Ancient Counterparts Research Articles

Temperate Carbonates on a Modern, Low-Energy, Isolated Ramp: The Balearic Platform, Spain

ABSTRACT This paper presents a depositional model for a temperate, low-energy carbonate ramp based on descriptive studies of five areas around the Balearic Islands of Mallorca and Menorca. This low-energy ramp differs significantly from other present-day temperate carbonate platforms that are primarily high-energy open shelves. It is characterized by the following lithofacies (from shore to basin): (1) lagoon, (2) barrier island, (3) shallow subtidal, (4) inner ramp, (5) middle ramp, and (6) distal ramp. Subaqueous carbonate dunes are present near slope breaks off Menorca and Cabrera, but they are not representative of the entire ramp. Balearic ramp sediments differ in composition, texture, biology, and degree of cementation from those on modern low-energy, tropical ramps. Balearic ramp carbon tes lack ooids and peloids, hermatypic coral buildups, and calcareous green algae (except in one restricted bay where Halimeda is relatively common). Red algal sands and gravels extend to depths of up to 90 m, and are coarser than their strandplain-equivalent lime sands. Skeletal allochems consist of the bryomol-rhodalgal association, marine cementation is rare, and the carbonate fraction of deep-water muds is mainly Mg-calcitic and calcitic in composition. Aragonite is rare. Except for the red alga Peyssonnelia, which is composed of aragonite, it is neither a dominant skeletal constituent nor common as a cement. The Mediterranean Sea off Mallorca and Menorca is a low-energy, temperate, oligotrophic, clear-water environment. The depositional model for the region is an isolated platform configured as a homoclinal ramp. Ancient counterparts of the Balearic ramp are present in the Neogene of the Mediterranean Tethys and the Paratethys, and though the constituents of fossil assemblages vary with time, the biota of the Balearic ramp, such as bryozoans, red algae, echinoderms, and mollusks, ranges from the Paleozoic Era to the present. Echinodermal bryomols passing to basinal muds on carbonate ramps are particularly characteristic of the Early Carboniferous in North America and Europe. This suggests that the Balearic Islands temperate ramp may be more representative of some ancient carbonate sequenc s than either temperate, high-energy, open shelves or tropical ramps in the present oceans.

Modeling of cyclic tidal rhythmites based on a range of diurnal to semidiurnal tidal-station data

Where individual tides deposit appreciable accumulations of sediment, cyclic tidal rhythmites can form that exhibit semimonthly tidal cyclicities. A number of modern analogs and ancient counterparts indicate the importance of such features for paleoenvironmental analyses. In this study, predicted, high-low, tidal-station data are used to simulate current speeds, which in turn are used to simulate sedimentation of tidal rhythmites. This modeling has been done to depict the patterns encoded in cyclic tidal rhythmites. In order to delineate the spectrum of different tidal regimes, the simulations are based upon a series of macrotidal, diurnal to semidiurnal tidal systems. Depending upon thresholds of relative tidal-current speed, only portions of semimonthly tidal cycles will be preserved within cyclic tidal rhythmites.

Holocene and Paleogene Arkoses of the Massif Central, France: Mineralogy, Chemistry, Provenance, and Hydrothermal Alteration of the Type Arkose

ABSTRACT Paleogene arkoses of the Auvergne region of France represent the type arkose originally described by Brongniart (1826). They are alluvial-fan and fluvial deposits including traction-current sandstones (arenites) and matrix-rich debris-flow deposits (wackes). Locally, they have been extensively altered by geothermal waters related to nearby Tertiary-Holocene volcanic activity. The alteration is typified by leaching of detrital grains and precipitation of ubiquitous chert cement. The average Gazzi-Dickinson composition of unaltered arenites is Q40F60L0, with K/F (potash feldspar/total feldspar) = 0.63. Altered arenites have an average composition of Q55F43L2, with K/F= 0.73, reflecting loss of plagioclase due to intense a teration. When secondary chert is included with detrital quartz and chert, the altered rocks have an average composition of Q66F32L1, emphasizing silica enrichment associated with alteration. Holocene sands derived from crystalline basement of the Massif Central have the same provenance as the Paleogene sandstones. They have an average Gazzi-Dickinson composition of Q46F49L5 with K/F = 0.52 and compare favorably with their unaltered ancient counterparts. Arkosic alluvium at Roosevelt Hot Springs, Utah is believed to represent a modern analog for alteration of the Auvergne sandstones. The alluvium has been altered and cemented with silica by geothermal waters at near-surface conditions in the epithermal zone. Chemical analyses of 130 sand and rock samples demonstrate original sediment compositions and changes due to hydrothermal alteration. Holocene sands and unaltered Paleogene clastics are compositionally similar and show variation trends similar to feldspathic clastics derived from granitic basement in other basins. Silica enrichment in the altered sandstones causes dilution of all other elements as an effect of constant summation. Consequently, most element abundances decline proportionately with silica dilution. Na and Rb, however, are reduced below the levels predicted by dilution due to plagioclase and biotite destruction. As a result, K/Rb ratios are higher in the altered sandstones. Ba, S, SO4, As, and Sb are enriched in the altered rocks by precipitation from hydrother al solutions. Barium is in barite as fracture-filling veins and scattered patches in sandstone matrix. Destruction of mafic minerals during alteration released Fe, Co, Cr, Ni, Zn, Pb, and Th that later precipitated as oxides and sulfides in fractures and locally in chert matrix in the sandstones.

Where's the reef?: a critical reevaluation of the role of framework

Despite suggestions to the contrary by Norman Newell, in 1971, the presence of in-place, interlocking framework remains primary among the list of criteria used to recognize “true reefs” in the rock record. This is a logical outgrowth of observations made in modern systems, emphasizing 1) high carbonate-production rates of modern corals, 2) their commonly upright and interlocking architecture, and 3) the abundance and diversity of closely packed organisms on the surface of many modern reefs.Applying a framework-dominated model to ancient reefs, however, has led to repeated frustration and the assumption that the problem is by and large related to the changing nature of reefs through time. Our present attempt to apply uniformitarianism to modern reefs and their ancient counterparts was perhaps best summarized by H. A. Lowenstam, “The present is the key to the Pleistocene… perhaps”.A growing data base from cores through modern reefs implies that much of the problem is related to real differences between what we think constitutes the interior of modern reefs and what is really there. Recognizable coral generally constitutes less than 30% of the reef fabric, based on examination of over 50 cores from a variety of reef types throughout the eastern Caribbean region. Volumetrically, loose sediment and void space comprise a significantly higher portion of the reef interior. Furthermore, obviously in-place and interlocking “framework” is conspicuously absent. Branching Acropora palmata is invariably found in a variety of orientations, implying largely detrital emplacement rather than the preservation of intact and undisturbed colonies. Head corals are generally separated by intervals of detritus or open cavities, rather than sitting atop older colonies. Where rigidity of the reef fabric can be observed, it is almost always a function of secondary encrustation by coralline algae and/or post-depositional marine cementation.The patterns seen in cores are supported by previous budget calculations of carbonate cycling in modern reefs around St. Croix. Bioerosion reduces original carbonate material to sediment, leaving recognizable coral to represent only a small part of the total reef volume. While still a significant component of the total reef fabric, few of the recognizable corals are bound together, and it is difficult to clearly demonstrate that the samples were recovered from life position (although some probably were). At Salt River, recovery was very low, and corals have been demonstrably moved from their original sites of growth.Effective modeling of ancient reefs must take into account the importance of secondary processes that reduce and redistribute the original carbonate material produced by corals. Combined with physical reworking, cementation and encrustation, these destructive processes result in a deposit that resembles more a pile of reef debris than the coherent assemblage of in-place, interlocking framework usually conjured up to represent the modern-reef interior. While still of great importance, constructional processes that are so evident on the reef surface must be viewed within the context of other physical, chemical and biological processes that are reflected in the growing body of core data from modern reefs in the western Atlantic and throughout the world.

Gravelly shoreface deposits: a comparison of modern and ancient facies sequences

ABSTRACTThe morphology and dynamics of modern gravel shorefaces are poorly documented. This hinders the interpretation of possible ancient counterparts. A comparative study of a modern (Chesil Beach, England) and an ancient (Baytree Member of the Cardium Formation, Alberta) gravel shoreface shows that the two systems are very similar close to and above sea‐level, with a high (about 1 m) gravel plunge step lying below plane‐bedded sands and gravels of the beachface. The shoreface at Chesil Beach is dominated by asymmetrical gravel wave ripples. These are oriented offshore near the toe of the shoreface, and onshore in shallower depths. This may reflect offshore movement during storms and landward reworking during fair weather. The Baytree Member is over 12 m thick and comprises over 80% conglomerate. Conglomerate is decimetre‐bedded, massive or cross‐bedded, with sets over 60 cm thick produced by gravel bedforms migrating alongshore. It is interbedded with discontinuous cm‐ to dm‐bedded sandstones which may be cross‐bedded. Pebble fabric and cross‐bed orientation both indicate strong alongshore sediment transport. Near the base of the section, pebble orientations suggest that gravel wave‐ripples developed below the zone of strong longshore flows. Differences between these two examples may be attributed to different directions of wave approach.

Constructional Azooxanthellate Coral Communities: An Overview with Implications for the Fossil Record

As evidenced by the success of zooxanthellate coral species today, symbiosis with algae confers great advantages to organisms inhabiting the reef environment. Although not as well known as their zooxanthellate counterparts, azooxanthellate coral communities, dominated by so-called corals, can and do build constructionalframework, harboring a variety of associated vertebrate and invertebrate fauna. As has long been noted, when it comes to reefs and constructional roles, zooxanthellate and azooxanthellate corals exhibit mutually exclusive relationships. This is traditionally explained in terms of competition. When zooxanthellate species are absent due to restrictive environmental conditions, azooxanthellate species may inhabit relatively shallow water, even building limited constructional framework. In some non-reef settings where zooxanthellate corals are not living under optimal conditions, azooxanthellate species can be found living alongside them. On the shallow-water reef as a rule, azooxanthellate corals are relegated to cryptic habitats not colonizable by the overwhelmingly dominant zooxanthellate species. A review of 16 well-documented examples of modern constructional azooxanthellate coral communities and their physical and biological characteristics reveals a wide variety of latitudinal and depth ranges, temperatures, species composition, thicknesses, and species diversity. In all major oceans, they are best developed in cold and relatively deep water, as deep as 1500 m, and from the equator to latitudes of over 70?. Besides having a predominantly coral composition, some deep-water, azooxanthellate coral communities with extensive constructional framework have unexpected faunal diversity and presumably also complex interactions. The paleobiogeography and evolution of some of these deeper-water, coral-dominated azooxanthellate communities, known to have inhabited atolls and oceanic seamounts, may have been influenced by plate tectonic seafloor movements. While we are becoming increasingly cognizant of Holocene examples, counterparts in the geologic record are much more poorly known. The apparent paucity of ancient analogs may stem from difficulties in their interpretation and recognition in the geologic record. Judging from the size, facies relationships, faunal content, and diversity of Holocene examples, ancient deep-water coral structures, when preserved in the rock record, could be easily misconstrued as features of shallow-water origin. More careful analysis of deep-water Holocene examples can greatly expand our awareness of ancient analogs. In some cases, depending on the amount and thickness of construction, they could hold potential as hydrocarbon reservoirs. Azooxanthellate scleractinian communities are common in today's oceans and examples of them extend back to the Middle Triassic period. Fossil examples, unfortunately, are rare and sporadic in stratigraphic distribution, with the most notable reports from the Tertiary. Our view of the relative abundance of ancient counterparts may be hampered by poor preservation and problems of recognition.

Conditions favourable for the formation of warm‐climate aeolian sand sheets

ABSTRACTAeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent. Sand sheets are ubiquitous to modern, warm‐climate sand seas, generally occurring marginal to dune fields, although they may exist within the interior of a sand sea or independent of a dune field. Sand‐sheet deposits are recognized in ancient aeolian sequences, where they may account for significant accumulations of low‐angle aeolian stratification. We suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets. A study of six modern sand sheets in North America (located at Great Sand Dunes, Gran Desierto, Dumont, Algodones, Padre Island, and Colorado River delta) indicates that the factors favourable for sand‐sheet development are: (1) a high water table, (2) surface cementation or binding, (3) periodic flooding, (4) a significant coarse‐grained sediment population, and (5) vegetation. These factors are reflected in the nature of stratification and the accessory features of sand‐sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the Triassic Dolores and Pennsylvanian‐Permian Rico formations.

The frequency and periodicity of preserved turbidites in submarine fans as a quantitative record of tectonic uplift in collision zones

The frequency and periodicity of preserved graded turbidite cycles in submarine fans in the Coral Sea and Sea of Japan are correlated to times of tectonic uplift in response to major collisions in the Owen-Stanley Range of Papua and the Hida Range of Japan, respectively. Large frequencies and shorter-term periodicities of turbidites at DSDP Site 210 were coeval with early Pliocene maximum tectonic-uplift rates which occurred in the Owen-Stanley Range in response to obduction. Similarly, large frequencies and shorter-term periodicities of turbidites at Site 299 (Toyama Submarine Fan) were coeval with the late Pleistocene uplift in the Hida Range; this uplift of 1000 to 1500 m occurred in response to collision tectonics. In both cases, trends of increasing frequencies and towards shorter-term periodicities of preserved turbidite depositional events correlate to trends of increasing rates of tectonic uplift. The role of sea-level fluctuations on changing denudation rates in these two collision zones is secondary. At Site 210, larger frequencies and short-term periodicities of preserved turbidites were coeval with early Pliocene high stands of sea level, whereas at Site 299, Pleistocene sea-level fluctuations are considered minor because at low stands of sea level, both relief and denudation rates were increased by about ten to 14%. At Site 286 (New Hebrides Basin), Eocene turbidite deposition is coeval with high stands of sea level, whereas at Site 297 (Northern Shikoku Basin), turbidite deposition was coeval with both rising and falling sea level. Analysis of both frequency and periodicity of turbidites by fan subenvironment at Site 299 indicates a record of continuous deposition, and maintainance of frequency and periodicity trends controlled by tectonic uplift. Late Pleistocene channel and overbank deposits showed periodicity differences of less than 28% of an order of magnitude, whereas Miocene-Pliocene overbank and distal turbidite periodicities differed by a 19% order of magnitude. Greater differences in magnitude occurred between distal turbidites or early Pleistocene age and Pliocene age than between Miocene-Pliocene overbank and distal turbidite deposition with a magnitude difference of 860%. These findings suggest that shifting depocenters and differences in sedimentation history in subenvironments of submarine fans are secondary to the role of tectonic uplift in this particular case. The minimal rate of tectonic uplift required to generate deep-sea fan turbidities appears to be approximately 400 m/million years. This figure is tentative and is based on very few observation points. Frequency and periodicity of preserved turbidite cycles in submarine fans in active continental margins and ancient counterparts should provide an independent measurement of rates and timing of tectonic uplift, particularly in collision terrains. Because this sediment parameter is a record of a single process from a single source and a record of “event stratigraphy”, its usage is preferable over standard and bulk sediment accumulation rates determined from age depth curves.

Taphonomy of Holocene cryptic biotas from St. Croix, Virgin Islands: Information loss and preservational biases

Surveys of submarine caves and overhanging ledges from St. Croix, Virgin Islands, provide new insights into the preservational processes active therein and into the taphonomy of ancient counterparts. Comparisons of preservable, skeletonized versus nonpreservable, unskeletonized components of these modern cryptic communities indicate that there is significant information loss in areals coverage, taxonomic richness, and diversity of fossilized examples. Quantitative estimates of such losses have been made. In addition, skeletal differences between early and late stage successional groups suggest biased representation of the former in the geologic record. Nonpreservable later colonizers may further erase the record of skeletonized forms through destructive life processes. Such forms of taphonomic information loss are probable in ancient counterparts and must be considered in accurate reconstruction of cryptic paleocommunities.

Precambrian geological evolution

Geologists from Brazil, the three Guianas, Venezuela, the northern Andean countries, North America, and Europe are participating in a new project of the International Geological Correlation Program, concentrating on the Precambrian geological evolution of the Amazonian region. Most of the major Precambrian rock units of the region are currently being studied by geologists in the various countries that have exposures of the Guiana and Guaporé shields, which make up the Amazonian craton. The craton is notable because of the great expanses of crust that formed in the Early Proterozoic. There are granite‐greenstone terranes as extensive as those of the Archean provinces of the North American shield and high‐grade gneiss and granulite terranes of both Archean and Proterozoic age. These provide opportunities to compare continental formation and evolution of Proterozoic age with their more ancient counterparts in other shields. Most of the craton was established in the Early Proterozoic and was affected by the Trans‐Amazonian orogeny; about half of this was then covered by Middle Proterozoic intracratonic sedimentray basins, associated felsic volcanics, mafic intrusives, and epizonal granitoid rocks. The felsic magmatic rocks of the Middle Proterozoic are particularly extensive and well exposed: This must rank as one of the prime regions in the world to study such rocks. Alkaline complexes with associated carbonatites occur in Brazil, Guyana, and Venezuela. All of the rock types mentioned have important ore deposits associated with them, and one of the goals of the project is to develop further understanding of the mineral potential and metallogenesis of the region.

Aragonite Crusts and Pisolites Beneath Dolomitic Tepees, Lake MacLeod Evaporite Basin, Western Australia: ABSTRACT

Research currently being conducted by the Sedimentology and Marine Geology Group, under Brian W. Logan at the University of Western Australia, has recently concentrated on Lake MacLeod, a 2,000 km2 (770 mi2) coastal salina on the western coast of Australia. This work has shown that this evaporite basin, which is 3 to 4 m (10 to 13 ft) below sea level, is separated from the Indian Ocean by a topographic barrier, but seawater under hydrostatic head, seeps freely through the barrier and discharges from several vents and springs in a carbonate mud flat at the north end of the basin. From there, seawater flows slowly across the basin, evaporating and depositing carbonate, gypsum, and ephemeral halite. About 10 to 12 m (33 to 39 ft) of evaporites have been deposited in the past 5,300 years. In July 1982, the authors visited the carbonate mud flats and discovered abundant aragonite pisolites and botryoidal-mammillary crusts of fibrous aragonite cement beneath lily-pad tepee slabs of cemented protodolomite. This protodolomite host-rock is well-lithified, intraclast, peloid packstone with abundant coarse fenestrae. Thick aragonite crusts cover both the undersides of lily-pad slabs and the lithified floors of tepees. Crusts covering the floors are more botryoidal and consist of both aragonite nubs and mounds (0.2 to 2.5 cm, 0.08 to 1 in., in diameter), and a few scattered, loose pisolites, several millimeters in diameter. Pisolites are composed of multi-generation fibrous layers of square-tipped aragonite rays surrounding peloid-intraclast nuclei. Thus, it seems that per odic deposition of a fine layer of carbonate mud, peloids, and intraclasts across the floor of a tepee is a prerequisite to pisolite growth. Stable isotope analysis of the host rock and aragonite cements gave expected marine values (^dgr18O = +0.25 to +1.14 PDB and ^dgr13C = -0.18 to +0.16 PDB) and reflect precipitation from ground water (marine composition) discharging from seeps in the carbonate mud flats. The manner in which crusts, pisolites, and tepees occur at Lake MacLeod raises the possibility that they and their ancient counterparts from the Permian basin share a common origin. Perhaps Permian pisolites and aragonite crusts formed beneath cemented slabs of peritidal sediments in tepees bathed by marine water which seeped across exposed portions of the shelf crest. End_of_Article - Last_Page 478------------

Men and citizens in international relations

Since Rousseau political theorists have had frequent recourse to a contrast between the fragmented nature of modern social and political life and the allegedly communitarian character of the Greek polis. At the heart of this opposition was the belief that the polis represented a condition of unsurpassable harmony in which citizens identified freely and spontaneously with their public institutions. Unlike their ancient counterparts, modern citizens exhibited less identification with their public world than resolution to advance their separate individual interests and pursue their private conceptions of the good. Nevertheless, the disintegration of the polis was not depicted in the language of unqualified loss. History had not been simply an unmitigated fall, because the individual's claim to scrutinize the law of the polis on rational grounds involved a significant advance in man's self-consciousness. The positive aspect of its decline was man's transcendence of a parochial culture in which neither the right of individual freedom nor the principle of human equality had been recognized. If the modern world had lost the spontaneous form of community enjoyed by the ancients, it surpassed that world in its understanding and expression of freedom.

Ripple-Drift Cross-Lamination in Tidal Deposits: Examples from the Precambrian Bhander Formation of Maihar, Satna District, Madhya Pradesh, India

Ripple-drift cross-laminations are described from tidal deposits of the Precambrian Bhander Formation, which belongs to the Vindhyan Supergroup of central India. Considering the transport processes operative in modern midflats, ripple-drift cross-laminations should be common in ancient counterparts. The generation of ripple-drift cross-laminations is, however, usually hindered by the absence of persistent, unidirectional current flow. These structures can only develop in areas of the midflat where there is persistent, unidirectional current. Because of periodic velocity changes of tidal currents, rhythmically size-differentiated foreset laminae appear to be a diagnostic feature of tidal ripple-drift cross-laminations. Size differentiation is normally absent in the ripple-drift cross-laminations of turbidites and fluvial sequences, for in the environments responsible, there exists no cyclic pattern of water movement (that is, currents that alternate with periods of slack water) as is found in the tidal-flat environment.

Formation of Red Beds in Modern and Ancient Deserts

Research Article| March 01, 1967 Formation of Red Beds in Modern and Ancient Deserts THEODORE R WALKER THEODORE R WALKER University of Colorado, Boulder, Colorado Search for other works by this author on: GSW Google Scholar GSA Bulletin (1967) 78 (3): 353–368. https://doi.org/10.1130/0016-7606(1967)78[353:FORBIM]2.0.CO;2 Article history received: 28 May 1965 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation THEODORE R WALKER; Formation of Red Beds in Modern and Ancient Deserts. GSA Bulletin 1967;; 78 (3): 353–368. doi: https://doi.org/10.1130/0016-7606(1967)78[353:FORBIM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New evidence supports the theory that the hematite pigment in many red beds, particularly those associated with evaporites and aeolian sandstones, formed after deposition in hot arid or semiarid climates. The evidence includes field, petro-graphic, and chemical data collected from studies of two stratigraphic sequences that contain red beds: (1) Recent, Pleistocene, and Pliocene deposits in the Sonoran desert of northeastern Baja California, Mexico, and (2) late Paleozoic deposits in Colorado.The sequence in the Sonoran desert contains examples of red beds forming today in a hot dry climate. They are associated with bedded evaporites and occur where regional faunal, floral, and pedological evidence indicates rainfall has been low throughout the postdepositional history of the sediments. The facies associations reflect deposition in fluvial and fluvial-marine transitional environments. Red arkose fanglomerates occur on the flanks of the highland source areas, and red muds, probably of intertidal and shallow subtidal origin, occur in the transition sediments. Both of these red-stained facies show progressive stages of in situ alteration of nonred sediments to hematite-stained red beds, and in each, the iron in the stain is derived from intra-stratal alteration of iron-bearing detrital grains, particularly iron silicates such as hornblende and bio-tite.The late Paleozoic red beds of Colorado contain rock types and facies associations similar to those of the Sonoran desert; they are interpreted as ancient counterparts of those red beds. Numerous lines of field and petrographic evidence indicate that the hematite pigment in the late Paleozoic red beds formed in place after the sediments were deposited in desert basins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Practical Classification of Reefs and Banks, Bioherms and Biostromes: ABSTRACT

End_Page 167------------------------------Reefs and banks form stratigraphic traps which hold more than 40 per cent of the total petroleum production in the world. They are of special importance in the Permian basin exploration. Yet there is much confusion concerning the classification and terminology of these skeletal deposits. It is the purpose of this paper to attempt to define these structures on a genetic and morphological basis and indicate their exploration potentials. Following Lowenstam (1950) and Nelson et al. (1962), the writer applies the terms and to denote the origin of the structures, whereas the terms and are used to designate the shape of the structures and their relation to the associated layered facies. A reef is a structure built by the in-place growth of organisms which have the ecological potential to act as frame-builders. It is a wave-resistant, prominent structure on the sea floor and, therefore, influences, and modifies sedimentation in its vicinity. A bank, in contrast, consists of organisms which did not have the ability to erect a rigid three-dimensional frame-work. Banks may be formed in place or by mechanical build-up following transport of the skeletal remains. Banks also are wave-resistant. They may or may not be prominent structures on the sea floor or influence the sedimentation around them. According to their formation, banks are subdivided into (1) mechanical and (2) biogenetic accumulations. Biogenetic accumulations are subdivided further into banks resulting from (a) biogenetic baffling of sediment, (b) biogenetic binding of sediment, and (c) gregarious local growth of organisms without erecting prominent structures. Thus, reefs and banks are distinctly different types of accumulations. In a reef, in situ growth of organisms is more important than sediment trapping and binding, and the organic productivity of the frame-building organisms alone is sufficient to elevate the structure above the surrounding sea floor. Frame builders in general are organisms that cement themselves to the substratum and form a rigid mass. Cementation appears to be very important in the resulting structure. Colonial hexacorals, stromatoporoids, calcareous sponges, rudistids, and crustose CaCO3-secreting red algae can act as frame builders in a reef. In contrast, in banks, the sediment-baffling and binding functions of the organisms are the main sources of sediment accumulation. Tetracorals, tabulate corals, bryozoans, crinoids, phylloid algae, pelecypods, gastropods, and brachiopods are organisms that can not erect a rigid framework. However, they can actively trap sediment and form banks. Furthermore, their local gregarious growth may lead to bank-type accumulations. A bioherm is a massive, mound-shaped structure which is in discordant relationship with the surrounding layered facies of different lithologic types. A biostrome is coarsely layered and grades concordantly into the surrounding layered sediments. According to these definitions, a reef, according to its shape and geological setting, represents a bioherm. A bank, however, may take the form of a bioherm or biostrome. Mechanically accumulated banks and biogenetic banks resulting from the sediment-baffling activity of organisms generally have the form of bioherms. Banks resulting from biogenetic binding of sediment may have the form of either bioherms or biostromes. Local gregarious growth of organisms usually leads to accumulations of the biostrome type. The fact should be stressed that the shape of a fossil structure is not necessarily the same as its shape at the time of formation. Differences in compaction capabilities of skeletal deposits versus clay or carbonate-mudrich deposits tend to exaggerate the relief of the organic bu ld-up. Examples of various types of Recent and ancient reefs and banks are illustrated and discussed. Recent coral reefs of the Florida Keys reef trend and Cretaceous rudistid reefs in Sierra El Abra, Tampico region, Mexico, are examples of true reefs. Little Molasses Reef, Florida Keys, is interpreted as a bank which was formed by mechanical accumulation of skeletal debris. Rodriguez Key, Florida Keys, and mud mounds in Florida Bay are examples of banks representing biogenetic accumulations resulting from the sediment-baffling functions of organisms. Phylloid algal mounds from the Hueco Tanks area (West Texas), Dry Canyon near Alamogordo, New Mexico, and Marble Canyon, Alamogordo, New Mexico, are ancient counterparts of these biogenetic banks. Algal stromatolites are representative of banks formed by the sediment-binding action of organisms. Cretaceous oyster beds are indicative of bank accumulations by local gregarious growth of organisms. It is very important to emphasize the fact that structures, which superficially appear to be very similar, may, in fact, have formed in very different environments and, therefore, require different genetic interpretations. End_of_Article - Last_Page 168------------

Recent turbidities in the Hardangerfjord, Norway : Holtedahl Hans, 1965. In: Submarine geology and geophysics, Colston Papers, W. F. Whittard and R. Bradshaw, editors, Butterworth, London, 17: 107–140.

The frequency and periodicity of preserved graded turbidite cycles in submarine fans in the Coral Sea and Sea of Japan are correlated to times of tectonic uplift in response to major collisions in the Owen-Stanley Range of Papua and the Hida Range of Japan, respectively. Large frequencies and shorter-term periodicities of turbidites at DSDP Site 210 were coeval with early Pliocene maximum tectonic-uplift rates which occurred in the Owen-Stanley Range in response to obduction. Similarly, large frequencies and shorter-term periodicities of turbidites at Site 299 (Toyama Submarine Fan) were coeval with the late Pleistocene uplift in the Hida Range; this uplift of 1000 to 1500 m occurred in response to collision tectonics. In both cases, trends of increasing frequencies and towards shorter-term periodicities of preserved turbidite depositional events correlate to trends of increasing rates of tectonic uplift.The role of sea-level fluctuations on changing denudation rates in these two collision zones is secondary. At Site 210, larger frequencies and short-term periodicities of preserved turbidites were coeval with early Pliocene high stands of sea level, whereas at Site 299, Pleistocene sea-level fluctuations are considered minor because at low stands of sea level, both relief and denudation rates were increased by about ten to 14%. At Site 286 (New Hebrides Basin), Eocene turbidite deposition is coeval with high stands of sea level, whereas at Site 297 (Northern Shikoku Basin), turbidite deposition was coeval with both rising and falling sea level.Analysis of both frequency and periodicity of turbidites by fan subenvironment at Site 299 indicates a record of continuous deposition, and maintainance of frequency and periodicity trends controlled by tectonic uplift. Late Pleistocene channel and overbank deposits showed periodicity differences of less than 28% of an order of magnitude, whereas Miocene-Pliocene overbank and distal turbidite periodicities differed by a 19% order of magnitude. Greater differences in magnitude occurred between distal turbidites or early Pleistocene age and Pliocene age than between Miocene-Pliocene overbank and distal turbidite deposition with a magnitude difference of 860%. These findings suggest that shifting depocenters and differences in sedimentation history in subenvironments of submarine fans are secondary to the role of tectonic uplift in this particular case.The minimal rate of tectonic uplift required to generate deep-sea fan turbidities appears to be approximately 400 m/million years. This figure is tentative and is based on very few observation points.Frequency and periodicity of preserved turbidite cycles in submarine fans in active continental margins and ancient counterparts should provide an independent measurement of rates and timing of tectonic uplift, particularly in collision terrains. Because this sediment parameter is a record of a single process from a single source and a record of “event stratigraphy”, its usage is preferable over standard and bulk sediment accumulation rates determined from age depth curves.

Computer Simulation of Marine-Organism Community Environments: ABSTRACT

Computers can be used to re-create the behavior of ancient marine-organism communities with surprising effectiveness. Organism communities and their environments have been represented symbolically in a three-dimensional mathematical model embodied as a series of computer programs for IBM 7090/7094 computers. Factors affecting environmental conditions, such as depth of water, distance from shore, water turbulence, deposition of sediment, and salinity, can be adjusted by changing the numbers fed into the End_Page 616------------------------------ computer, thus regulating the model. By advancing the model through increments of geologic time, the response of the organism communities can be observed. This approach provides an experimental means of dealing with problems in which experimentation has, heretofore, been largely or wholly closed. The model may be used by initially populating the sea floor with organism communities whose behavior is to be studied. Each community is assigned certain properties which affect its response to different water depths, presence of mud and sand, and other environmental factors. These properties can be finely adjusted so that the communities behave more or less like their actual ancient counterparts, competing with each other and adapting to changing conditions. The adjustments can be made on a trial-and-error basis until satisfactory results, as determined by comparison with observed distribution of fossils, are obtained. Recent work with the model has dealt with environmental responses of leaf-like calcareous algae of the late Paleozoic. These algae were widespread in shallow Pennsylvanian and Early Permian seas, locally creating thickened banks or reefs. Today, some of these algal deposits serve as large oil reservoirs in southeastern Utah, northern Oklahoma, and West Texas. Exploration for these reservoirs will be enhanced if the environmental response of the organisms that created them can be determined experimentally and this knowledge used effectively in an exploration program. End_of_Article - Last_Page 617------------

Isoprenoid Hydrocarbons in Recent Sediments: Presence of Pristane and Probable Absence of Phytane

Pristane (2,6,10,14-tetramethylpentadecane) has been isolated from two recent marine sediments. Unlike their ancient counterparts, these sediments contain no detectable phytane. These two facts suggest a biochemical origin for at least a fraction of the sedimentary pristane and a later, geochemical formation of phytane. Commercial reagent-grade solvents (pentane, isooctane, methanol) contain appreciable quantities of fossil pristane and probably phytane.

Bar-Finger Sands of Mississippi Delta: ABSTRACT

Elongate, lenticular sand bodies, termed bar fingers, characterize the Mississippi birdfoot delta sedimentary complex. They underlie the 15-20-mile-long principal distributaries or passes of the river which radiate from Head of Passes, and have formed in response to long continued distributary-mouth-bar deposition. These sand bodies attain a thickness of more than 250 feet and a width of as much as 5 miles. Their thickness results in part from subsidence brought about by compaction of underlying clays, and the width beneath a given pass is comparable to that of the actively forming distributary-mouth bar. The sand bodies comprise beds of well sorted, fine to very fine sand or silt and occasional thin layers of clay and silty clay. Diagnostic features are numerous cross-be ded thin layers in which the principal elements dip gulfward, laminae composed of root and wood fragments, sand-size grains of lignite, and an absence of faunas. The bar-finger sands grade downward into delta-front silty clays which rest upon deeper marine prodelta clay deposits. Laterally they interfinger with extensive thick clay sections which accumulated in delta-front, bay, and marsh environments. The sands are transitional with overlying natural-levee, marsh, and bay deposits. Locally, the bar fingers have been disturbed by the upward movement of clays from underlying deposits to form mud lumps. Bar fingers are distinguished from other sand bodies in the Recent deposits of the Mississippi deltaic plain by their greater thickness and their pattern of distribution. Their ancient counterparts have been recognized in the Pennsylvanian Booch sands of Oklahoma. Abnormally thick sand masses present locally within the Eocene Wilcox and Sparta formations of Louisiana may also be deltaic bar-finger sands. End_of_Article - Last_Page 1249------------