Shallow-water Biota Research Articles

Formation mechanism of high-quality Palaeogene “compositional mixing” reservoirs in lacustrine rift basins: A case from the offshore Bohai Bay Basin, China

Mixed siliciclastic-carbonate reservoirs are currently considered a unique type of reservoir for hydrocarbon exploration. However, to date, research on “compositional mixing”, an important type of mixed sediment, remains relatively scarce. In recent years, significant breakthroughs in hydrocarbon exploration have been achieved in compositional mixing deposits, which are found within a lacustrine setting of the offshore Bohai Bay Basin in China. Despite the lack of systematic studies on their formation mechanisms, these discoveries are insightful case for understanding compositional mixing reservoirs (CMRs). In this study, an interdisciplinary approach that integrates petrographic analysis, cathodoluminescence analysis, reservoir characterization, and high-pressure mercury injection tests from drilled wells, cores, and thin sections, is employed to elucidate the characteristics of CMRs and the factors controlling reservoir modifications. Our findings reveal that compositional mixing deposits predominantly occurred with fan delta systems adjacent to uplifts. Two distinct reservoir types, namely, mixed slide deposits and mixed sandy debris flow deposits, were formed by coexistence of fan delta front siliciclastics with shallow-water biota (carbonate components). A comparative analysis of reservoir properties suggests that CMRs are of equivalent or superior quality to fan delta systems, challenging existing paradigms. This superiority is attributed to two primary factors: (1) the unique internal texture of compositional variations significantly influences subsequent diagenesis, leading to the extensive development of selective dissolution porosity and a reduction of clay cementation; and (2) distinctive early diagenetic events, including micrite envelopes and early dolomitization, markedly enhance the reservoirs’ resistance to compaction. In contrast, fan delta deposits lacking carbonate components do not undergo selective dissolution and their siliciclastic composition facilitates clay cementation. This study offers novel insights and a model for understanding the formation mechanism of CMRs, providing implications for future global petroleum exploration in mixed sediment reservoirs.

Short-term middle Eocene (Bartonian) paleoenvironmental changes in the sedimentary succession of Olivetta San Michele (NW Italy): the response of shallow-water biota to climate in NW Tethys

This study focuses on the paleontological content of the middle Eocene (Bartonian) carbonate–siliciclastic sediments of the Capo Mortola Calcarenite Formation from Olivetta San Michele (Liguria, Italy). Along the succession, there are significant paleoecological changes triggered by the variation in neritic input as a consequence of tectonic and climatic instability. Among microfossils, nummulitids prevail, followed by orthophragmines, smaller benthic, and planktonic foraminifera, whereas mollusks and ichnofossils are the most abundant macrofossils. The sudden changes in the benthic communities due to the progressive increase in fluvial input are recorded throughout the sedimentary succession. An increase in water turbidity caused stressful conditions for autotrophic taxa, reducing their size and abundance. In contrast, filter feeders became dominant, suggesting an increase in dissolved and suspended nutrients. Ichnological analysis shows environmental fluctuations controlled by the transport of neritic material offshore, thus confirming the general deepening trend of the studied succession. In the upper part of the succession, we recorded an alternation between gravity flows and marly sediments that are interpreted as short-term alternations between low and intense precipitations. The gravity flows yield taxa such as larger benthic foraminifera (LBF), smaller benthic and planktonic foraminifera, mollusks, and corals. In turn, marls display only a few LBF and abundant smaller benthic and planktonic foraminifera. In these intervals, the increase in planktonic foraminifera also suggests a deepening of the carbonate ramp coinciding with a reduction of light that did not favor the development of LBF. These changes are probably related to the climatic dynamics that occurred in the Bartonian in the western Tethys.

Estimating Water Depths of Upper Cretaceous Pilot Knob Volcanic-Related Strata: The McKown and Pflugerville Formations and Pyroclastic Ash at the Lower Falls Section, McKinney Falls State Park, Austin, Texas

The McKown and Pflugerville formations and pyroclastic ash beds at McKinney Falls State Park in Austin, Texas, are closely associated with the Lower Campanian Pilot Knob volcano. This volcano is one of over 400 known Late Cretaceous volcanoes in the Balcones Igneous Province. The center of the Pilot Knob volcano is 1.5 mi (2.4 km) southeast of the McKinney Falls State Park. The McKown strata at the Lower McKinney Falls within the park have been interpreted as a beach complex deposited on the shallow-water north flank of the Pilot Knob volcano. Previous researchers based this interpretation on the presence of a shallow-water macrobiota and the coarse-grain texture of the limestones. However, a new investigation of the Lower McKinney Falls outcrops has developed evidence that the McKown limestones were deposited in deeper water (150 to 300 ft [50 to 100 m]) on the flank of the Pilot Knob volcano by gravity-flow processes that formed carbonate debrites and hyperconcentrated-density-flow deposits. The new evidence comes from micropetrography data that demonstrates the matrix of the packstones is dominated by coccolith hash and that there are calcispheres and planktic foraminifers in both the packstones and grainstones. The composition of the McKown limestone is a mixture of shallow- and deep-water biotas, indicating the shallow-water biota were resedimented into a deeper-water setting. Also, McKown bedding relationships with the pyroclastic ash beds below indicates deposition in a low-energy setting. The pyroclastic volcanic beds contain coccoliths and are interpreted as being deposited in a deeper-water, lower-energy setting. Also, the Pflugerville limestone section is very similar to the McKown limestone section and evidence indicates Pflugerville limestones were not deposited in a shallow-water setting. The evidence produced by the present investigation substantiates that the carbonate strata at the Lower McKinney Falls were not deposited in a beach or shoaling complex but were deposited on the deeper flank of the Pilot Knob volcano. The concepts developed by this investigation of these deeper-water carbonates can be applied to interpreting the depositional setting of other carbonates associated with the Balcones Igneous Province volcanic mounds.

Spatial heterogeneity in carbonate-platform environments and carbon isotope values across the Paleocene–Eocene thermal maximum (Tethys Himalaya, South Tibet)

The Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) is a large negative carbon isotope excursion (CIE) that testifies to a massive perturbation of the global carbon cycle and has been considered to be an ancient, deep-time analogue for present and future climate change. However, the environmental and carbon isotopic response to the PETM in shallow-water carbonate platforms has remained largely elusive. This study presents new sedimentological, biostratigraphic, and carbon isotopic data to evaluate the impact of the PETM on a shallow-water carbonate platform from more proximal southern parts of the northern Indian continental margin. Detailed biostratigraphy and carbon isotope data indicate that the PETM occurs 3 m below the boundary between nodular and thin-bedded limestones, is maintained up to the thin-bedded limestone and calcareous marl interval, and is followed by recovery at the base of thick-bedded limestone interval. Microfacies analysis testifies to a regression from open to restricted shallow-marine environments at the transition from PETM onset to PETM core, and shallowing continued through the PETM core. Restricted lagoonal deposition was renewed during PETM recovery. These environmental changes were associated with two major turnovers of shallow-water biota. We infer that the first sudden biotic change at the PETM onset may relate to intensified continental weathering, whereas the second biotic change at PETM recovery may have been caused by sea-level fall. The smaller magnitude of the CIE observed in proximal, shallower-water than in distal, deeper-water environments is ascribed to increased primary productivity due to increased nutrients' supply associated with intensified continental weathering. • A regression at the PETM onset from open to restricted shallow-marine environments, and restored to restricted lagoon at the recovery • The sudden environmental changes across PETM were associated with major changes of shallow-water biota • The smaller magnitude of the CIE observed in shallower-water than in deeper-water strata is ascribed to increased primary productivity

Main biotic and climatic events in Early Permian of the Western Urals, Russia, as exemplified by the shallow-water biota of the early Kungurian lagoons

The paper deals with the Early Permian (mostly early Kungurian) biotas of the Cis-Urals (Perm region, Russia). Taxonomic composition of the early Kungurian biota of the stratotype area (close vicinity of the City of Kungur) includes algae Algites philippoviensis Naugolnykh, A. shurtanensis Naugolnykh, tracheophytes (higher plants), i.e., equisetophytes Paracalamites spp., conifers Shaidurodendron columnaris Naugolnykh, Walchia appressa Zalessky, vojnovskyaleans Rufloria derzavinii (Neuburg) S. Meyen etc.; invertebrates: coelenterates of uncertain affinity, bivalves Permophorus costatus (Brown), Permophorus sp., Netschajewia sp. cf. N. tschernyshowi (Licharew), gastropods Goniasma angulata (Stuckenberg), terebratulid brachiopods Dielasma sp. cf. D. moelleri Tschernyschew, arthropods (limulids Paleolimulus kunguricus Naugolnykh); vertebrates: chondrichthyan and actinistian fishes.Paleogeographically, the studied area belonged to the near-shore zone of a large lagoon basin disposed along the western-southern part of the Paleo-Urals during the Kungurian time. The main events in evolution of this basin reflect the final phases of the Artinskian sea basin with normal salinity, the transition to an early Kungurian (Philippovian) lagoon, then the appearance of intercalations of evaporate conditions and episodic incursion of marine faunas, and finally a gradual transition to semi-terrestrial environments with the cyanophyte communities in early Ufimian (Solikamskian, early Roadian) time. Climatic conditions in this area changed from semi-humid to semi-arid. A new genus and species of algae Dichothallus divaricatus Naugolnykh n. gen. n. sp. is described on the basis of material originated from the stratotype of the Philippovian Horizon of the Kungurian stage, Lower Permian.

Microproblematica, calcareous algae, and microbialites at the Frasnian-Famennian boundary interval in the Šumbera section (Moravian Karst, Czech Republic) and their significance in the context of the Kellwasser Crisis

The Kellwasser Crisis represents one of the most severe extinction events in the Phanerozoic. The Sumbera section (Moravo-Silesian Basin, Moravian Karst) spans the Upper rhenana to Palmatolepis minuta minuta (or younger) conodont zones and corresponds to the upper part of a carbonate ramp with material derived from shallower areas. A rich association of calcareous algae, cyanobacteria, and microproblematica, comprising renalcids (Izhella), Girvanella, Rectangulina cf. tortuosa, Rothpletzella, "solenoporaceans", "Keega"-like microfossils, udoteacean alga Paralitanaia, volvocean and radiospherid calcispheres, palaeoberesellids, Umbellina bella, and Wetheredella, is similar in diversity to associations known from China, Australia, Canada, and Belgium. The Sumbera section represents a unique locality where changes in shallow-water biota at the Frasnian-Famennian boundary are well calibrated by detailed conodont biostratigraphy. We track in detail the evolution of shallow-water biota and the features that are often discussed in the context of "anachronistic" facies, such as flourishing microbial structures, enhanced early marine cementation, and flat-pebble conglomerates. Some features which could be indicative of environmental stress such as flat-pebble conglomerates developed during the Frasnian part of the Kellwasser Crisis (Upper rhenana to linguiformis Zones), whereas a renalcid boom started at the Frasnian-Famennian boundary.

Conodont biostratigraphy of Tournaisian shallow-water carbonates in central Guangxi, South China

The conodont faunas of Tournaisian shallow-water carbonates from central Guangxi are described mainly for biostratigraphic purposes. A complete series of samples was collected from the Long’an and Du’an formations in the Long’an section. These formations are characterized by lime-mudstone, skeletal and peloidal wackestone, packstone and grainstone with typical shallow-water biota. Overall, these samples produced 809 identifiable Pl elements, belonging to 50 species in 11 genera, of which one species and one subspecies are new. The fauna enables the establishment of seven biozones (in ascending order): Polygnathus spicatus, Siphonodella homosimplex, S. sinensis, S. dasaibaensis, Polygnathus communis carina Acme, Gnathodus cuneiformis, and P. communis porcatus zones. Based on these new collections from central Guangxi and on data from the literature, a conflated Tournaisian conodont zonation is proposed for shallow-water successions in South China. Most of the conodont zones correlate well with their counterparts recognized in Western Europe, which may be of greater significance in stratigraphic correlation than previously thought.

Australian Barnacles (Cirripedia: Thoracica), Distributions and Biogeographical Affinities

Currently, 279 barnacle species are recognized in Australia waters. The barnacle fauna of tropical Australia exhibits high species diversity (221), with a high incidence of tropical species (87 Indo-west Pacific [IWP], 16 West Pacific and 65 Indo-Malayan), a low species endemicity (8), and 44 cosmopolitan and 1 Australasian species. Conversely, that of temperate Australia shows lower species diversity (129), with a lower incidence of tropical species (26 IWP, 10 West Pacific and 25 Indo-Malayan), higher species endemicity (23), 37 cosmopolitan, 6 Australasian species, and 3 Australasian/Antarctic species. Distributions corroborate the general patterns demonstrated by the shallow-water biota of northern tropical and southern temperate Australian biogeographic provinces. Tropical and temperate provinces grade into each other in a broad overlap zone along both the western and eastern Australian coasts. This overlap zone is essentially a transitional region, with the gradual replacement of a tropical barnacle fauna in the north by a predominantly temperate barnacle fauna in the south. Both western and eastern Australian coasts are bounded by major poleward-flowing warm currents that have considerable influence on the marine flora and fauna, distributing tropical species of many taxa much farther south than could be predicted by latitude. Currently, 16 barnacle species introduced into Australian waters are identified, although this number may increase in the future due to new port developments and increased shipping arrivals.

A route to recovery: The early Silurian shallow-water shelly fauna in the northern Oslo basin

The shelly fauna of the lowest part of the Saelabonn Formation in the Hadeland district of the Oslo Region provides a rare insight into the shallow-water biota of the earliest Silurian and hence the start of biotic recovery after the end Ordovician extinction event. It is dominated by the brachiopods Dalmanella cf. pectinoides Bergstrom, Coolinia cf. columbana (Reed), Leptaena cf. haverfordensis Bancroft and Zygospiraella scotica (Salter) together with the trilobite Acernaspis elliptifrons (Esmark). The first three species in this unique association belong to eurytopic Ordovician survivor genera which were also present in the underlying unit in Hadeland and continued to thrive during the Silurian. Significantly, Zygospiraella and Acernaspis have no unequivocal Ordovician record but diversified rapidly and became common during the early Silurian (Rhuddanian) in many parts of the world. Moreover, Acernaspis and Zygospiraella were highly eurytopic, pioneer taxa that were among the first elements of the Rhuddanian shelly fauna to appear in many parts of the world especially around the margins of the remnant Iapetus Ocean, following the drowning of areas previously emergent during the late Ordovician sea-level lowstand. The Hadeland fauna provides evidence of a previously undocumented route (among several) by which life ultimately returned to the global diversity plateau established in the mid-Ordovician.

Evolution of shallow benthic communities during the Late Paleocene–earliest Eocene transition in the Northern Tethys (SW Slovenia)

A paleoecological and sedimentological study was carried out on shallow-water carbonates of the Kras Plateau (SW Slovenia) with the goal of reconstructing paleoenvironmental conditions and evolution of foraminiferal communities on the northwestern Adriatic Carbonate Platform (AdCP) during the Late Paleocene–earliest Eocene. Three facies have been recognized and summarized in a carbonate ramp model. Within these facies, six foraminiferal assemblages, representing different ramp sub-environments, have been defined: during the Late Paleocene sedimentation took place in a protected innermost ramp with (1) smaller miliolids- and (2) small benthic foraminifera-dominated assemblages thriving on partly vegetated, soft substrates. In the Uppermost Paleocene, sedimentation primarily occurred along a mid ramp. The upper mid-ramp was sporadically influenced by storms/currents and occupied by (3) Assilina-dominated assemblage occurring on a soft sandy substrate. The deeper mid-ramp was characterized by (4) ‘bioconstructors’- and (5) orthophragminids-dominated assemblages, colonizing biotopes with substrates of different nature. During the earliest Eocene, deposition occurred in an inner-ramp setting with (6) alveolinids-nummulitids assemblage thriving on muddy and sandy substrate, partly covered or close to seagrass beds. The Late Paleocene–earliest Eocene environmental conditions, coupled with the long-term evolution of larger benthic foraminifera (LBF), seem to have favored this low-light dependent group as common sediment contributors. By comparing the evolution of the shallow-water biota from the Adriatic area with data from the Pyrenees and Egypt, a general latitudinal trend can be recognized. However, on a smaller geographical scale, local conditions are likely to have played a pivotal role in promoting the evolution of biota characterized by suites of unique features.

Phototoxicity in tropical reef animals

Phototoxicity refers to the toxic effects of chemicals caused or enhanced by light. Long known in man and terrestrial animals, it was first reported in freshwater animals in 1983. Our purpose was to test the process in marine species. Laboratory and outdoor trials in animals, representing Porifera, Cnidaria, Annelida, Arthropoda, Mollusca and Chordata, showed μg/liter levels of the polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, to be phototoxic to most of them, although non-toxic in the dark and light-colored animals were more sensitive than were dark ones. Uptake of the PAHs followed by absorption of ultraviolet (UV) radiation appeared necessary for phototoxicity, which also depended on PAH concentration, wavelength and UV intensity. Dissolved oil and tar components may be particularly hazardous to shallow-water biota exposed to sunlight.

Danian Mollusks from the Prince Creek Formation, Northern Alaska, and Implications for Arctic Ocean Paleogeography

The marine molluscan fauna of the Prince Creek Formation near Ocean Point, northern Alaska, is of Danian age. It is the only diverse and abundant Danian molluscan fauna known from the Arctic Ocean realm, and is the first evidence for an indigenous Paleocene shallow-water biota within a discrete Arctic Ocean Basin faunal province.A high percentage of endemic species, and two endemic genera, emphasize the degree to which the Arctic Ocean was geographically isolated from the world ocean during the earliest Tertiary. Many of the well-preserved Ocean Point mollusks, however, also occur in Danian faunas of the North American Western Interior, the Canadian Arctic Islands, Svalbard, and northwestern Europe, and are the basis for relating this Arctic Ocean fauna to that of the Danian world ocean.The Arctic Ocean was a Danian refugium for some genera that became extinct elsewhere during the Jurassic and Cretaceous. At the same time, this nearly landlocked ocean fostered the evolution of new taxa that later in the Paleogene migrated into the world ocean by way of the northeastern Atlantic. The first Cenozoic occurrences are reported for the bivalvesIntegricardium(Integricardium),Oxytoma(Hypoxytoma),Placunopsis, Tancredia(Tancredia), andTellinimera, and the oldest Cenozoic records given for the bivalvesGari(Garum),Neilo, andYoldia(Cnesterium). Among the 25 species in the molluscan fauna are four new gastropod species,Amauropsis fetteri, Ellipsoscapha sohli, Mathilda(Fimbriatella)amundseni, andPolinices(Euspira)repenningi, two new bivalve genera,ArcticlamandMytilon, and 15 new bivalve species,Arcticlam nanseni, Corbula(Caryocorbula)betsyae, Crenella kannoi, Cyrtodaria katieae, Gari(Garum)brouwersae, Integricardium(Integricardium)keenae, Mytilon theresae, Neilo gryci, Nucula(Nucula)micheleae, Nuculana(Jupiteria)moriyai, Oxytoma(Hypoxytoma)hargrovei, Placunopsis rothi, Tancredia(Tancredia)slavichi, Tellinimera kauffmani, andYoldia(Cnesterium)gladenkovi.

Ternate Formation, a Late Eocene Bioclastic Submarine Fan of the Lombardian Basin (Southern Alps): ABSTRACT

The Ternate Formation consists of some 150 m of redeposited bioclastic limestones, channelized rubble beds, and pebbly mudstones and subordinate hemipelagic marls with a late Eocene fauna of planktonic Foraminifera (Globorotalia cerroazulensis cerroazulensis Zone). Parallel-bedded massive or graded bioclastic limestones are overlain by channelized deposits. Large channels are up to 12 m deep; some contain pebbly mudstones with large slabs and boulders of older marls and limestones in a matrix of middle Eocene marls. More typically, the channels are filled by pebbly calcirudites grading into calcarenites and laterally wedging out over very short distances. Interchannel deposits are thin-bedded, graded calcarenites and thin green marls. The bioclastic limestones contain mainly coralline algae, larger Foraminifera (nummulitids, Discocyclina), bryozoans, echinoderms, and other shallow-water biota. All lithic fragments are of local, south-Alpine origin. By far the most abundant are spiculitic limestones and marls of Liassic age and Upper Cretaceous to middle Eocene limestones and marls. Most of the redeposited calcareous sand and gravel was deposited in ephemeral channels which cut across each other in a very complex way and were briefly active. This depositional system matches the suprafan environment of a prograding submarine fan of low transport efficiency. The facies of the conglomerates, themore » local source of the lithic fragments, and comparison with other lower Tertiary bioclastic deep-water deposits suggest short transport and deposition in one (or a series) of local bioclastic submarine fans. The benthic Foraminifera associated with the hemipelagic marls indicate deposition in the upper bathyal zone (600-1,000 m).« less

Deep Sea Drilling Project, Leg 77, southeastern Gulf of Mexico

In January 1981, R/V Glomar Challenger drilled five holes in the southeastern Gulf of Mexico to provide ground data for extensive seismic surveys and to document the pre-Tertiary history of the Gulf. Holes 535 and 540 were drilled in a basinal terrane for maximum penetration of the Cretaceous-Tertiary sequence. Rhythmic alternations of light bioturbated and dark laminated carbonaceous limestone represent the Early Cretaceous interval. Some of the dark layers are rich but immature oil source rocks. The limestones resemble the Blake-Bahama Formation in the North Atlantic but their stratigraphic age overlaps in part with the Hatteras Shale. Late Cretaceous rocks are almost totally missing in the basin sites and the Cenozoic section consists of chalk and marly carbonate ooze. Holes 536,537, and 538A were drilled on high-standing fault blocks. Hole 537 recovered phyllite that records 40 Ar/ 39 Ar plateau ages of about 500 m.y. and is overlain by an Early Cretaceous deepening sequence of alluvial to littoral elastics and oolitic-oncolitic limestones, capped by a thin sequence of Cretaceous and Cenozoic pelagics. In Hole 538A, basement consists of mylonitic gneiss and amphibolite, intruded by several generations of diabase dikes (that is, “transitional” crust). 40 Ar/ 39 Ar dates of hornblendes and biotite from the regional metamorphic rocks suggest a 500-m.y. (“Pan-African”) age with mild late Paleozoic thermal overprint. 40 Ar/ 39 Ar whole-rock dates from the dikes suggest intrusions between 190 and 160 m.y. ago. Basement is covered by a thin layer of pelagic chalk, followed by Early Cretaceous skeletal-oolitic limestones and, finally, Cretaceous-Tertiary pelagics. The oolitic-oncolitic limestones at both sites represent either parts of a shallow-water carbonate platform or platform talus deposited in deep water. Hole 536 bottomed in shallow-water dolomite (Jurassic or Permian), overlain by middle Cretaceous skeletal limestones with shallow-water biota and intercalations of pelagic chalk, interpreted as Cretaceous talus at the foot of the Campeche Bank. Cretaceous-Tertiary chalk and carbonate ooze cap the sequence. Among the most significant results of the leg are: (1) recovery of “transitional” crust with early Paleozoic (Pan-African) metamorphic rocks, (2) recovery of Early Cretaceous deep-water limestones with immature petroleum source beds, (3) recovery of mid-Cretaceous platform talus resembling the reservoirs in the Poza Rica and probably some of the Reforma fields of Mexico, and (4) discovery of a Late Cretaceous hiatus of 30 m.y. that corresponds approximately to the “mid-Cretaceous unconformity” recognized widely on seismic records in the Gulf of Mexico.

Utopia Algal-Bank Complex and Its Potential in Shallow Gas Exploration in South-Central Kansas: ABSTRACT

The Utopia Limestone (Upper Pennsylvanian) was deposited as a series of algal banks forming a narrow arcuate band approximately 25 mi (40 km) long on the shelf margin of Sedgwick basin in south-central Kansas. Utopia banks may attain a maximum thickness of 70 ft (21 m) and are encased in a dark gray shale, creating excellent reservoirs for hydrocarbon accumulation. Having porosities of up to 22%, the Utopia produces economic quantities of gas from depths of 2,000 to 2,500 ft (610 to 762 m). The Utopia contains three distinct facies: (1) biomicrite, the core facies; (2) calcilutite, an Osagia-coated biosparite, the mound facies; and (3) an oolitic facies associated with the mound. The facies distribution was controlled by basin geometry and the variations in the subtidal and intertidal environments of the shelf. Sedimentation rate approximately equaled subsidence rate, maintaining the position of the bank tops within the shallow intertidal range. The banks are slightly asymmetrical and thicken basinward. The initial mound accumulation began with the deposition of a biomicritic mound core, in a quiet-water environment. The major constituents of the calcilutite facies are green algae (Osagia and Epimastopora), along with a wide assortment of shallow-water biota. The ool te shoal facies formed contemporaneously beside the mound facies. Subaerial exposure and subsequence leaching of the banks during successive marine transgressions created the intraparticle and oomoldic porosity. Locally, dolomitization of the algal fragments occurred. Results of this study indicate that further exploration in this area and basinward should prove profitable. End_of_Article - Last_Page 637------------

Biogeographical Analysis of Intertidal Macrophyte Floras of Southern California

The macrophyte floras of California mainland and island sites south of Point Conception form five discrete biogeographic units based on indi- vidual between-site similarity comparisons, hierarchical classification (cluster) analysis and principal coordinates ordination. These site assemblages correlate well with the complex patterns of surface water circulation and temperature. Sites bathed more regularly by the warm waters of the Southern California Countercurrent tend to group and exhibit relatively low overlap in species composition with the cold-temperate flora characteristic of the central California coast north of Point Conception. Greater cold water floristic affinities were found for the sites proximal to cold California Current flow, while sites located in the region of variable mixing between cold and warm water currents showed intermediate affinities. The apparent relationship between the groups established by classification analysis and ordination, the degree of overlap with the cold- temperate central California flora, and our understanding of surface circulation and temperature gradients, provide indirect evidence implicating temperature in the control of the distributions of southern California intertidal macrophytes. Additionally, the changing species composition along the coast south of Point Conception clearly demonstrates the transitional nature of the southern California shallow-water biota and underscores the importance of this unique region to eastern North Pacific biogeography.

Plate tectonics and the evolution of intertidal and shallow-water benthic biotic distribution patterns of the southwest Pacific

A brief summary is given of recent information on the changing palaeogeography of the southwest Pacific region following the break-up of Gondwanaland and the parallel palaeoceanographic changes that occurred. Present intertidal and shallow-water benthic marine provinces in Australasia can be grouped into tropical—subtropical, warm-temperate, transitional warm-temperate, cold-temperate, transitional subantarctic and subantarctic regions. A notable feature is the marked faunistic attenuation that occurs with increasing latitude in most groups. Historical explanations of biotic distribution patterns fall into two classes: dispersal explanations and vicariance explanations. The problems involved in evaluating the relative roles of dispersal and vicariance in the evolution of the biota of a region are outlined and the conditions which must be met before larval dispersal across wide stretches of ocean can be successful are reviewed. It is concluded that rafting of adults is restricted to a few groups of animals and is of limited importance as a dispersal mechanism. An examination of the external relationships anddfossil record of Australasian intertidal and shallow water biota has identified a number of elements of which the predominant one is a Malayo-Pacific one comprising plants and animals of tropical affinities. The evolutionary history of the biota is re-interpreted in the light of palaeogeographic and palaeoceanographic changes with reference to a series of palaeogeographic reconstructions showing successive positions of Antarctica, Australia and New Zealand which have been used as a geographic base upon which speculative patterns of oceanic circulation and positions of oceanographic fronts have been depicted. Tectonic events, coupled with changes in circulation patterns and sea temperatures and the establishment of the Antarctic and Subtropical Convergences with a series of water masses forming belts from the poles to the tropics have been controlling influences of the distribution and evolution of the marine biota of the region. A major factor has been the migrations that have occurred in the position of the warm-water/cold-water boundary, the Subtropical Convergence. This migration coupled with changing island archipelago nature of the New Zealand land area has played a major role in speciation processes in the New Zealand region. It also led to the extraction of many warm-water genera and species as it moved north. In Australia the larger land mass extending further to the north provided opportunities for species to migrate up both the east and west coasts. This, coupled with the Pleistocene land bridges which connected Tasmania and New Guinea to Australia, led to isolation and subsequent allopatric speciation, followed by sympatry when the derived species migrated along the coast as the Subtropical Convergence retreated south to its present position and the Torres and Bass Straits barriers disappeared with rises in sea level.

Global Tectonics and the Fossil Record

The theory of plate tectonics implies greatly different continental geographies in the past which can be reconstructed from a variety of geophysical, structural, petrologic, and stratigraphic evidence. Platetectonic processes affect relative sizes, the relative emergence, and the latitudinal and longitudinal patterns of continents. These factors affect the patterns of trophic-resource regimes in shallow marine water and also determine the patterns of provinciality of the shallow-water biota, and these patterns are probably chiefly responsible for the regulation of species diversity in shallow waters. The diversity patterns are partially the products of the adaptive strategies followed by populations in different resource regimes; high diversities correlate with stable regimes and low diversities with fluctuating regimes. The proportions of feeding types vary among types of regime also, resulting in qualitative differences in the faunas. Precam-brian and Cambrian radiations from which the higher invertebra...