Siliciclastic Tidal Flats Research Articles

Challenges to carbonate-evaporite peritidal facies models and cycles: Insights from Lower Cretaceous stromatolite-bearing deposits (Oncala Group, N Spain)

Peritidal carbonate-evaporite successions, since they are developed in the transition between continental and marine realms, provide essential keys for palaeobathymetric and palaeoclimatic interpretations. As a result, several facies models have been proposed to assist on the interpretation of ancient tidal flat deposits, and peritidal successions have been extensively used for cyclicity analyses. In this study, well-exposed, Lower Cretaceous peritidal deposits (Oncala Group, Cameros Basin, N Spain) are analysed and compared with the most commonly-used present-day analogues (from Shark Bay, the Arabian Gulf and the Bahamas) and with ancient peritidal successions, providing their palaeoenvironmental and palaeoclimatic interpretation, assessing the usefulness and limitations of the facies models, and evaluating the suitability of these deposits for analysis of decimetre to metre-scale cycles.The studied peritidal deposits consist of thinly-bedded to laminated dolostones, dolomitic stromatolites, stromatolite breccias, flat-pebble and edgewise breccias, and calcite and quartz pseudomorphs after anhydrite nodules. Abundant resemblances of the peritidal deposits of the Oncala Group with those of Shark Bay, including that they are largely composed of microbialites and intraclasts, makes the peritidal deposits of the Oncala Group one of the best fossil analogues of this present-day setting. However, the presence of anhydrite nodules indicates pervasive evaporite precipitation in the supratidal zone, which is a feature that does not occur in supratidal flats of Shark Bay, but is characteristic of arid sabkhas of the Arabian Gulf. Nevertheless, the fact that carbonate-evaporite tidal flats of the Oncala Group were laterally related with siliciclastic tidal flats with large freshwater input and broadly inhabited by dinosaurs, suggests that anhydrites precipitated under less arid climates than those of the Arabian Gulf nowadays, pointing to semiarid climatic conditions during deposition. Moreover, the fact that peritidal deposits with anhydrite nodules were exclusively formed in a low-subsidence area of the Cameros Basin suggests that the rate of accommodation space creation also played an important role in their development. Regarding the comparison with other fossil peritidal sediments, the studied deposits show more abundant similarities with Proterozoic and Cambrian successions, composed mainly of stromatolites, microbial laminites, and intraclasts, than with other Mesozoic peritidal deposits, in which bioclasts and burrowing are usually more abundant. This highlights the difficulties for assigning specific features to certain geological ages. Finally, peritidal facies of the Oncala Group may change laterally and vertically to any other facies, showing a patchy lateral distribution of facies and an unsystematic vertical stacking pattern. The sedimentary features of the stromatolite, breccia and thinly-bedded to laminated dolostone facies do not allow their assignment to a unique tidal zone. Moreover, sedimentary features indicative of subaerial exposure, such as anhydrite nodules formed in the capillary zone, occur within any of the carbonate facies and show limited lateral extent. This results in a succession that cannot be clearly subdivided into subsequent shallowing-upward cycles not even by using erosive surfaces or the anhydrite nodule layers as marker horizons of the upper part of the cycles, because their limited lateral extent prevents reliable correlations. Similar composite lateral and vertical facies relationships have been documented both in the present-day analogues and in ancient successions, which suggests that this kind of facies relationships may be common in peritidal successions and highlights the caution that must be taken when trying to perform cyclicity analysis on them.

Coastal wetlands as markers of transgression in proximal extensional systems (Berriasian, W Cameros Basin, Spain)

The early stages of intraplate extensional systems commonly are recorded by deposition of continental sediments. In this context, given appropriate tectonics and eustasy, transgressions can be well recorded in the areas of the basins located close to the sea, but they may be difficult to recognize in the innermost landwards areas of the system. This situation occurs in the innermost Upper Jurassic-Early Cretaceous Cameros Basin, part of the Iberian Extensional System (N. Spain), where a Berriasian transgression is recorded. The Berriasian succession in this area consists of siliciclastic deposits (sandstone and mudstone) of the Salcedal Formation and of carbonate and mixed carbonate-fine siliciclastic deposits (limestone and marl) of the San Marcos Formation. The sedimentological analysis of this depositional succession indicates that a Berriasian carbonate coastal wetland system occupied that sector of the Cameros Basin during deposition of the San Marcos Formation. This carbonate coastal wetland system consisted of shallow and quiet water bodies including some with marine influence others with no to very little marine influence, and palustrine areas. A semiarid climate characterized by the seasonal alternation of short wet and long dry periods caused water bodies of the system to undergo episodic desiccation and subaerial exposure. Moreover, this complex mosaic of sub-environments was connected laterally with a distal zone of a distributive fluvial system that was rimmed by siliciclastic tidal flats during phases of greater marine influence. The paleogeographic arrangement of this coastal wetland depositional system indicates that the marine influence came from the Basque-Cantabrian Basin to the north. During the period of Berriasian maximum marine influence, accommodation linked to the eustatic rise added to accommodation generated by tectonic subsidence from the extensional reactivation of late Variscan strike-slip faults. All these factors favored marine incursion into the west Cameros Basin from the Basque-Cantabrian Basin to the north. The example of the Berriasian transgression recorded in the W Cameros Basin by establishment of coastal wetland systems matches the interpretations of previous studies in neighboring areas. In those areas, complex coastal systems record transgressions in the innermost parts of the intraplate extensional basins of the Iberian Plate. This observation suggests that this paleogeographic and sedimentological arrangement may be common in the innermost parts of intraplate extensional basins during transgressive episodes throughout the geological record.

Sequence development of a latest Devonian–Tournaisian distally-steepened mixed carbonate–siliciclastic ramp, Canning Basin, Australia

The sequence development and evolution of latest Devonian–earliest Carboniferous Fairfield Group in the Canning Basin have been established through integration of detailed sedimentological analysis of core, petrophysical data, existing biostratigraphic data and new seismic interpretations. The Fairfield Group on the Lennard Shelf was deposited on a mixed carbonate–siliciclastic distally-steepened ramp with a broad inner ramp, narrow mid ramp and steepened outer ramp. The majority of facies associations (FA1–FA8) were formed in intertidal–shallow subtidal conditions in proximal to distal inner ramp including siliciclastic tidal flats (FA1), carbonate intertidal flats (FA2), tidal flats and channels (FA3), lagoons (FA4–FA5), and shallow subtidal (FA6), backshoal (FA7) and fore-shoal areas (FA8). Bioclastic muddy sandstone (FA9) and bioclastic mudstone (FA10) are the dominant mid-ramp facies. Recognition of turbiditic facies of middle to lower slope of the outer ramp (FA11–FA13) led to the identification of a distally-steepened ramp. Antecedent topography exerted a significant control on platform morphology and the development of the widespread inner-ramp facies on the Lennard Shelf.A sequence-stratigraphic analysis reveals that the Fairfield Group ramp deposits consists of four third-order sequences (S1–S4) that were largely deposited during sea-level highstands (HST) characterized by progradational trends and dominant shallow subtidal inner-ramp facies associations. Transgressive systems tracts (TST) are well developed in S1 and S3 and have a retrogradational facies pattern with dominant deep subtidal mid-outer ramp facies associations. Lowstand systems tracts, characterized by lowstand wedges and turbiditic facies, are identified in the lower parts of S2 and S3. Coarse and fine-grained siliciclastic facies are mixed with carbonate facies as a result of coeval deposition on the inner and mid ramp, and reciprocal deposition on the outer ramp. A temporal variation in the types of carbonate grains and the amount of siliciclastic facies, from the mixed peloidal-skeletal carbonate–siliciclastic ramp in S1 and S2, to the mixed skeletal carbonate–siliciclastic ramp in S3 and a siliciclastic-dominated ramp in S4, is interpreted as the change from a greenhouse to an icehouse state.

New insights on stratigraphy and sedimentology of the Oncala Group (eastern Cameros Basin): implications for the paleogeographic reconstruction of Iberia at Berriasian times

The Berriasian Oncala Group of the eastern Cameros Basin is a key element for the paleogeographic reconstruction of NE Iberia during this period of time, due to its intermediate position between the Basque-Cantabrian, southern Iberian, and Pyrenean Basins. The Oncala Group, which comprises up to 2500 m of sediments, consists of siliciclastic deposits in western areas of the basin, which pass gradually eastwards and upwards to carbonate-evaporitic deposits. The siliciclastic deposits consist of an alternation of laterally extensive laminae of siliciclastic mudstone and sandstone, and meander loop bodies. The carbonate deposits are made up of an alternation of laminae of carbonate mudstone and pseudomorphs after gypsum, interbedded with shales and carbonate breccias. These deposits had been traditionally interpreted as formed in fluvial and alluvial systems, and laterally related lakes, but recent and new sedimentological data discussed in this paper suggest that they were deposited in broad, siliciclastic tidal flats, laterally associated with a fluvial system to the west, and with shallow, coastal, carbonate-sulphate water bodies to the east. The development of these coastal environments in the Oncala Group indicates that a wide, shallow, coastal embayment was developed in the eastern Cameros Basin during the Berriasian. This new paleoenvironmental interpretation makes necessary a revision of the paleogeographic reconstruction of this area of Iberia during the Berriasian because it implies that the connection of the Cameros Basin with marine areas was much more important than previously considered. The presence of very similar facies associations in the Basque-Cantabrian Basin, which include siliciclastic tidal and lagoonal deposits in the western areas, and evaporites and carbonates of restricted marine environments in the eastern areas, the similar sulphur isotope compositions of the sulphates of the Basque-Cantabrian and Cameros Basins, and the similar vertical evolution of the Berriasian deposits of both basins, suggest that the Cameros Basin was connected with marine and transitional areas of the Basque-Cantabrian Basin during Berriasian times. Nevertheless, a connection with the southern Iberian Basins, which also contain siliciclastic and carbonate tidal deposits, cannot be excluded.

Characterization of Microbial Mats from a Siliciclastic Tidal Flat (Bahía Blanca Estuary, Argentina)

Biofilms and microbial mats cover the tidal flats of the central zone of the Bahía Blanca estuary (Argentina), creating extensive layers. The objective of this study was to characterize the microphytobenthic communities in these biofilms and mats from sediment cores taken in March, June, September and December 2010. Microorganisms were identified and enumerated by microscopy, and their biomass (chlorophyll a, biovolume) quantified at two different stations in the lower supratidal zone, located ∼210 m apart from each other (namely S1 and S2). Additionally, the colloidal carbohydrates produced by these microbial communities were quantified, together with physical parameters such as temperature, granulometry, moisture and organic matter content of the sediment layers that comprise a typical epibenthic mat. On the other hand, changes in biomass and colloidal carbohydrate content were studied through a half-tidal cycle (7 h). There were significant seasonal differences in microphytobenthic biovolume (P < 0.001) with a considerably lower biomass in summer, but no significant differences in microalgal biovolume between stations (P = 0.454). Cyanobacterial biomass (largely composed of the filamentous Microcoleus chthonoplastes) was dominant on all dates at both stations, followed by pennate diatoms. Chlorophyll a and colloidal carbohydrate contents in sediment presented a similar pattern to that of microalgal biovolume; with a 5-fold variation in chlorophyll a for S1 between consecutive sampling events on September and December. There were significant differences between sampling dates in colloidal carbohydrates (P < 0.001) with the lowest values recorded during fall and winter; conversely there were no significant differences between stations (P = 0.324). Silt was the dominant sediment fraction at S1 while sand dominated throughout the uppermost 20 mm at S2. Chlorophyll a contents did not show significant differences throughout a half-tidal cycle, likely the product of vertical migration along the section sampled. Conversely, the content of colloidal carbohydrates varied 5-fold, showing a significant (P < 0.001) and steady increase with time of exposure to air and pointing to the rapid metabolic rates of the community. In conclusion, the microphytobenthic community of the Bahía Blanca estuary presented marked seasonality in its biological parameters and overall physiognomy, also showing elevated metabolic rates when subject to tidal fluctuations.

Biostabilization of sediments by microbial mats in a temperate siliciclastic tidal flat, Bahia Blanca estuary (Argentina)

Extensive microbial mats have been found in the siliciclastic tidal flats of the temperate Bahía Blanca estuary in the Atlantic coast of Buenos Aires province, Argentina. Several microbially induced sedimentary structures (MISS) occur especially in the upper intertidal and lower supratidal flats, among which the most conspicuous are erosional pockets, gas domes, microbial mat chips, and polygonal oscillation cracks. Biostabilization processes by epibenthic and endobenthic mats are also analyzed. Endobenthic mats occur in the upper intertidal area stabilizing ripples that despite the occurrence of severe storms persist in a fixed position for at least 2 months. Epibenthic mats occurring in the lower supratidal area also protect the substrate forming a thick microbial cover through the studied period. This thick cover was only affected by a strong storm that formed areas with erosional pockets and mat pieces. Nevertheless, the loose sediment within the erosional pockets was quickly colonized by microorganisms that developed a thin biofilm layer after a week. Changes in sediment accumulation were also recorded all over the upper tidal flat during a year, showing an important increase due to bioturbation activities of crabs. This situation also affected microbial mat growth, which evolved from a thin microbial biofilm into a thick, stratified microbial mat community in almost 2 years, mainly in the lower supratidal areas. The results of this study not only help us to better characterize the complex interactions between the microorganisms forming microbial mats, the tidal-flat sediment and the physical parameters that control this setting, but also have important implications for the understanding of analogous fossil sedimentary successions.

Kinneyia-Type Wrinkle Structures--Critical Review And Model Of Formation

Kinneyia structures are among the most typical wrinkle structures observed on ancient siliciclastic sediment surfaces since the Archean. Recently, Kinneyia structures have been grouped together with other microbially induced, crinkly decorations on ancient bedding surfaces as wrinkle structures. They are mainly preserved on upper surfaces of ancient siliciclastic-event deposits and are characterized by millimeter-scale, winding, flat-topped crests separated by equally sized round-bottomed troughs and pits. The structure resembles small-scale interference ripples including crest-dominated linear and pit-dominated honeycomb-like patterns. The steep slopes of the crests, however, exclude their formation at the air or water-sediment interface. Thin sections across Kinneyia structures reveal their formation beneath microbial mats. They formed at an early stage and do not arise from loading and other processes related to burial. Based on the close relationship to event deposits, a genetic model considering the specific hydraulic conditions on siliciclastic tidal flats after storms or floods is proposed. Numerical calculations show that, after microbial mats have been reestablished on the new sediment surface and groundwater is still flowing downslope, the top portion of the sediment confined beneath mats may be liquefied, thus allowing grains to move with the groundwater. Oscillations of groundwater flow owing to periodic reversals of flow direction at rising tides, and a tidal signal of oscillating pore pressure may enhance formation of ripplelike structures along the boundary with the overlying mat. The model applies primarily to Kinneyia structures presumed to be formed beneath cohesive microbial mats in peritidal zones.

A new window into Early Archean life: Microbial mats in Earth's oldest siliciclastic tidal deposits (3.2 Ga Moodies Group, South Africa)

Newly discovered sedimentary structures produced by ancient microbial mats in Early Archean sandstones of the 3.2 Ga Moodies Group, South Africa, differ fundamentally in appearance and genesis from Early Archean stromatolites and bacterial cell fossils preserved in chert. Wrinkle structures, desiccation cracks, and roll-up structures record the previous existence of microbial mats that effectively stabilized sediment on the earliest known siliciclastic tidal flats. In thin-section, the sedimentary structures reveal carpet-like, laminated fabrics characteristic of microbial mats. Negative d 13 C isotope ratios of 220.1 to 221.5 6 0.2‰ are consistent with a biological origin for the carbon preserved in laminae. The biogenicity of the sedimentary structures in the Moodies Group is substantiated by comparative studies on identical mat-related features from similar tidal habitats throughout Earth history, including the present day. This study suggests that siliciclastic tidal-flat settings have been the habitat of thriving microbial ecosystems for at least 3.2 billion years. Independently of controversial silicified microfossils and stromatolites, the newly detected microbially induced sedimentary structures in sandstone support the presence of bacterial life in the Early Archean.

Discrimination of Local and Global Effects on Upper Mississippian Stratigraphy, Illinois Basin, U.S.A.

ABSTRACT Interpretive cross sections based on detailed descriptions of 33 outcrops and cores are used to better understand the relative effects of tectonics, eustasy, tides, and climate on Upper Mississippian (middle Chesterian) stratigraphy in the tectonically active, tide-dominated Illinois basin. The cross sections show that five mixed carbonate-siliciclastic, high-frequency sequences in the Bethel through Glen Dean formations can be correlated around the outcrop belt through areas with very different subsidence histories. The sequence boundaries are marked by paleosols and incised valleys and can be correlated basin-wide within a framework of distinctive marker beds. Because of its updip position, lowstand systems tracts are not preserved in the Illinois basin. The transgressive systems tracts generally consist of one or two parasequences that are typically composed of tidally influenced quartz sandstone filling incised valleys at the base overlain by open-marine skeletal limestone, shallow-marine shale, and heterolithic siliciclastic tidal-flat deposits. The maximum flooding surface (MFS) for the sequences is picked at the base of the deepest water limestone facies. Highstand systems tracts are composed of 1 to 6 regressive parasequences that consist of basal offshore skeletal limestone capped by laterally extensive shale and heterolithic siliciclastic tidal-flat facies. The basin-wide extent of the sequence boundaries and maximum flooding surfaces across tectonic highs and lows suggests that the sequences were produced by eustatic sea-level changes rather than local tectonics or autogenic processes. The sequences were likely produced by moderate- to high-amplitude (30-100 m) fourth-order ( 400 ky) glacio-eustatic sea-level changes driven by the transition from the greenhouse conditions of the Early Mississippian to the icehouse conditions of the late Paleozoic. The lateral extent and frequency of component parasequences suggests that they were likely produced by fifth-order sea-level changes (10-100 ky). The sequences may be bundled into third-order composite sequences, but the third-order signal is obscured by the magnitude of the fourth-order sea-level changes a feature typical of ice-house stratigraphies. The sequences can be used as time slices to identify spatial and temporal variations in differential subsidence between the Cincinnati Arch and the more rapidly subsiding Basin Interior. Episodes of high and low differential subsidence occurred every two to three sequences. These subsidence variations had a major impact on lithofacies distribution and onlap and offlap geometries in sequences and parasequences. The occurrence of some widespread seismically disturbed beds suggests that active faulting occurred during deposition. Normal faulting appears to have occurred during periods of high differential subsidence and reverse faulting during periods of low differential subsidence. Differential subsidence and related normal and reverse faulting may have occurred in response to phases of thrust loading and quiescence in the Appalachian orogenic belt to the east. Even in this tectonically active setting, however, it is the eustatic signal that generates basin-wide, mappable stratigraphic sequences.

Fluvial-shallow marine-glaciofluvial depositional environments of the Ordovician System in Jordan

The Ordovician System, cropping out in southern and west-central Jordan, consists entirely of a 750 m thick clastic sequence that can be subdivided into six formations. The lower Disi Formation starts conformably above the Late Cambrian Umm Ishrin Formation. According to Cruziana furcifera occurring in the upper third of the Disi Formation, an Early Ordovician age is confirmed. The Disi Formation, consisting mainly of downstream accretion (DA) fluvial architectural element, was deposited in a proximal braidplain flowing N–NE from the southerly-located Arabian–Nubian Shield towards the Tethys Seaway. The braidplain depositional environment evolved into a braidplain-dominated delta through the middle and upper parts of the Disi Formation and the lower part of the overlying Um Saham Formation. The delta was replaced by siliciclastic tidal flats, that in turn evolved into an upper to lower shoreface environment through the upper part of the Um Saham Formation. The depositional environment attained the maximum bathymetric depth during the deposition of the lower and central parts of the third unit, the Hiswa Formation, where offshore graptolite-rich mudstone with intercalated hummocky cross-stratified tempestites were deposited. The Tethys Seaway regressed back through the upper part of the Hiswa Formation promoting a resumption of the lower–upper shoreface sedimentation. Oscillation between the lower to upper shoreface depositional environment characterized the entire fourth unit, the Dubaydib Formation, as well as the Tubeiylliat Sandstone Member of the fifth unit, the Mudawwara Formation. The depositional history of the Ordovician sequence was terminated by a glaciofluvial regime that finally was gradually replaced by a shoreface depositional environment throughout the last unit, the Ammar Formation.

A microscopic sedimentary succession of graded sand and microbial mats in modern siliciclastic tidal flats

Microscopic studies of thin sections from modern siliciclastic tidal flat sediments in the southern North Sea demonstrate the significant role of microbial mats in the buildup of sedimentary sequences. This is documented by a unit only a few millimetres thick. It starts at the base with a fine- to medium-grained quartz sand often containing secondary pores (‘fenestrae type’) merging gradually into finer sediments. The lower siliciclastic part is superposed by an upper organically dominated layer built by microbial mats. Within the organic material, single quartz grains without any contact to each other are oriented with their long axes parallel to the bedding planes. Each siliciclastic part in the lower section of the unit indicates the initial deposition of coarser grains in a stronger flow regime followed by gradually decreasing flow velocities. Each microbial mat in the upper part essentially represents a period of low sedimentation rate. During its growth, grains still settle down onto the mat and become bound in the organic matrix. The orientation of these grains with their long axes parallel to the bedding plane points to an energetically suitable position to gravity achieved by the friction reduction of the soft organic matter. Repeated depositional events followed by low-rate deposition cause the buildup of various units. There is no visible reworking of the former surfaces, since the microbial mats prevent erosion during periods of increased flow. The buildup is characteristic of siliciclastic sediments repeatedly occupied, stabilized, and fixed by microbial films or mats.

El Jurásico superior marino en el Sector Demanda-Cameros (La Rioja-Soria)

Este trabajo aborda los aspectos litoestratigraficos y paleogeograficos del Malm marino de la Cuenca de Los Cameros, depositado en un estrecho surco de direccion NW-SE limitado entre los Macizos del Ebro e Iberico. Ademas ha sido interpretado como dos secuencias deposicionales en el sentido de Mitchum ci al (1977). Estas dos secuencias de deposito se corresponden con las dos unidades litoestratigraficas con rango de Eormacion que se han diferenciado: Calizas negras de Aldealpozo, de edad Oxfordiense medio y superior, y Calizas con corales de Torrecilla en Cameros, de edad Kimmeridgiense inferior. Se describe de forma detallada cada una de ellas, analizando sus variaciones laterales de facies y su interpretacion sedimentologica. La Formacion inferior se interpreta como depositada en una rampa carbonatica, basculada hacia el NW, donde hay facies mas profundas y con situaciones muy someras, con continuas emersiones, en el SE, donde se realizaria su union con el Surco Iberico. En los margenes SW y NE la influencia siliciclastica fue grande. La unidad superior constituyo un episodio con abundancia de bioconstrucciones. La plataforma muestra dos sectores con diferentes tipos de margenes bioconstruidos: Arrecifes franja, con predominio de la acreccion frontal, en el NW, y arrecifes barrera, con predominio de la acreccion vertical en S y SE. Estos margenes separan un surco sedimentario mas profundo, al NE, de la plataforma somera situada al SW. El otro margen del estrecho no muestra tendencia a la bioconstruccion, por el contrario, los aportes siliciclasticos son abundantes. En el trabajo se analiza ademas el caracter de las discontinuidades que limitan las secuencias deposicionales. La inferior constituye un nivel de removilizacion, de parada de la sedimentacion y me-elaboracion. Incluiria al Calloviense superior p.p.-Oxfordiense inferior. La superior constituye una discordancia cartografica, asi como un cambio paleogeografico y paleoambiental radical en la cuenca. La discontinuidad intermedia es de menor rango, estando generalmente caracterizada por senales de emersion (paleokarstificaciones) y de forma mas local por la existencia de un hardground.

Cyclic Sedimentation in the Mixed Siliciclastic-Carbonate Abo-Hueco Transitional Zone (Lower Permian), Southwestern New Mexico

ABSTRACT In southwestern New Mexico, Lower Permian (Wolfcampian) rocks grade southward from nonmarine siliciclastics (Abo and Earp Formations) to marine carbonates (Hueco and Horquilla Formations). A transitional zone between siliciclastic and carbonate facies trends east-northeast across southwestern New Mexico and consists of 64 to 186 m of cyclically interbedded siliciclastic and carbonate rocks, which were deposited in tidal-flat and shallow-marine environments. Shallow-marine facies include fossiliferous limestone and olive-gray shale. Tidal-flat facies consist of 1) ripple-laminated sandstone, which was deposited on intertidal sandflats near mean low tide, 2) mixed sandstone-shale, which was deposited on an intertidal flat shoreward of the ripple-laminated sandstone facies, and 3) nodula shale, which is characterized by pedogenic calcareous nodules and was deposited in a supratidal setting. The intertidal facies are truncated by or grade laterally into rare channel sandstones, which represent tidal-creek or estuarine facies. In addition to siliciclastic tidal-flat deposits, a few beds of laminated carbonate also were deposited in the intertidal zone. Vertical sequence analysis aids in delineating three types of depositional cycles. Asymmetrical cycles display the vertical sequence: basal fossiliferous limestone--olive-gray shale--ripple-laminated sandstone--mixed sandstone-shale--nodular shale, and record shoreline progradation. The asymmetrical cycle is always overlain by fossiliferous limestone, which indicates a major transgression that inhibited siliciclastic sedimentation. A common symmetrical cycle consists of fossiliferous limestone--olive-gray shale--ripple-laminated sandstone--olive-gray shale--fossiliferous limestone, and indicates systematic seaward and landward migration of facies zones associated with small-scale sea-level changes. A less common symmetrical cycle involves laminated carbonate--fossiliferous limestone-- aminated carbonate. Cyclic sedimentation in Abo-Hueco transitional strata is most likely the result of glacial eustatic sea-level fluctuations.

Anatomy of a Modern Marine Sabkha in a Rift Valley Setting, Northwest Gulf of California, Baja California, Mexico: ABSTRACT

An extensive sabkha, 100 by 20 km (62 by 12 mi), caps Holocene siliciclastic tidal flats along the coast of Baja California in the northwest Gulf of California structural trough. This sabkha, bounded on the west by alluvial fans, is a complex of sand flat, saline mud flat, gypsum-halite pan, and supratidal mud flat subenvironments. The sand flat, transitional between the fans and the sabkha, consists of horizontally to wavy laminated sand washed onto the flats by sheetfloods from the fans under upper-flow-regime conditions. The sand flat passes into saline mud flats characterized by massive mud crowded with discoidal gypsum crystals grown within the sediment, destroying the layering. This intrasediment growth of gypsum is driven by evaporative pumping of brine from a shal ow subsurface brine body. A gypsum pan occupies a wide, shallow depression within the saline mud flat. After storm flooding (either marine or meteoric) the pan becomes a temporary saline lake with an algal mat overgrowing a siliciclastic storm layer. Precipitation of gypsum needles from the open lake brine and bottom growth of gypsum prisms produce a gypsum layer that mimics the algal mat topography. Evaporation of the lake leads to (1) vadose diagenetic growth of gypsum that distorts and polygonally disrupts the gypsum layers (2) deposition of a surface halite layer in the center of the pan. These layered pan deposits are the evaporites of the sabkha. Seaward of the saline mud flats are supratidal mud flats underlain by millimeter-thick laminites with deep prism cracks, sheet cracks, an scattered gypsum discoid crystals. Pervasive mud cracking during long periods of nondeposition completely disrupts layering, creating a massive arid soil. A beach ridge separates this sabkha complex from the burrowed and ripple cross-laminated intertidal sediments. End_of_Article - Last_Page 460------------