Flank Deposits Research Articles

Isolated stationary carbonate platforms: the Middle Triassic (Ladinian) of the Marmolada area, Dolomites, Italy

ABSTRACTAs a result of a phase of extensional tectonics in the western Tethyan region, a horst and graben topography formed during the Middle Triassic (Ladinian) in northern Italy. Horsts were sites of shallow water carbonate sedimentation, while pelagic and volcaniclastic sediments were deposited in the grabens. Two carbonate platforms approximately 500 m thick can be distinguished in the Marmolada area of the Dolomites: the Marmolada platform proper, which covered an area of 6 km2, and the Costabella platform, which extended for about 12 km in a NW‐SE direction and was about 3 km across.The facies of these isolated platforms reflect the influence of storms from the SW. Windward platform margins were characterized by a marine sand belt of skeletal and aggregate grainstones with a dominant platform directed cross‐stratification. The central portions of the platforms were occupied by supratidal sand cays which are made up of storm washovers. Leeward parts of the platforms are composed of shallow subtidal sand flat deposits. Laterally discontinuous reefs chiefly composed of various calcareous algae are developed at the outer margins of the platforms. Along windward margins, reefs may form a belt several hundred metres wide; along leeward margins their width is commonly reduced to some tens of metres. Foreslope talus breccias surround the platforms. Clinoform bedding showing basinward dips of 30°‐40° is typical of this facies belt, which is approximately 2 km wide. Basinal sediments, only some tens of metres thick, are radiolarian micrites. Abundant sediment‐gravity‐flow deposits expand the basinal sequence at the toes of windward margins and were probably triggered by storm return flows.Synsedimentary faults striking both NNE‐SSW and NW‐SE separate the bedded platform limestones from flank deposits and reefs. They account for the stationary nature of the platforms. Neptunian dykes show preferred NNE‐SSW and E‐W trends. Sinistral displacements are associated with NW‐SE trending faults. Depressions in the basins, filled with red, turbiditic pelagic sediments, show N‐S trends and are probably compressional in origin. The structural pattern may have resulted from oblique, NW‐SE oriented extension of the E‐W trending Middle Triassic graben zone of the Dolomites.In the Ladinian of the Dolomites, the stationary platform type can be distinguished from a retrograding type, whereas continuously prograding platforms apparently did not develop.

Facies variation in Waulsortian buildups, Part 1; A model from Belgium

AbstractA composite cross‐sectional model of a Mid‐Dinantian Waulsortian buildup and its lateral equivalents is assembled from 10 measured sections. Five of these are in the Furfooz reef southeast of Dinant and the others, well correlated, are in non‐reef facies in the Dinant and Namur synclinoria (ranging up to an original 100 km or more from the area of the buildups).This model provides the framework for a petrographic study (over 700 samples) in which 25 components, mainly allochems, were recorded. The distributions of 15 of the most interesting of these are figured and discussed. Major patterns, extending well beyond the limits of the Waulsortian facies, can be discerned. Four component assemblages (A‐D) are distinguished. Broadly arranged in strati graphic order (A at the bottom; D at the top), their constituents are.Echinoderm (mainly crinoid) debris, fenestellid bryozoans, and ostracods.As A plus sponge spicules (notably the large, hyalosteliid ones) and moravamminids.As B plus plurilocular foraminifera, small gastropods, intraclasts and peloids.As C plus micritization, cryptalgal coatings on grains, and calcareous algae.The simple A assemblage typifies the classic Waulsortian facies–and many of its lateral equivalents. The others have increasing compositional diversity, with B and C occurring in the Waulsortian buildup facies and assemblage D mainly limited to flank deposits.Many components of these assemblages (calcareous algae, micritization, plurilocular foraminifera, etc.) have distribution patterns showing marked diachronismdownslopeon the flank of the buildup. This is interpreted as representing a lowering of critical depth thresholds for individual components due to a major drop in sea level late in Tournaisian times. Different components shift downwards ‘in step’ at slightly different stratigraphic levels and this allows construction of a relative depth scale. Taking the lower limits of the calcareous algae as a reference level (0 m), other lower limits are, approximately: micritization and cryptalgal coating (‐90 m), plurilocular foraminifera (‐120 m), hyalosteliid sponges (–150 m). This relative scale is tentatively converted into a water depth scale using as criteria the distribution of the calcareous algae, the limit of micritization and cryptalgal coating, and evidence indicating that the upper parts of the buildup were in the shallows at the end of Tournaisian times when the buildup had a sea floor relief of about 220 m.Changes in the depositional environments throughout the formation of the buildup are then traced, from initiation of growth in subphotic depths, probably exceeding 300 m, through shallowing phases possibly leading to emergence.Acknowledgements.We gratefully acknowledge: E. Groessens of the Service Géologique de Belgique for much unpublished information on conodonts, particularly for the sections at Pont‐à‐Lesse and Furfooz 2, 5, and 6; R. Conil and M. Laloux for help with various aspects of the micropalaeontology; V. Leclère for sampling and initial study of the Pont‐à‐Lesse section; L. Hance for information on biostratigraphic problems in the Lower Viséan and for useful comments on the manuscript; and M. Hennebert and J. Miller for critically reading the manuscript.

Late Paleozoic Foraminifera as Depth Indicators: ABSTRACT

Many late Paleozoic foraminifera consistently occur in particular rock types for which depths of deposition may be inferred. A few genera appear randomly in many different rock types and were possibly pelagic. Textulariina: In the Ammodiscacea, most genera of Hippocrepininae (Astrorhizidae) and Ammodiscidae are widespread and End_Page 542------------------------------ common in dark gray shales, siltstones, fine to very fine sandstones, and silty, calcareous mudstones deposited in basinal, slope, and deeper shelf setting below effective wave base. In the Lituolacea, the Hormosinidae, Lituolidae, Textulariidae, Trochamminidae, and Ataxophragmiidae include only a few late Paleozoic genera, which locally are abundant in dark to medium gray, silty shales, siltstones, silty sandstones, and carbonate wackestones and packstones; all deposited near wave base. Miliolina: Late Paleozoic representatives are primitive (tubular, nonseptate) genera of Hemigordiopsidae, Fischerinidae, and Nubeculariidae. These occur mainly in shallow, warm water calcareous wackestones. Fusulinina: These were the most taxonomically diverse of the late Paleozoic foraminifera and were adapted to a wide range of depth habitats. Parathuramminacea locally were very abundant in deeper water, dark gray calcareous wackestones formed in basins and slopes below wave base. Endothyracea (s.l.) included many genera and families that dominated most of the Early Carboniferous shallow water, calcareous depositional environments. Nodosinellidae preferred open shelf facies and may have extended to depths below wave base. Colaniellidae, Ptychocladiidae, Paleotextulariidae, Tetrataxidae, Tournayellidae, Endothyridae, Loeblichiidae, and Lasiodiscidae locally were common in shallow shelf, shoal, and lagoonal carbonate wackestones, packstones, and as displaced fossils in some grainstones, uch as oolites. Bradyinidae were globose, had pseudoalveolar walls, and were widely scattered in a number of different lithologies suggesting a pelagic or planktonic habitat. Archaediscidae, which have recrystallized wall structure, were common in shallow water, carbonates and calcareous shales. Fusulinacea, most of which probably had photosynthetic symbionts, were adapted to shallow carbonate depositional habitat at depths less than 15 to 20 m (49 to 66 ft). Carboniferous Fusulinidae and Ozawainellidae apparently occupied most of this depth range because of their adaptation to Middle Carboniferous cool surface waters. Permian Schubertellidae and Oazawainellidae became adapted to shallow, warm water lagoons and shelves. Verbeekinidae and Neoschwagerinidae were common in reef cores and upper flank deposits of Permian Tethyan reefs. Schwagerinidae also adapted to shallow to very shallow water carbonate environments, such as reef edges, shallow lagoons, tidal flat channels, margins of algal shoals and banks, and other shallow nearshore areas. For example, Eoparafusulina formed e tensive skeletal grainstones in many cross-bedded, subtidal deposits. Several globose lineages within the Fusulinacea possibly were pelagic, such as Robustoschwagerina, Pseudoschwagerina, Verbeekina, and many of the Permian Staffellidae. End_of_Article - Last_Page 543------------

Phosphorus accumulation rates in metalliferous sediments on the East Pacific Rise

Based on phosphorus, iron and manganese analyses in 16 cores (5 dated) from the crest and flanks of the East Pacific Rise and the Bauer Deep we estimate that phosphorus is being deposited about 20 times faster in metalliferous sediments near the rise crest than in adjacent flank deposits, and about 40 times faster on the crest than in the Bauer Deep. Almost all of the phosphorus on the rise crest is contained in poorly crystallized hydrothermal iron oxyhydroxides, supporting Berner's (1973) proposal of phosphate sorption by these phases. The phosphate is probably derived from seawater, but some hydrothermal contribution cannot be excluded at this time. Flux estimates indicate that metalliferous sedimentation could remove 15–40% of the pre-agricultural river input of dissolved phosphate.

アルミニウム-珪素系鋳物合金の切削における工具付着物に関する検討

During the cutting operation, the authors observed the deposits of the tool face and flank by an X-ray microanalyzer and the like on the aluminium-silicon casting alloys which showed comparatively severe tool wear, generated much tool deposit and were difficult to obtain good finished surfaces among aluminium alloys, and then searched for the relationship of tool wear to deposit formation. Cutting specimens are F-material which are cast metallic mold and T6 treatment material after casting of AC4B (Copper silumin) and AC8A (Lo-ex), and the tests consist of dry cutting with fhe sintered carbide (P20) and high speed steel (SKH4) tools. The results obtained are as follows.Flank deposits are divided into the stratum type and the ribbon type which is followed by rapid growth. The stratum type deposit is formed with flank wear as its basis and adhere rigidly to the tool flank 2 to 4 times greater than the flank wear width. The ribbon type deposit grows rapidly as the cutting time elapses at a speed faster than the cutting speed at which the built-up edge disappears, and the faster the cutting speed the more remarkable the growth of the ribbon type deposit. In this deposit T6-material generates earlier than F-material, but it is easier to generate F-material in terms of flank wear width. The flank deposits are divided into the stratum type part containing a large a mount of Al, the part with piled-up Si, Mg, Ni, etc. and the part to which Al seems to have readhered. On the other hand, the tool face deposit protects rake face as a comparatively thick stratum for a low cutting speed, but for a high cutting speed, adhesion wear advances and at that time Si particles may play an important role in the advance of wear.

Age and Stratigraphic Implications of Major Fault in Llano Region, Central Texas: GEOLOGICAL NOTES

Illustrative of the northeastward-trending normal faults in this region is the Hall uplift, a horst that straddles the eastern boundary lines of McCullough County. Fault related flank deposits occur 2-3 miles west along a low escarpment of Smithwick(?) shale and Strawn sandstone capped by flat Cretaceous deposits. The flank deposits consist of limestone cobble and chert pebble conglomerate, which occurs just above and is interbedded with clay shale referred to as Smithwick(?) shale, and is believed to represent downslope failure of structurally elevated detritus. The fossil assemblage indicates a lower Strawn assignment of the beds in question. Similar deposits may occur on the downthrow side of other faults.

Geometry, Paleontology, and Petrography of Thornton Reef Complex, Silurian of Northeastern Illinois

A detailed study of the geometry, paleontology, and petrography of a Silurian Reef Complex, which lies about 10 miles southwest of the southern tip of Lake Michigan, demonstrates that the complex was a true reef 575 feet thick at its maximum development. The reef was characterized by an organic framework or core and a surrounding, preponderantly bioclastic, flank deposit on which many satellite reefs developed. Initial reef development was probably from coalescing coral growth centers on the floor of a Silurian epicontinental sea. Upon attainment of wave base by the beginning of the Niagaran Epoch, a prevailing west-by-southwest wind, in conjunction with a surrounding water depth of about 200 feet, controlled the development of a definite southwest-to-northeast ecologic zoning in the form of distinct biotopes and lithotopes. The reef core, represented by a preponderance of skeletal material in growth position, is in successive order the site of a windward wave-resistant stromatoporoid ridge, a coral rampart, a dead-reef flat, a lagoon beach, a lagoonal crinoid meadow, and a leeward wave-resistant stromatoporoid ridge. The core is surrounded by predominantly bioclastic flank deposits, consist ng of rhythmic alternations of fossil fragments, calcareous muds, and argillaceous sediments. The flank was the site of organic communities dominated by brachiopods, gastropods, or sponges. Other areas of the flank were characterized by a lack of a distinctive fauna. Continued rise of sea-level allowed further upward and lateral reef growth with the result of an atoll stage of reef development by early Cayugan time.