Bermuda Carbonate Platform: Quantitative Analysis of Depositional Environments: ABSTRACT

Flume experiments with fine-grained carbonate particles (< 62.5 µm) show that they form floccules that travel in bedload, form current ripples, and deposit laminated sediments. In order to compare our flume experiments to work on sand transport, we ran flume experiments with medium sand and observed that the velocities at which sand grains started to move and form ripples were in the same range as those where floccules move and form ripples. These ripples are in essence identical to those formed by clay-mineral floccules or sand grains under similar conditions. In light of previous experiments with clay minerals, these results indicate that, of the key controls on mud deposition, flocculation and suspended-sediment concentration are more important than particle mineralogy or water chemistry, and are about as important as bottom shear stress for deposition. Suspensions of carbonate mud show the same pattern of flocculation, ripple formation, and bed accretion as observed previously in experiments with clay-mineral suspensions. The resulting carbonate mud deposits show internal low-angle (2–5 degrees) laminae, and in plan view a pattern of ripple foresets that is identical to rib-and-furrow structure in sandstones. Just as previously assumed for terrigenous muds, there has been a long-standing notion that accumulation of abundant carbonate mud reflects quiescent conditions of offshore and deeper-water environments. These experiments demonstrate unequivocally that carbonate muds can also accumulate in energetic settings. In the sedimentary record of carbonate rocks, interbedded grainstones and lime mudstones may thus not necessarily reflect shifts in depositional energy (or water depth), but alternatively may imply a shift in supplied sediment type. The observations we report suggest that published interpretations of ancient lime muds and derived paleoceanographic conditions may need to be reevaluated.

HISTORY: Twenty-five years ago, a 17-year-old healthy, right-handed male presented with right hemiparesis and sensory loss after scuba diving. The patient descended to 37 meters and ascending following typical dive safety protocol. Upon reaching the dive boat, he had a seizure and lost consciousness. Upon regaining consciousness, he noted new onset right-sided hemiplegia and hemiparesis. Due to the remote dive location, it took four hours to reach the nearest medical center. PHYSICAL EXAMINATION: Right-sided hemiparesis and sensory loss to light touch. Regular rate and rhythm without murmur. DIFFERENTIAL DIAGNOSIS: Decompression illness. Cardioembolic stroke from atrial fibrillation. Ischemic stroke from cerebrovascular disease. Epilepsy complicated by Todd’s Paralysis TEST AND RESULTS: Cerebrovascular imaging revealed stroke and spinal cord injury with normal cerebral vessels. Transesophageal echocardiogram (TEE) demonstrated a patent foramen ovale (PFO) with a right-to-left shunt and normal biventricular function. ECG was without atrial fibrillation FINAL WORKING DIAGNOSIS: Stroke and spinal cord injury due to decompression illness TREATMENT AND OUTCOMES:100% oxygen and one week of recompression therapy were administered. Extensive physical therapy partially improved his paralysis and paresthesia. However, diminished strength (4/5) and light touch sensation (1/2) in his right leg persisted. Despite recommendations to stop diving, the patient continued to dive at shallow depths (4.5 to 6 meters). Twenty-five years later, the patient was referred by his primary care provider to cardiology to discuss the safety of scuba diving. Repeat TEE demonstrated PFO with moderate right-to-left shunt as well as atrial septal aneurysm, normal biventricular function and mild right ventricular enlargement. Cardiac CT revealed normally connected pulmonary veins. The patient’s PFO anatomy was amenable to percutaneous closure. Given his history of decompression illness, percutaneous closure was considered to decrease the risk of adverse events associated with scuba diving. After a shared decision-making discussion, the patient elected against closure of his PFO and continues to dive recreationally at shallow depths.

Bioerosion and carbonate mud production on high-latitude shelves

A bar isolates the hypersaline pool from the Gulf of Aqaba. Finely laminated algal mats carpet the shallow shelf of the pool and gypsum floors the slope and bottom. Algae secrete pellets, ooids, oncolites, grapestones, flakes, and carbonate laminites. The ooids have a radial texture; hence, contrary to statements in the literature, ooids with a radial texture are formed in the depositional environment. The carbonate laminites occur between the gray and black algal mats. Although some of them are fibrous, most are cryptocrystalline. Cryptocrystalline laminites which consist of high-Mg calcite mimic the micrite of the geologic rock record; these laminites can preserve the morphology of the mats even after the organic matter has disappeared. Scanning electron micrographs sho the laminites to consist of a mosaic of micron-size rhombohedrons which, during diagenesis, would stabilize to low-Mg calcite. Hence, the origin of some ancient stromatolitic limestones (pelmicrites) may be explained in terms of secretion of cryptocrystalline high-Mg calcite laminites. These laminites are lithified within the algal mats; hence, their origin does not necessitate the introduction of later cement and establishes algal secretion as a potent force in lithification. This inference may supersede the concept that all micrites result from neomorphic replacement of aragonite. The algae create a microenvironment in which Mg becomes enriched in the organic matter, and in which high-Mg calcite with up to 40% molecular MgCO3 is secreted. The total molecular percent of MgCO3 in the Mg-organic complex and high-Mg calcite combined may reach 60. This preferential concentration of Mg may explain the high level of dolomitization of stromatolitic rocks in the geologic record. Amino acids devoid of sulfur, especially aspartic acid, as part of the biologic system may exert considerable influence in the precipitation of the carbonate laminites and particles. End_of_Article - Last_Page 618------------

Coastal reef environments support a great diversity of benthic species, which are physically arrayed according to their adaptations to biotic factors such as competition and to abiotic factors such as salinity and desiccation. Few studies have described adaptive strategies on rocky shores in tropical regions. This contribution deals with the spatial distribution of three species of zoanthids (Palythoa caribaeorum, Protopalythoa variabilis and Zoanthus sociatus) on flat sandstone reefs in north-eastern Brazil. Zoanthid distribution and abundances were determined as the percentage of cover along six transects, where 50 × 50 cm quadrats were sampled. Biotic data (influence of macroalgal cover) and abiotic data (relief, substrate type, temperature and salinity) were also taken along the transects to attempt to explain the zoanthid distribution. These species tended to colonize the middle and low intertidal zones, which are least exposed to air. Zoanthus sociatus apparently resists desiccation better than P. caribaeorum. Protopalythoa variabilis showed the highest abundance and frequency, occurring in the entire middle intertidal zone and colonized areas with high proportions of unconsolidated sediment. Desiccation was the main factor limiting the spatial distributions of the three species, although substrate composition and probably interspecific competition with macroalgae were also important. The results suggest that the zoanthids use a variety of these adaptive strategies, which account for their success on consolidated substrates. Continuous monitoring of changes in distribution patterns can be useful for indicating human impacts on marine biodiversity, providing insights for monitoring and conservation programmes.

Knowledge of structure, composition and texture, the three fundamental attributes of sedimentary deposits, is necessary for a robust analysis and interpretation of depositional environment. In the absence of core information; borehole images, "elemental-capture spectroscopy" and NMR logs all contribute to understanding the structure, composition and grain-size attributes. Grain-size is considered the most important textural parameter because it reflects the processes and energy levels active at the time of deposition. Early studies show that depositional processes and environment can be inferred from size distributions (Krumbein1934 and Sahu1964).These inferences are based on deductions and experimental data and deliver acceptable accuracy. Although Krumbein (1941) shows other textural (e.g. shape, roundness, roughness) and compositional information enhances certainty.). Building on early work, we present a methodology and examples where NMR derived grain-size-distributions are used to infer depositional environment. In contrast to spot-core analysis, NMR provides a continuous along hole profile of grain-size-distributions. This study is the first to utilize a continuous grain-size-distribution profile. The study was conducted blindfolded, without initial reference to core studies, to test the robustness of the methodology. By using statistical parameters from grain-size distributions that are characteristic of depositional environments, and applying Sahu (1964) linear-discrimination functions, depositional systems for two wells were inferred to be shallow marine-to-fluvial deltaic. Linear-discriminate analysis of geostatistical variables showed bimodal distributions of sediments dominated by fine-grain sands and silts. The studied sandstones were concluded to be mainly fine grained, moderately to poorly sorted, fine skewed, mesokurtic, leptokurtic occasionally platykurtic in nature. Our conclusion was "shallow-marine deposition" and was compared and confirmed with core-driven studies as "shallow marine". In summary, profiles of grain-size-distributions from NMR logs provide important information about changes in depositional energy level from which we infer depositional setting and reservoir quality. A precise depositional environment interpretation is crucial for optimal/economical field development.

Sands penetrated at shallow depths (&amp;lt; 1 km) in the Gulf of Mexico can flow to the surface and undermine seafloor installations. Here we explore aspects of natural fluid expulsion structures from these "shallow-water sands". Highresolution seismic data shows no obvious sand injection structures from the Blue Unit, a shallow-water sand, in the Ursa area, but perhaps nascent dike formation. Also, channel sands stratigraphically above the Blue Unit are overpressured, with a possible pressure source in the latter. In the Ursa area the maximum principal stress is vertical and constrained by the integrated unit weight of the sediments at IODP Site U1324. The stress necessary to form hydrofractures (the minimum principal stress plus negligible tensional strength of the sediment) is about 85% of the maximum principal stress indicating an approximately isotropic state of stress. High Poisson's ratios (0.46-0.49), determined from wireline compressional and shear wave velocities at Site U1324, are also consistent with a nearly isotropic state of stress at shallow depths. Estimated maximum fluid pressures are about 80% of the total overburden stress, less than necessary to hydrofracture the sediment. Therefore, the fluid pressure is insufficient to form sand injectites. The formation of sand injectites, as observed in the geologic record, may require very high fluid pressures, approaching lithostatic values. Diverse orientations of ancient injectites support the necessity of very high fluid pressures and a nearly isotropic state of stress. Introduction Drilling operations in the Gulf of Mexico, especially in deep water, have encountered unconsolidated and overpressured sand at shallow depths. Once penetrated these sands flow into the wells, washout, erode around the exterior of casing strings, and locally flow to the surface [1]. Surface features associated with this ?shallow water flow' behavior include cracks, faults, and gully-like features [2]. In areas where this behavior has occurred, the seafloor appears to be undergoing subsidence associated with sediment loss at depth. In the Ursa drilling area (Figure 1) the shallow-water sands are overpressured which initiates the flow into the boreholes [1, 3]. The shallow-water sands in the Ursa area readily flow into boreholes and to the surface if the borehole is normally pressured (i.e. the hole is uncased or an underbalanced mud is used). Therefore, an obvious question is whether there are any natural injections from the shallow-water sands or other evidence of fluid flow. Given the overpressured nature of the shallow-water sands [3], naturally occurring injections and other fluid migration features could be expected. These "injectites" are common in some petroleum systems where they complicate sand reservoir geometry and have important implications for hydrocarbon production [4]. Because fluid pressure and sand geometries are well-constrained, the Ursa area may be an important natural laboratory to ascertain the limiting fluid pressure and stress conditions for the occurrence of injectites and other fluid flow features. Fluid Migration and Sand Injection from Blue Unit Sand in IODP Drilling Area During the last 70 ka, rapid deposition, directly south of the Mississippi River accumulated a sand and mud sequence, the Blue Unit [5].

Sandstone reservoirs are the results of long and commonly complex histories of geologic evolution. The combined processes of deposition, burial, compaction, diagenesis, and structural deformation yield final reservoir bodies of widely varied geometries, permeability-porosity characteristics, and structural configurations that are difficult to predict. In unraveling the evolution of sandstone reservoirs it is necessary to have detailed knowledge of their initial depositional characteristics and of the postdepositional modifications impressed upon them. This knowledge can provide a rational basis in predicting the characteristics of reservoir bodies away from areas of data control. Little information pertaining to the reservoir characteristics of freshly deposited sand bodi s has been available. In an API sponsored study, permeability, porosity, and textural parameters were derived from 992 oriented and undisturbed sand samples of river bars, beaches, and dunes undergoing active sedimentation. River point-bar samples have permeabilities ranging from 4 md to more than 500 darcys and average 93 darcys. Porosities in the river point-bars range from 17 to 52 percent and average 41 percent. Beach sand samples have a permeability range of 3.6 to 166 darcys and average 68 darcys. Porosities in beach sands range from 39 to 56 percent and average 49 percent. Permeability values in dune sands range from 5 to 104 darcys and average 54 darcys. Dune sand porosities range from 42 to 55 percent and average 49 percent. Permeability values in river-bar sands are extremely varied in comparison to those of beaches and dunes. In river bars, permeability decreases systematically downstream and bankward. Although of low variability, permeabilities on beaches are low on the beach faces, high on t e beach crests, and variable on the beach-berm areas. Both river-bar and beach sands have well organized directional permeabilities, parallel with the length of the bodies in river bars and perpendicular to the length of the bodies in beaches. Dunes are characterized by low variability in permeability and porosity and show no significant patterns or trends. There is greater variability within bedding and lamination packets than between them. In addition the boundary conditions between bedding and lamination packets are important factors in determining the effective reservoir characteristics of sand bodies, to the extent that a bedding unit of higher permeability completely surrounded by units of lower permeabilities will not demonstrate its ultimate through-flow capabilities, but will have an effective permeability influenced by and largely determined by the lower permeabilities of the bounding units. River-bar sand bodies have significantly different arrangement and variability between bedding units from those of beaches or dunes. The ideal relations between permeability-porosity and textural parameters that have been set forth by various authors for artificially packed particles are demonstrated only slightly by these natural sands from different depositional environments. In all three depositional environments permeability increases with increase in grain size and porosity increases with increase in grain sorting. However, in river-bar sands permeability increases as grain sorting increases and porosity increases as grain size increases, just the opposite of the relations in beach-dune sands and in the artificially packed grain experiments. The underlying cause of these deviations is the different style of grain packing in the river-bar sands.

Quaternary climate and sea-level research in coastal karst basins (caves, cenotes, sinkholes, blueholes, etc.) generally focuses on analyzing isotopes in speleothems, or associating cave elevations prior sea-level highstands. The sediments in coastal karst basins represent an overlooked source of climate and sea-level information in the coastal zone, but to accurately interpret these sediments first requires an understanding of the forcing mechanisms that emplace them. In this study, we hypothesize that coastal karst basins transition through vadose, littoral, anchialine, and finally into submarine environments during sea-level rise because groundwater and sea level oscillate in near synchrony in the coastal zone, causing each environment to deposit a unique sedimentary facies. To test this hypothesis, the stratigraphy in twelve sediment cores from a Bermudian underwater cave (Green Bay Cave) was investigated and temporally constrained with twenty radiocarbon dates. The results indicate that we recovered the first succession spanning the entire Holocene from an underwater cave (~ 13 ka to present). The sediments were characterized with X-radiography, fossil remains, bulk organic matter, organic geochemistry (δ 13C org, C:N), and grain size analysis. Four distinct facies represent the four depositional environments: (i) vadose facies (> 7.7 ka, calcite rafts lithofacies), (ii) littoral facies (7.7 to 7.3 ka: calcite rafts and mud lithofacies), (iii) anchialine facies (7.3 to 1.6 ka: slackwater and diamict lithofacies), and (iv) submarine facies (< 1.6 ka: carbonate mud and shell hash lithofacies). The onset and duration of these sedimentary depositional environments are closely linked to Holocene sea-level rise in Bermuda, indicating that sea level controls environmental development in coastal karst basins. Finally, we present a conceptual model for interpreting the sediments and environments in coastal karst basins as a result of sea-level change.

Three classic carbonate models (carbonate bank, oolitic shoal, and tidal flat) are applied to interpret the depositional environments and diagenesis of the Caddo limestone (Middle Pennsylvanian). The study area is located in west-central Stephens County, approximately 4 mi southwest of Breckenridge, Texas. Phylloid algal mounds developed on a lower Caddo ramp, along the transition of the Concho platform and Fort Worth basin. Within the area defined as the Curry unit, phylloid algal mounds and intermound facies aggraded to sea level. This led to the development of an areally restricted carbonate bank. Some phylloid algal buildups underwent subaerial exposure, and fresh water leached unstable carbonate minerals, creating minor secondary porosity (<10%). An oolitic shoal (0-9.2 m thick) overlies and partially intergrades with the phylloid algal bank. Ooids were deposited along the margin of the preexisting bank, whereas fine-grained oolitic and peloidal packstone were deposited on the bank. The bank margin oolite exhibits oomoldic porosity in excess of 15%. The uppermost 6.1-24.4 m of the Caddo represents intertidal deposition. Sediment, primarily algal remains and carbonate mud, was deposited on a preexisting surface of ooids and peloids. Fenestral fabric. rhizoliths, brecciated zones, and small-scale channels are common features. A wide variety of poremore » types are present in the intertidal zone; intraskeletal, framework, biomoldic, and vuggy pores are most important. Throughout this zone, porosity exceeds 10% and permeability averages 12 md. The Curry unit has produced approximately 28 million bbl of oil (primary and secondary) from the upper Caddo. Depositional environments and diagenesis of the Caddo are complex. A greater understanding of these complexities may enable geoscientists to evaluate remaining future potential within the Caddo and in other similar carbonate accumulations.« less

Although admired and examined since antiquity, carbonate sediment and rock research really began with Charles Darwin who, during a discovery phase , studied, documented and interpreted their nature in the mid‐19th century. The modern discipline, however, really began after World War II and evolved in two distinct phases. Breakthroughs during an advancement phase came about via the numerous studies by many researchers globally on both modern and ancient carbonates that were eventually merged into carbonate sedimentology as a whole in the 1970s. Formulation of the factory concept and integration of recurring facies into facies models solidified the discipline and allowed integration to move ahead. Stromatolites were discovered growing in modern environments and it was realised that the spectrum of diverse fossil structures called reefs also existed in the modern world. Internal reef structure was largely interpreted as ecological. At the same time, development of radiogenic and stable isotopes allowed Pleistocene carbonate stratigraphy and processes to be accurately dated, and precipitates related specific formation waters. Cementation, long postulated as mostly meteoric, was also realised to be a common synsedimentary phenomenon. Breakthroughs during the more recent refinement phase were likewise synthesised into books, but incrementally as the science progressed. Research on the genesis of critical components such as ooids, stromatolites and carbonate muds was at last convincing. The whole new realm of cool‐water carbonates was revealed with resulting different interpretations of rock record facies and the critical role of aragonite in diagenesis. Reconciling the relationship between sea water nutrient levels, factory type and taphonomy was a major advancement to interpreting the stratigraphic record. Sequence stratigraphy, coupled with shallow drilling and isotopes, led to progressively more dynamic interpretations, especially of reefs. Use of stable isotopes became more sophisticated, and development of clumped isotopes held out possibilities of even more precise interpretations. Finally, although prediction of a future phase is futile, much more needs to be done on mixed carbonate‐siliciclastic systems, and the implications of mixed carbonate ironstone and chert in Precambrian accumulations, especially using new geochemical techniques. Perhaps, philosophically, it is time to use attributes of the exquisite carbonate rock record to tell us about the future of our rapidly changing world.

To infer the depositional environment, thirty-eight samples of the Orangi Sandstone-the uppermost unit of the Upper Oligocene Nari Formation, exposed at Hub Dam were collected. Granulometric data acquired from standard sieving techniques. The computed graphic mean's average value, was 1.38. Of the 38 samples, 24 were categorized as medium-grained, and the remaining 14 as coarse-grained. According to the sorting values, 36.84% of the samples were categorized as moderately sorted to moderately well-sorted, whereas 63.15% of the samples were poorly sorted. Skewness values showed a wide range from -2.17 to 2.65 suggesting a mixed population of coarse and fine skewed sediments. Kurtosis ranged from -2.46 to 2.97, showed situation of both leptokurtic and platykurtic. Bivariate plots created employing various textural parameters suggested predominantly fluvial origin with very few deviations. This interpretation is further supported by linear discriminant function analysis, which indicates a fluvial to shallow marine depositional setting for most samples. The study represents an initial attempt to appraise the vertical and lateral variability of the Orangi Sandstone using cement composition and textural parameters; skewness and kurtosis. The studied samples significantly reflect the impact of Upper Oligocene Period's tectonics, climate shift, and sea level fluctuations, and sediment transport dynamics. A model is constructed to describe the preponderant fluvial environment outlining the role of beach and tidal processes along with fluvial channels in the deposition of the sandstone.

Drowning is likely to result from impairment of consciousness when scuba diving. Causes include toxic effects of breathing gas, including nitrogen narcosis and oxygen toxicity, and arterial gas embolism. Review of the medical records of scuba divers who had impaired consciousness underwater that could not be attributed to toxic effects of breathing gas or arterial gas embolism. Four scuba divers had episodes of impaired consciousness when at shallow depths (8-18 m) underwater. The descriptions of the episodes were very similar. Three had histories of recurrent episodes of vasovagal syncope on land. Absence of other causes for their impaired consciousness underwater leads to the conclusion that the probable cause was vasovagal syncope.

Drs. R. V. Tyson and T. H. Pearson convened a symposium on modern and ancient shelf anoxia in May, 1989 under the aegis of the Gologica] Society Marine Studies Group. This volume is the product of that meeting. The meeting was multidisciplinary including geologists, biologists and chemists. The examination and interpretation of anoxic sedimentary environments is of significance in our understanding of the interplay of organic production, water circulation and sediment accumulation. When the flux of degradable organic material exceeds the oxygen supply, then anoxia and the exclusion of macrobenthos removes the more common bioturbation effects preserving primary depositional structures. It has also been postulated that the anoxic environment permits preservation of a larger proportion of the incoming organic matter. Studies in a variety of modern environments has tended to argue against that hypothesis although certainly the role of sulfate reducing bacteria versus aerobic bacterial processes is a major factor in the ultimate sedimentary product. Within the broad spectrum of anoxic environments, shelf anoxia is a special case. One complicating factor for the stratigrapher is that the anoxic condition in several shelf environments discussed in these papers is temporary and ephemeral. Thus a primary question is what artifacts of these periods of low or zero oxygen will remain in the sedimentary record? As the convenors point out in their opening review, the modern effects impact fisheries and with the overprinting of anthropogenic effects produce some complex questions for the marine biologist. They also aver that much of the hydrocarbon-rich sedimentary resources were laid down and developed in organic-rich and oxygen-depleted depositional environments. As noted earlier, the sometimes higher organic content of such sediments reflects the common association of high productivity regions (upwelling regions) with oxygen-deficient depositional environments. Even if degradation and removal rates are similar for all depositional environments, certainly the net input under high production areas will be larger. Major factors influencing oxygendeficiency in shelf regions are the higher biological production in margins as compared to open ocean, the short fall path in margins compared to open oceans and resulting higher sediment input rates, higher sediment accumulation rates and thus thicker mass accumulation and higher burial rates for carbon. Bioturbation is also operating at higher rates, and fluctuations in depositional regime are larger and faster. The editors also introduce a suggested standard terminology for such environments in which the environmental facies are termed oxic, dysoxic (with subdivisions), suboxic and anoxic with respective bottom water oxygen contents of > 2.0 ml/L, 0.2-2.0 ml/L, 0-0.2 ml/L and <0.0 ml/L (increasing dissolved H2S). Biofacies terms for these are respectively aerobic, dysaerobic, quasi-anaerobic and anaerobic at the same oxygen boundaries. Physiological regimes would be Normoxic, Hypoxic (no matching term for Suboxic) and anoxic respectively. The volume is subdivided into modern shelf anoxia studies, and ancient shelf anoxia studies. The papers in the two sections number 11 and 16 respectively and space does not permit review of each paper. The modern studies examine the northern Gulf Of Mexico shelf associated with or adjacent to the Mississippi River discharge (papers by Boesch and Rabalais; Rabalais, Turner, Wiseman and Boesch; Harper, McKinney, Nance and Salzer)) which discuss effects on benthos, recovery of communities and general oceanographic characteristics of the environment. This area may well be one in which the anoxia has an anthropogenic source as noted in the paper by Rabalais and others. Contrasts with other areas are noted. The effects are of the order of 4-5 months in duration in the delta front area. A paper by Van der Zwaan and Jorissen compares the effects in the northern Adriatic, Orinoco-Paria shelf and the Mississippi locale. All are influenced by high river discharges and pollution effects. The authors utilize benthic foraminifera as indi-

Criteria for identifying alluvial-valley fills in the geological record have commonly been derived from coastal areas, where high-magnitude glacioeustatic fluctuations generate predictable systems tracts. In contrast, inland alluvial valleys formed in response to climatic, tectonic, and intrinsic events of varied magnitude and duration. Sequence boundaries and architecture reflect a variety of causes, typically mediated through changes in the river’s equilibrium profile and equilibrium floodplain height. We evaluate valley and sequence models using Quaternary examples from southern Asia and Australia. For these areas, proxy records are well established for climate and sea level, but the scarcity of records for tectonic activity and intrinsic channel changes hamper an interpretation of stratigraphic events. On the large alluvial plains of northern India and central Australia, climatic changes have radically affected sediment and water discharge on timescales as short as 10 to 10 years. Periods of incision and aggradation generate discontinuity-bounded sequences (aggradational–degradational rhythms), and reaches may vary between valley and channel morphology over short periods and distances. Across Asia, changes in monsoonal precipitation have caused near-synchronous incision and aggradation in varied tectonic settings. However, mature interfluve paleosols correlative with valley bases may be local or absent on these large plains, where small interfluve channels, eolian dunes, and lakes aggrade while mainstem valleys incise. Gullied floodplains, adjacent to valleys and tributaries, and terrace deposits are useful indicators of incised systems. Climatic effects appear to be underrepresented in interpretations of ancient alluvial sequences in general. Angular unconformities, channel diversion, and incised and tilted fans near major faults reflect tectonic controls on architecture, and soft-sediment deformation and rare surface ruptures testify to earthquake events. In smaller basins such as the Rio Grande Rift of southwestern USA, alluvial architecture reflects both tectonic and climatic controls. Channel-body amalgamation may not be a good proxy for tectonically induced subsidence, especially for megafans at upland exits where a low degree of amalgamation may reflect rapid accumulation rather than rapid subsidence and accommodation creation. Multistory valleys and extensive multistory and single-story sheets are present in inland and coastal alluvial areas. However, below the Australian continental shelf, seismic profiles identify entrenched channels and channel belts rather than the valley fills that are commonly assumed to form under such conditions. This example shows that sea-level fall and river extensions need not generate deeply incised fluvial valleys in coastal settings.