Algal Reefs Research Articles

Coastal geomorphology: an introduction

Preface and Acknowledgements. List of Figures. List of Tables. Introduction. Coastal Processes. Land and Sea Level Changes. Cliffs and Rocky Shores. Beaches. Spits and Barriers. Coastal Dunes. Intertidal Landforms, Salt Marshes and Mangroves. Estuaries and Lagoons. Deltas. Coral and Algal Reefs. Future Coasts. Appendix: Classification of Coastal Landforms. References. Index.

Composition and community structure of the coralline algal reefs from Atol das Rocas, South Atlantic, Brazil

Coralline-algal frameworks from Atol das Rocas reefs were sampled along windward and leeward transects, and multivariate statistical analysis was used to investigate the composition and community structure of the encrusting community. The following components of the reefs were identified in slabbed and polished reef blocks sampled along each transect: encrusting coralline algae dominated by Porolithon cf. pachydermum, vermetid gastropods, Homotrema rubrum, acervulinid foraminiferans, molluscs, corals, polychaete tubes, clionid sponge borings, polychaete/sipunculid borings, cemented sediments, and growth cavities. Q-mode cluster analysis correctly classified 78% of all windward samples and 69% of all leeward samples, and R-mode separated reef components adapted to high wave energy environments from those adapted to low wave energy. Separate classification and ordination of samples from each transect indicate that seriation breakdown occurs in the windward reef between 25 and 45 m from the reef front. In the leeward reef between 75 and 90 m from the leeward reef front, seriation breakdown was not found to be significant. These results confirm that seriation (zonation) is best developed in the windward reef, as has been documented in coral-dominated reefs.

Paleoclimatic significance of Phanerozoic reefs

A database of pre-Quaternary Phanerozoic reefs is used to test the significance of ancient reefs as paleoclimatic tracers. The compilation of reef paleolatitudes through time and comparison with published paleoclimate curves shows that neither the width of the tropical reef zone nor the total latitudinal range of reefs is correlated with published estimates of paleotemperature. However, reefs trace paleoclimate indirectly: Algal reefs tend to prevail during icehouse climatic intervals, and distinct high-latitude reefs are only developed in cold intervals, whereas during greenhouse episodes, the reef zone usually ended abruptly at a particular latitudinal boundary in the subtropics. Additionally, different biotic reef types tended to be concentrated in different latitudes. The lowest latitudes have usually been occupied by coralline sponge or microbial reefs, coral reefs tended to grow in intermediate latitudes, and bryozoan reefs constantly occupied the highest latitudinal position.

Geochemical evidence for the formation of a large miocene “travertine” mound at a sublacustrine spring in a soda lake (Wallerstein castle rock, nördlinger ries, Germany)

“Travertines” (tufa pinnacles) of the Miocene Riescrater basin have been investigated to test whether carbon, oxygen and strontium isotopes can be used for the recognition of fossil subaquatic spring deposits in high-alkalinity settings. The Ries basin “travertines” have so far been interpreted as a product of subaerial to sublacustrine artesian springs discharging calcareous groundwater into a freshwater or slightly saline lake. However, recent studies on microfacies and fabric development propose a formation at Ca2+-supplying sublacustrine springs of a soda lake. Geochemical analysis of “travertines” of the castle rock Wallerstein, including “sickle-cell” limestones, thrombolites, non-skeletal stromatolites, and speleothems, now support the latter interpretation. High Sr contents surpassing that of the contemporaneously formed dolomitic algal biocherms of the lake shore point to an aragonitic composition of primary precipitates. the δ13C values of diagenetically moderately to weakly altered “travertine” facies types are in the same range of the impact-brecciated Upper Jurassic limestones, thus, are inconsistent with a mixture of soil-derived CO2 and CO3 2− from the Jurassic limestones. In addition, the δ18O values are too high to support a significant contribution of CO3 2− by meteoric waters seeping through marine Jurassic limestones. Instead the δ13C and δ18O values indicate an origin of the CO3 2− from a lake water body characterized by evaporation. This is consistent with a sodium-rich lake water as indicated by high sodium contents of aragonitic algal bioherms of the lake shore. The87Sr/86Sr isotope ratio of the “travertine” mound carbonates are consistent with calculated mixing of spring waters discharging from the crystalline basement and lake water high in dissolved inorganic carbon. This points to an origin of the divalent cations from sublacustrine spring waters. In turn,87Sr/86Sr isotope ratios of green algal reef carbonates of the lake shore are closer to that of the Upper Jurassic carbonates, due to surface run-off from surrounding limestone uplands.

Effect of n00084ent salts eluted from hedoro blocks on growth and survival rate of gametophyte and juvenile Eisenia bicyclis

SUMMARY: The effects of nutrient salts eluted from artificial algal reef blocks made with sea-bottom sediment (hedoro) and cement to the growth and survival rate of the gametophytes and juveniles of Eisenia bicyclis were investigated. When each sample block was submerged in filtered seawater for 28 days, the nutrient salt content from the hedoro increased with time and, in particular, the amounts of NO3−− N and NO2−− N reached a constant level on day 10. We observed that the lower the cement ratio, the higher the concentrations of eluted NH4+− N and PO43−− P. The growth of gametophytes as well as juveniles was highest in the elution of the block with the lowest cement ratio of 30%. In contrast, the survival rate of gametophytes and juveniles decreased linearly with time and the survival rate was higher for the hedoro blocks than for the block made of mortar.

Growth of rigid high‐relief patch reefs, Mid‐Silurian, Gotland, Sweden

Patch reefs up to 35 m thick and generally 100–150 m wide, separated by bedded inter‐reef sediment, dominate the Högklint Formation (Lower Wenlock) of north‐west Gotland. The spacing between adjacent patch reefs is variable, but is commonly 150–350 m. The Högklint is a shallowing sequence, and the patch reefs exhibit a well‐developed vertical succession: (1) Axelsro‐type patch reefs developed in the underlying Visby Formation; (2) halysitid tabulates capped by laminar stromatoporoids; (3) domical and bulbous stromatoporoids and red algae; (4) cyanobacterial–algal reef crest. The patch reefs expand upwards from an initial bioherm phase with a small base to a laterally extensive biostrome phase. This gives them a thumb‐tack appearance. In stage 2 of the bioherm phase, rigid framework development and high reef relief resulted in breakage of angular blocks up to 15 m long, which were incorporated into the reefs or fell into adjacent sediments. Poorly sorted talus haloes (Millingsklint Member) also developed adjacent to stage 2 of the bioherm phase. These include angular blocks and exhibit depositional slopes up to 40° away from the reefs. Stage 3 biostrome development was mainly non‐rigid cluster reef, which shed skeletal debris (Domkyrka Member) but few lithified blocks. Stage 4 biostrome development was a reef crest with open to closed frame structure. Storm breakage and overturning produced large blocks with complex cavity fill sequences including double geopetals. Relief during the bioherm phase, indicated by fallen blocks and talus slopes, was up to at least 15 m; during the biostrome phase, it was up to 10 m.

Differences Among Fish Assemblages Associated with Nearshore Posidonia oceanica Seagrass Beds, Rocky–algal Reefs and Unvegetated Sand Habitats in the Adriatic Sea

Fish assemblages associated with nearshore Posidonia oceanica seagrass beds, rocky–algal reefs and unvegetated sandy substrates were studied at two sampling localities, Otranto (Apulian coast) and S. Domino (Tremiti Islands), located in the Southern and Central Adriatic Seas (Mediterranean Sea), respectively. Data were collected in situ by using non-destructive diver visual census methodology. A higher species richness and fish density were observed over thePosidonia seagrass and, in turn, the rocky–algal reef and unvegetated sand habitats. Planktivorous fish species (Spicara maena, Spicara smaris and Chromis chromis) showing patchy distributions were dominant in terms of abundance at both localities. The two former species did not show any particular habitat preference, while C. chromis was common over both the Posidonia and rocky–algal habitats and was only occasionally recorded over sand.Symphodus ocellatus , Diplodus annularis and Spondyliosoma cantharus were the most common species in theP. oceanica seagrass beds. Symphodus roissali, Symphodus tinca and Diplodus sargus were mainly associated with the rocky–algal reef habitats, while other species, such as Gymnammodytes cicerellus,Lithognathus mormyrus , Gobius geniporus, Mullus barbatus and Uranoscopus scaber, were censused over the unvegetated sand habitats. A cluster analysis performed on the entire fish density data set showed distinct groupings for seagrass, rocky–algal and bare sand fish assemblages, regardless of season and sampling site. Fish assemblages in the more structured (seagrass and rocky–algal reef) habitats were relatively similar to each other but quite different from those of the unstructured bare sands. With regard to small-sized specimens (juvenile stages including recruits), those of S. ocellatus, Symphodus mediterraneus, Serranus cabrilla, D. annularis, S. cantharus andSarpa salpa were mainly censused over P. oceanica beds, while juveniles of C. chromis inhabited predominantly rocky–algal bottoms rich in crevices. Small individuals of Coris julis were censused over both P. oceanica and rocky–algal habitats. No juveniles of any species were observed over bare sand. These results suggest that differences in fish species richness and abundance are primarily related to habitat structure. Different habitat preferences were evidenced for the juveniles and adults of several fish species. The ecological importance and need for protection of such shallow inshore habitats are discussed in relation to their crucial role as nurseries for many fish species, and their overall importance in maintaining littoral fish populations and species diversity is emphasized.

The chemical composition of shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea and their effect on ambient seawater

Abstract Submarine, hydrothermal venting occurs at Tutum Bay in shallow (5–10 m) water along the inner shelf that contains a patchy distribution of coral–algal reefs. Two types of venting are observed. (1) Focused discharge of a clear, two-phase fluid from discrete orifices, 10–15 cm in diameter. Discharge temperatures are between 89 and 98°C and estimated flow rates are as high as 300 to 400 l/min. (2) Dispersed or diffuse discharge that consists of streams of gas bubbles ubiquitous in the area. The composition of the gas is mainly CO 2 (92.6–97.9%) with minor amounts of N 2 (2.2–4.7%), O 2 (0.43–0.73%), CH 4 (0.6–2%) and He (∼0.01–0.02%). Based on their geographic position and chemical composition, the vents have been divided into two groups, A and B. Area B vents have higher K, Rb, Sb, Cs, Tl, and As and lower Ca, Li, Mn, Fe, and Sr concentrations. Their chemical difference is likely caused by subsurface mixing of a CO 2 -rich water with a deep reservoir neutral chloride fluid in varying proportions. A two- or possibly three-step process controls fluid evolution and final chemical composition: (1) phase separation in the deep reservoir beneath Ambitle Island produces a high temperature vapor that rises upward and subsequently reacts with cooler ground water to form a low pH, CO 2 -rich water of approximately 150–160°C. (2) The steep topography causes lateral movement of this CO 2 -rich fluid towards the margin of the hydrothermal system where it mixes with the marginal upflow from the deep reservoir. This produces a dilute chloride water of approximately 165°C. A third step may be the entrainment of minor amounts of ground or seawater during its final ascent. Based on a B–Rb/Cs mixing model, it has been estimated that approximately 10% of the deep reservoir fluid reaches the surface. Compared to seawater, the hydrothermal fluids are depleted in Cl, Br, SO 4 , Na, K, Ca, Mg, and Sr and enriched in HCO 3 , B, Si, Li, Mn, Fe, Rb, Cs, Sb, Tl and As. Although some elements are significantly enriched, they do not have a clear impact on ambient seawater composition because their concentration is buffered by mixing and uptake into secondary minerals. Only the surface water in Tutum Bay carries a clear imprint of the hydrothermal fluids.

Middle Pennsylvanian, late Atokan-early Desmoinesian echinoderms from an intermontane basin, the Central Colorado Trough

Middle Pennsylvanian, Atokan and Desmoinesian, crinoids and echinoids are described from the Belden and Minturn Formations of the Central Colorado Trough. The echinoderms from the Minturn Formation were living in an active intermontane trough setting. They are found in two environments, on the flanks of phylloid algal reefs and in patches on abandoned delta lobes on the gently sloping substrate. Faunas are dominated by cromyocrinids and are similar to stable platform Terrigineous Facies Belt faunas of the Midcontinent.New species described areDicromyocrinus beldenensis, Neoprotencrinus rockensis, andSciadiocrinus wipsorum. Goleocrinus, and the columnal generaBlothronagma, Cyclocaudex, andPlatyplateiumare reported for the first time from the Minturn Formation. The columnal taxaBlothronagma cinctutumandFloricyclus angustimargoare believed to be cromyocrinids.B. cinctutummay be synonomous withSynarmocrinus molasensis, andFloricyclus angustimargomay be synonymous withUlocrinus rockymontanus.

Modern nearshore cold-temperate calcareous sediments in the Troms district, Northern Norway

ABSTRACT During the past 6400 years rocky coastal platforms and outer fjord troughs in the Troms District, northern Norway (70°N) became sites of considerable production and deposition of skeletal carbonate in a cold-temperate environment under Arctic solar radiation conditions. Because of poleward heat transfer by the Norwegian Current along the eastern margin of the North Atlantic Ocean, the cold-temperate marine climate persists beyond the Arctic Circle. In the shallow subtidal zone, carbonate generation is confined to benthic macroalgal ecosystems. Along the wave-exposed margins of the coastal platforms, kelp forests provide habitats for numerous carbonate-secreting organisms such as barnacles, echinoids, and bivalves. Kelp with attached calcareous organisms is involved in many redistributional processes such as rafting and phylloid drift. Reef growth of coralline algae, together with maerl deposits and rhodolith pavements, dominate in wave-sheltered platform areas. This windward-leeward zonation of carbonate production along the rocky coasts reflects specific properties of coralline algae versus kelp to withstand physical disturbances of episodic storms. Carbonate sediment in gullies or outer fjord troughs consists of molluscs, bryozoans, benthic foraminifers, and verrucid barnacles. Terrigenous sediment loads delivered as spring meltwater discharge is trapped in the inner fjord troughs. Hydrodynamic concentration of calcareous skeletons is fostered by a complex topography that offers numerous sediment-trapping areas along the rocky coastline and outer fjord troughs. Physiological adaptation of coralline algae to cope with extreme seasonality in solar radiation is a prerequisite for the formation of coralline algal reefs beyond the Arctic Circle, which have carbonate production rates near those of subtropical coralline algal grounds.

Depositional History of the Smackover Formation, Appleton Field, Escambia County, Alabama: ABSTRACT

Appleton Field is a Smackover field situated above two pre-Mesozoic paleohighs near the updip limit of the Smackover in the Conecuh Embayment of southwestern Alabama, Smackover deposition in Appleton Field was influenced by both pre-Smackover paleotopography and sea level fluctuation. Fourteen lithofacies were identified in the Smackover and the overlying Buckner Anhydrite Member of the Haynesville Formation. These lithofacies were deposited in three depositional stages (early, middle, and late Smackover) that correspond to periods dominated by marine transgression, aggradation, and ultimately progradation. Early Smackover deposition accompanied a rapid sea level rise that inundated the paleohighs. Rapid sedimentation produced a system that was first transgressive and then aggradational in nature. Algal patch reefs developed around the periphery of the paleohighs and onlapped the structures as sea level rose. Middle Smackover deposition was the result of a decrease in the rate of sea level rise. Tidal flat, lagoon, and shoal complexes formed across topographically higher parts of the field, while subwavebase sediments were deposited off structure. Short-term sea level fluctuations produced seven shallowing-upward packages. During Late Smackover deposition long term sea level was relatively stable allowing the Smackover to aggrade and prograde. Upper Smackover deposits are peritidal dominated. Short-term sea level fluctuationsmore » again produced shallowing upward packages capped in crestal locations by exposure surfaces. With continued sedimentation, supratidal sabkhas formed over the crests of the paleohighs and prograded offstructure during early Buckner deposition. Short term fluctuations in sea level produced a series of shallowing upward sabkha cycles. Development of coastal salinas or restriction of the northern Conecuh Embayment led to deposition of subaqueous evaporites in the upper Buckner.« less

Coralline algal maerl frameworks-Islands within the phaeophytic kelp belt

In the subtropical belt highly productive ecosystems are formed by coral reefs in oligotrophic seas. Towards more eutrophic conditions, coral reefs diminish and are subsequently replaced by highly productive kelp forests. In high latitudes framework constructing carbonate production is enhanced by the growth of branching coralline algae which predominantly generate maerl-type deposits. On a global view, these coralline algal ecosystems show an island-like distribution pattern within the phaeophytic kelp belt. Compared to kelp ecosystems, coralline-algaldominated ecosystems have low rates of productivity. Therefore, it is reasonable to seek the pronounced competitive value of the extremely slow-growing corallines. Due to their low annual growth increment, the coralline algae studied are very endangered by abiotic physical disturbances and by overgrowth of rapidly growing filamentous algae or sessile invertebrates. To overcome fouling pressure and storm-triggered physical disturbances, coralline algae thrive well in wave-sheltered headlands or skerry areas and generate characteristic ‘denuded areas’ by intense herbivory. This general distributional pattern is also true for high-boreal to subarctic coralline algal bioherms in northern Norway. Such a complex biological feedback maintains a high potential of self-regulation or self-organization in the algal reef bioherms. The different proponents involved in feedback processes include bacterial colonization, diatom microfouling and selective induction of larval metamorphosis. The negative impact of diatom microfouling and the important role of herbivores are relevant activities in the feedback system on a microscopic scale. Macroscopically, intense herbivory on coralline algae create denuded conditions, which are a widespread phenomenon in coralline algal ecosystems.

Fusiline foraminiferal biostratigraphy and carbonate facies of the Permian Ratburi Limestone, Saraburi, central Thailand

Abstract. A succession of Permian carbonates outcropping along the highway north of Saraburi, central Thailand, has yielded a prolific and diverse fusiline-algal assemblage of Early Permian (Sakmarian) to early Late Permian (Midian) age. Six major units representing dominantly carbonate platform environments are recognised: turbidite and basin slope deposits, a platform margin algal reef, a back reef, an interior platform with patch reefs, a protected lagoon inner platform, and supratidal, dolomitised algal mats. Archaeolithoporella and Tubiphytes form major reef frameworks analogous to those described from the Middle Permian reefs of Trogkofel (southern Austria) and El Capitan (western Texas). The associated dasycladacean floras are assignable to the Eastern Circum-Pacific Realm, whilst the fusiline fauna has Arctic-Tethyan affinities in the Early Permian and Tethyan affinities in the Middle Permian. Eight fusuline assemblage zones are recognised and the Robustoschwagerina-Nagatoella Zone, representing the Sakmarian (early Artinskian) stage, is recorded for the first time from central Thailand. Phylogenetic studies of the fusulines, coupled with an examination of the diagenetic fabrics and field observations, indicate the presence of an unconformity during the late Early Permian-early Middle Permian, which may be correlatable with a worldwide eustatic sea-level fall or may be due to local tectonic movements.

Upper Jurassic Carbonate Sediments and Reefs in the Carpathian Foreland of Poland

Paleogeographic analyses and plate reconstructions indicate that the Upper Jurassic series of the Carpathian foreland accumulated on the Eurasian shelf of the Tethys Ocean. In the Carpathian foreland of southern Poland, marine Upper Jurassic carbonate series, exceeding 1000 m in thickness, are covered by Tertiary Molasse series or by thrust units of the Outer Carpathians. From a lithostratigraphic point of view, six facies units can be recognized in these Oxfordian to Tithonian carbonates, namely calcareous plankton, calcareous sponge, algal-oolitic, calcareous-marly coquina, dolomitic-calcareous, and upper algal series. During the lower part of the Upper Jurassic, deeper neritic conditions prevailed (calcareous plankton series). These gave way to upward shoaling conditions (calcareous sponge series, deeper basin calcareous-marly series) and a final shallow water environment (algal-oolitic and younger series). Thickness changes of the calcareous-marly series and the diachronous character of the facies boundary between the Oxfordian and Kimmeridgian show that the basin deepened to the south. Barrier reefs and associated fore-reef and back-reef facies occur within the algal-oolitic series. These trend northwest-southeast and reach 30-50 m in thickness. The reefs were mainly built by algae. Algal reefs under the Molasse and Carpathian thrust sheets are potential hydrocarbon exploration targets. The paleogeographic position of themore » Jurassic of the Carpathian foreland lies within the Tethys realm, which contains a large part of the world's producible oil reserves; this lends encouragement to this play.« less

Multikilometer-sized reefs built by foraminifera ( Solenomeris) from the early Eocene of the Pyrenean domain (S. France, N. Spain): Palaeoecologic relations with coral reefs

Solenomeris (Foraminifera) reefs of the early Ypresian Pyrenean Gulf were previously mistaken for red algal or incertae sedis algal reefs. Solenomeris has recently been referred to the acervulinid Foraminifera. These Foraminifera buildups are described in detail and their palaeoecology is discussed. The distribution of these reefs within the Languedoc Basin demonstrates their preference for bathymetric highs of external platforms, in common with coral reefs to which they locally succeed during the early Ypresian transgression. All Solenomeris reefs examined are located either at the outer periphery of a lagoon with small Operculina or on the external margin of a carbonate platform characterized by various Miliolida. They extend into deeper waters than the associated coral reefs, even in turbid environments unfavourable for encrusting coralline algae. The reconstruction of the palaeoecology of Solenomeris shows many similarities with corals. However, corals probably require higher light and oxygen levels, especially when present near their northern limit. The global distribution of Solenomeris reefs can not be established until their correct identifications have been made instead of “algal reefs”.

Do predatory fishes affect the structure of an epiphytal amphipod assemblage on a protected algal reef in Hawaii?

On a sheltered Hawaiian algal reef seaweeds are abundant and large herbivorous fishes are absent. Epiphytal amphipods are abundant on their seaweed habitats. Predator exclusion experiments were conducted to determine if predatory fishes affect the abundance, diversity, and size distribution of the epiphytal amphipod community living on plant substrata. Results suggested that amphipod abundance, expected number of species, and size distribution were all independent of predation pressure.

Miocene lacustrine algal reefs—southwestern Snake River Plain, Idaho

The Hot Spring limestone is a shallow-water algal carbonate within a late Tertiary transgressive lacustrine sequence exposed in the southwestern Snake River Plain. This 5 m thick lensoid sequence crops out over an 80 km 2 area that closely approximates original areal extent of nearshore carbonate accumulation. Reefal bodies consist of closely packed algal cylinders, several decimeters in height, each of which includes a dense laminated carbonate wall surrounding porous digitate carbonate that radiates outward and upward from one or more hollow tubes. These coalesce upsection into separate vertical columns several meters in diameter. Moderately well-sorted terrigenous and molluscan debris deposited between columns during growth indicates these structures were resistant to wave erosion and, therefore, were true reefs. Thick rings of littoral carbonate surrounding the upper walls of each column record the final stages of reef development. Structural attributes exhibited by these Miocene carbonate bodies are also common to a number of Tertiary and Quaternary algal buildups reported from other lacustrine settings. Although features within the Hot Spring limestone are complex in gross morphology and structural detail, both columnar reefs and algal cylinders display little variation in size, shape, or internal structure between areas of varying water depth and wave energy, thus reflecting the importance of biological processes as well as physical processes during reef development.

Crustose Coralline Algae from the Flower Garden Banks, Northwestern Gulf of Mexico: Controls on Distribution and Growth Morphology

ABSTRACT The East and West Flower Garden Banks in the northwestern Gulf of Mexico represent the northernmost tropical coral reefs and Caribbean coralline algal flora on the North American continental shelf. Coralline algae are the dominant framework builders and sediment contributors below 50 m on the banks. On the basis of growth form and taxonomic composition, coralline algae on the Flower Garden Banks can be divided into four overlapping depth zones: upper coral reefs (20-30 m), Madracis-algal ridges (30-40 m), algal nodule zone (45-80 m), and deep algal reefs (50-90 m). Corallines on the upper coral reefs are dominated by Hydrolithon, Lithophyllum, and Porolithon. Their growth form as thick, laterally restricted crusts and knobs results from intensive grazing by parrotfish and urchins. Grazing is diminished on the Madracis-algal ridges below the upper reefs, and corallines grow here as laterally extensive plates, binding coral rubble. Dominant corallines on the ridges are Lithophyllum, Paragoniolithon, and Tenarea. Algal nodules (rhodoliths) o the deeper bank slopes are the most widespread algal growth form on the Flower Garden Banks. On the basis of shape, internal structure, and composition, rhodoliths are divided into an upper zone (where they have a spherical shape with concentric laminae) and a lower zone (discoidal shape with irregular laminae). Diminishing current velocity with increasing depth controls nodule morphology. The red alga, Peyssonnelia, and the foraminifer, Gypsina, are two non-coralline components that contribute substantially to nodule growth. Reduced current velocity in the lower nodule zone enables corallines and Peyssonnelia to develop laterally extensive pavements and 1-2 m high algal reefs. Taxonomic composition, high framework porosity, and irregular internal growth morphology f the deep algal reefs distinguish these structures from shallow algal ridges described elsewhere. The 90 m isobath at the Flower Garden Banks approximates the upward limit of a turbid nepheloid layer composed of continually resuspended sediment. Reefal structures within this water layer are dead and are now covered by fine sediment.

A Structurally Controlled Fan-Delta Complex at the Southern Margin of the Peninsular Range Forearc Basin Complex (Baja California): ABSTRACT

A confined trunk fan delta and its structurally controlled tributary fan deltas are extremely well exposed along the southern margin of the Rosario embayment. This fan-delta complex consists of nonmarine to deep marine deposits almost continuously exposed for over 20 km in the down-paleoslope direction. Facies and stratal patterns in the fan-delta complex were controlled by local faulting, climatic variation, and eustasy, resulting in relative sea level fluctuations. Basin bathymetry and drainage patterns were controlled by a series of half-grabens that formed along north-south-trending faults that lay along the northern margin of an east-west-trending depression. Breccias were initially shed into the north-south-trending half-grabens; axial drainage systems were later established within the grabens, making up the tributary fan deltas. These tributary fan deltas fed a voluminous trunk fan delta confined to the east-west-trending depression. The trunk and tributary fan delta deposits show two major progradational to retrogradational cycles that record relative sea level fluctuations. Progradation in the shallow-marine environment is represented by conglomerate channels cut into ripple-laminated or bioturbated siltstone and HCS sandstones, overlain by conglomerate mouth bar deposits interstratified with nearshore sandstone deposits. Retrogradation in the shallow marine environment is recorded by either a vertical clastic facies transition or amore » clastic-carbonate facies transition. The clastic facies transition consists of nearshore sandstone deposits overlain by offshore bioturbated siltstones. The clastic-carbonate facies transition consists of the development of red algal patch reefs and rhodoliths on top of fan-delta conglomeratic lobes, mudstone and sandstone bank channel margins, or paleobasement highs. The clastic-carbonate facies transition reflects low sediment supply controlled by climatic conditions.« less

Carbonate Reservoirs Deposited During Sea Level Lowstands, Permian Basin: Occurrence, Geometry, Facies, and Origin of Porosity of In-Situ Buildups: ABSTRACT

Carbonate reservoirs composed of in-situ reefs and associated facies, mostly deposited basinward of shallow platforms during sea level lowstands, are common in Pennsylvanian rocks in the Permian basin. Specific examples include some Atokan and Strawn fields in the Delaware basin, and Canyon-Cisco fields in the Midland basin. Such reservoirs are conspicuous by their absence in Permian rocks, where lowstand facies are instead siliciclastics and detrital carbonates. In-situ lowstand carbonate reservoirs are represented by phylloid algal reefs (Atokan, Canyon-Cisco), crinoid banks locally with some Chaetetes and phylloid algal reefs (Strawn), and bryozoan-algal boundstone reefs (Canyon-Cisco). These facies are associated with bioclastic and, locally, oolitic sandstones. Lowstand reservoirs are both underlain and overlain by deep-water facies, and field geometries range from equidimensional to slightly elongate, generally parallel to platform margins. Porosity in many fields is due to extensive leaching and karsting accompanying subaerial or shallow subsurface meteoric exposure. Accordingly, porosity predictions are best based on knowledge of reconstructed sea level curves. However, many reservoir pore systems have resulted from deep-burial dissolution accompanying chemical compaction and the migration of fluids out of the basin. Porosity predictions in such cases must rely on knowledge of diagenesis and aspects of basin hydrodynamics.