ABSTRACT The autochthonous Southern Basin of Trinidad has been strongly influenced during infill by changing tectonic regimes. The continuous oblique collision between the South American and Caribbean plates since the Cretaceous led to a diachronous series of events across the northern margin of South America. This review traces events from passive margin to oblique collision, with the latter occurring since the late Oligocene because of the Southern Basin’s far eastern location from the migrating thrust front. The basin’s passive margin phase lasted from the Early Cretaceous to late Oligocene, and the fill was dominated by deepwater slope and basin deposition. The initial boundaries of the basin were established in the late Oligocene by the onset of oblique collision culminating in the middle Miocene, as reflected in the foredeep by coarse-grained turbidites. By late middle Miocene, the angle of plate convergence changed as the tectonic regime shifted from oblique collision to east–west strike-slip tectonics. The key element since late Miocene was the west-to-east transiting paleo-Orinoco delta constructing the new Atlantic Margin by the deposition of four large-scale clastic wedges across the Southern and Columbus basins, by early Pleistocene. These wedges were likely driven by glacio-eustatic changes on the order of 100 k.y. The most outstanding aspect of the Neogene margin is the high-sedimentation rate, which generated eastward-migrating growth-faults across the shelf segment allowing greater than 10 km (33,000 ft) of sediment to accumulate, largely overprinting the transpressional regime. In the late Pleistocene, a late stage of renewed compression occurred, which eventually defined the Southern Basin.

Abstract: Deltas represent the major sediment source for tectonically confined, tide-dominated seaways or straits. Modern examples show how along-shore tidal currents are able to modify the impinging delta shape, generating asymmetrical coastal plains, deflected delta fronts and elongate sandbanks. Seaway or strait deltas can become become tide-influenced or tide-dominated, assuming physical attributes that may depart from classical models. Ancient deltas in seaways and straits can also reveal unexpected facies stacking and stratigraphies, which can be misinterpreted or attributed to different depositional settings. Two ancient analogues of deltas that prograded into elongate basins dominated by amplified tidal currents are presented here. A common element in these deltas is the progressive-upwards change in the dominant process of sediment dispersion recorded in the delta facies. Early stages of progradation are dominated by river- and wave-influenced lithofacies, whereas late deltaic advancements occur with a dominance of tidal current circulation on the delta fronts and the consequent morphologies are deflected/elongated in the direction of tidal flow. This study provides the basis for a preliminary stratigraphic framework for the depositional style of these types of delta. The studied deposits also suggest analogies with the spatial distribution of many hydrocarbon reservoirs investigated along the margins of confined, narrow, linear basins, the interpretation of which is still debated.

Abstract Recent sequence stratigraphic debate on the Brent system have focused on the interpreted nature of the progradational trajectory (horizontal, slightly upwards or downwards) of the shoreline (Rannoch/Etive Formations) through time, as this gives a direct measure of how late Aalenian-Bajocian relative sea level changed during regression. Early interpretations emphasized the unified shallowing-upward nature of the Rannoch-EtiveNess depositional system, and implicitly accepted a uniform shoreline progradation, i.e. a shoreline trajectory that was horizontal or slightly rising, implying a stable or slightly rising relative sea level. No irregularities of the trajectory were noted, and unusual shifts in facies, grain size etc. were normally related to autocyclic processes. More recent work has suggested that in some instances there is evidence for more irregular shoreline progradation at certain times, and for fall(s) in relative sea level and forced regression. This evidence comes from incised valleys and deep erosion/subaerial exposure surfaces from the landward (Etive-Ness boundary) and basinward (Rannoch-Etive) reaches of the Brent system respectively. However, it is currently unclear if any of these downshift surfaces recognized in the strandplain/coastal plain and shoreface environments are in time-equivalent strata. Current debate is mostly handicapped by a lack of agreement on the origin and depositional facies of the Etive Formation. There is significant debate about the relative amounts of fluvial, tidal and wave influence detected in the strata of this formation, with some authors arguing for a dominance of fluvial distributaries and mouth-bar deposits, whereas others propose either tidal-channel and inlet deposits or wave-dominated shoreface and strandplain settings. The stratigraphy is impacted by this disagreement. The character and sharp base of the Etive Formation can be argued to be consistent with normal shoreline processes, where wave or tidal conditions can produce significant erosion in the shoreface, without the necessity of any forced regression. Other interpretations, particularly where the Etive Formation is seen in terms of fluvial facies and processes, require a significant basinward shift of the shoreline to explain the Rannoch-Etive superposition, and a fall of sea level to cause the erosive boundary between the two formations. However, there is now ample evidence, including new evidence presented here, that both of the end-member scenarios for the progradation of the Brent system are incorrect. The notion that the overall progradation was entirely a product of normal regression, during stable and/or slightly rising relative sea level, is negated by local evidence of incised valleys, of subaerial exposure and plant growth in lower shoreface strata in the Rannoch Formation, and of repeated erosion surfaces with coarse-grained lags at the base of the Etive Formation. On the other hand, the idea of continuous sea level fall or of a single, late-stage fall, such that there was regional valley incision of the Etive into the Rannoch Formation and that the former is entirely younger than the latter, is negated by local evidence of gradual upward facies change between the formations, of stratigraphic interfingering between the formations, and of time lines passing through the Etive into the Rannoch Formation. It is perhaps not surprising that the system’s overall regressive trajectory varied in time from being forced to being normally regressive, and that further detailed local studies are required before regional generalisations can be made.

Abstract Subaerial and subaqueous debris flow, streamflood, sheetflood, sieve and low-energy shoreline deposits have been documented within six alluvial fan-fan delta bodies, representative of the various fanglomerate sequences fringing the fault margins of Hornelen Basin (Devonian), western Norway. Waterlaid beds are recognised largely by their internal stratification and sorting, sieve deposits by their geometry, sorting and ‘open’ or bimodal texture, while shoreline-modified conglomerates are well sorted and have a “fitted” fabric. Subaerial and subaqueous debris flows are distin-guished from each other on the basis of bed thickness/grain size ratio, texture, degree of grading and imbrication, and association with other facies. Mapping and analysis of six representative fan bodies shows that: (1) In mixed fans the debris flows tend to give way distally to sheetflood and stream deposits. (2) In debris flow dominated fan deltas the subaqueous portion occurs as a distal facies of diamictite-like deposits generated by mixing with and resedimentation in the impinging lacustrine environment. (3) Coarseness (maximum particle size) and thickness of beds generally increase both proximally and upwards within the fan bodies, though this happens much more evenly in fluvial fans. (4) All fan bodies thin distally, though the geometry and manner of thinning varies, dependent primarily on the relative rates of aggradation of fan and adjacent alluvium. Conglomerate beds and sequences of beds have been used to draw some tectonic inferences for the accumulation of Hornelen Basin's 25 km thick sediment pile. The all-pervasive, small-scale (5-25 m) C-U sequences reflect a general background of rapid basin subsidence. The larger scale (100-200 m), basinwide, onlapping C-U sequences are thought to record progradational intervals between periods of lateral migration of the basin and may therefore be an important characteristic of basins generated by strike-slip faulting. The proposed horizontal component of movement is recorded either by hiatuses (and occasionally a marked unconformity) between adjacent major C-U sequences or by a F-U capping to such sequences. Differences in fan facies, geometry and thickness between northern and southern margins of the basin indicate more rapid subsidence against the northern margin suggesting that this was the major tectonic zone.