern half of the Peralta Belt, which contains only Upper Cretaceous rocks, remains relatively poorly understood. Coincident with Eocene deposition of the Peralta Group, the middle Eocene epiclastic, volcaniclastic, and volcanic rocks of the Cerrillos Belt of Puerto Rico were deposited in two short-lived intra-arc basins. In contrast to coeval Peralta Group deposits, the Cerrillos Belt rocks were associated with active arc magmatism, possibly along both margins of the belt. Late Eocene, northeast-verging thrust faults and folds that cut the Cerrillos Belt may record closure of the basins during the last stages of the Greater Antilles-Bahamas collision. Alternatively, basin opening and closure may reflect periods of transtension and transpression in an intra-arc, strike-slip fault zone associated with an obliquely convergent margin. Paleocurrents indicate that the basinal rocks of the El Mamey Belt of northern Hispaniola were deposited in an elongate, arc-parallel basin(s) active from late Eocene to early Miocene time. A basal, middle Eocene angular unconformity probably records deformation of island-arc basement during collision between the Greater Antilles arc and the Bahamas Platform. Paleoflow within the El Mamey basin apparently reversed during late Oligocene time, from an early period of southeasl-directed paleoflow to later northwest-directed paleoflow. An angular unconformity separates folded El Mamey Belt rocks from upper Miocene, flat-lying marls of the overlying Villa Trina Formation, reflecting an early(?)-middle Miocene period of folding, uplift, and erosion. This deformation may be related to the development of a restraining bend within the northern Hispaniola portion of the left-lateral northern Caribbean Plate boundary zone. The Tavera Belt of north-central Hispaniola underwent two distinct periods of basin development during early Oligocene (Velazquitos and Inoa Formations) and late Oligocene to early Miocene (Represa and Janico Formations) time, interrupted by a middle Oligocene period of folding and erosion. Complex paleocurrent patterns from the Represa and Janico Formations suggest that sediment may have been derived from at least three sides of the Tavera basin during late Oligocene-early Miocene time. Basinal sedimentation ended during the early Miocene, when Tavera Belt rocks were gently folded, eroded, and covered by shallow-marine conglomerates of the Cercado Formation. We tentatively interpret both the El Mamey and Tavera Belts as developing within a broad “California-type” strike-slip borderland.

The Cordillera Central of Hispaniola is a Cretaceous-Eocene island-arc terrane that was uplifted in Miocene to Recent time by oblique convergence between the North America and Caribbean plates. Island arc rocks at the southeastern topographic termination of the Cordillera Central plunge beneath three distinct Cenozoic marine clastic sedimentary sequences that are well exposed in a semi-arid climate. Major and minor structures and unconformities in the three sedimentary sequences record four Cenozoic deformational events that place important constraints on the tectonic history of Hispaniola. The first deformational event occurred in Late Eocene time and is marked by zones of syn-sedimentary stratal disruption and thrust imbrication of different lithologies of the Early-early Late Eocene Peralta Group (Witschard and Dolan, 1990). Witschard and Dolan (1990) proposed that Late Eocene syn-sedimentary deformation occurred in a Late Eocene accretionary wedge that formed as a result of early transpression along the North America-Caribbean strike-slip boundary and/or collision between the Hispaniola island arc and the Bahamas platform. Oblique northeastward underthrusting and accretion beneath southern Hispaniola may have continued until the Early Miocene and resulted in deposition of a thick (2 to 8.5 km), clastic sedimentary sequence (Rio Ocoa Group) in an elongate basin between an outer high (Peralta Group) and the extinct island arc of central Hispaniola. The second deformational event incorporated the previously deformed rocks of the Peralta belt and the Eocene to Early Miocene rocks of the Rio Ocoa Group in a southwest-verging fold and thrust belt. These rocks are unconformably overlain by unfolded rocks of the Middle Miocene-Pleistocene(?) Ingenio Caei Group that constrain the age of deformation to Early Miocene time. The deformational style of the Rio Ocoa Group suggests thrusting along a basal thrust fault inferred at depths of 2 to 3 km beneath the present-day land surface. We propose that the fold and thrust belt formed as the result of collision between Late Cretaceous oceanic plateau rocks of southern Hispaniola with the island-arc rocks of northern Hispaniola. The third deformational event occurred in Late Miocene(?) to the present and is marked by the uplift of the Cordillera Central and a resulting southeastward tilting of the Ingenio Caei Group and of fold axes within the Rio Ocoa Group. We attribute this uplift event to the formation of a strike-slip restraining bend in central Hispaniola. A fourth and final event affecting the area is marked by northeast-striking faults with apparent right-lateral offsets of Early Miocene folds and relatively greater post-Early Miocene shortening in the northwest corner of the study area. We attribute these effects to localized indentation of the southern margin of Hispaniola by northeastward displacement of the Beata Ridge.