Large Carbonate Platform Research Articles

EVIDENCE OF A GUADALUPIAN AGE FOR THE KHUFF FORMATION OF SOUTHEASTERN OMAN: PRELIMINARY REPORT

The Guadalupian succession of the Huqf area (Sultanate of Oman) represents a mega-sequence comprising the fluvial terrigenous Gharif Formation and the overlying marine Khuff Formation. The Khuff Fm. is subdivided into four members and is composed of marls and bioclastic limestones. The Khuff Fm. yields a rich fauna of brachiopods, conodonts, foraminifers, bivalves, gastropods, ostracods and cephalopods. The brachiopod fauna of the Khuff Fm. includes strophomenids, productids, orthids and terebratulids. The associated conodont fauna includes Hindeodus excavatus Behnken, Merrilina sp., M. praedivergens Kozur & Mostler, and Sweetina n. sp. (systematic descriptions of conodonts are given in the Paleontological Appendix). Foraminifers are represented by species of Miliolina and Rotaliina. The Khuff Fm. is given a Wordian age, based on brachiopods and conodonts. The depositional environment of the Khuff Fm. of southeastern Oman corresponds to the outer shelf of a large carbonate platform covering most of the Arabian Platform. The Khuff Fm. is interpreted as a major transgressive-regressive cycle related to differential subsidence.

The role of high-energy events (hurricanes and/or tsunamis) in the sedimentation, diagenesis and karst initiation of tropical shallow water carbonate platforms and atolls

Karst morphology appears early, even during carbonate sediment deposition. Examples from modern to 125-ka-old sub-, inter- and supratidal sediments are given from the Bahamas (Atlantic Ocean) and from Tuamotuan atolls (southeastern Pacific Ocean), with mineralogical and hydrological analyses. Karstification is favoured by the aragonitic composition of bioclasts coming from the shallow marine bio-factory. Lithification by aragonite cements appears as a rim around carbonate deposits and dissolution and non-cementation start at the same time on modern supratidal deposits (Andros micrite or atoll coral rudite) and provoke the formation of a central depression on small or large carbonate platforms. In fact, this early solution of the centre of platforms is closely related to the location of each of the studied examples on hurricane tracks. High-energy events, such as hurricanes and tsunamis, affect sediment transport but hurricanes also affect diagenesis as a result of the enormous volume of freshwater carried and discharged along their paths. This couple, lithification–solution, is localised at sea level and accompanies sea-level fluctuations along the eustatic curve. Because of the precise location of hurricane action all around the Earth, early karstification by aragonite solution, cementation and supratidal carbonate sediment accumulations (high-energy trails) act together on all the platforms and atolls located inside the Tropics (23°27′) between roughly 5°–10° and 25° on both hemispheres. However, early karstification acts alone on shallow carbonate platforms including atolls along the equatorial belt between 5°–10°N and 5°–10°S. These early steps of karstification are linked to the ocean–atmosphere interface due to the bathymetrical position of shallow carbonate platforms, including atolls. They lead to complex karstified emerged platforms, called high carbonate islands, where carbonate diagenesis, together with the development of bauxite- and/or a phosphate-rich cover and phreatic lens, will occur.

Controls on facies architecture of a large Triassic carbonate platform; the Great Bank of Guizhou, Nanpanjiang Basin, South China

ABSTRACT The Great Bank of Guizhou (GBG) is an exceptionally well exposed isolated Triassic platform in the Nanpanjiang Basin of South China. The platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth, evolution, and demise. The GBG formed near the southern margin of the Yangtze Platform during a deepening event that expanded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian. Initial accumulation of the GBG began in the latest Permian with small reef mounds and open-marine skeletal packstones. Following the end-Permian extinction, cyanobacterial boundstones grew over the bank top. During the Early Triassic the GBG developed a low-relief bank profile with mobile oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and gentle slopes dominated by pelagics, debris-flow deposits, and turbidites at the basin margin. In the Middle Triassic (Anisian) the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose. In the Middle Triassic (Ladinian) an erosional escarpment up to 1700 m high developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior, producing an atoll-like morphology. Later, a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening contributed to termination of the GBG before siliciclastic turbidites and shales were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundant muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture. The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries. The GBG's larger size increased mud production and protected it from extensive winnowing of mud, which in turn resulted in off-bank shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes which progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their angle-of-repose geometries.

Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China

Controls on Facies Architecture of a Large Triassic Carbonate Platform: the Great Bank of Guizhou, Nanpanjiang Basin, South China

On the secular variations in the composition of Phanerozoic marine potash evaporites

In a recent paper, L.A. Hardie proposed that the secular changes in the mineralogy of marine nonskeletal limestones and in the mineralogy of marine potash evaporites during the Phanerozoic are the result of changes in the composition of seawater caused primarily by fluctuations in the flux of seawater through mid-ocean ridges. He suggested that even quite small variations (25%) in this flux have a profound effect on the composition of seawater. We show that the effects of changes in the hydrothermal flux are much smaller than he proposed. Nevertheless, a doubling of the hydrothermal flux could probably alter the composition of seawater sufficiently to affect the mineralogy of marine potash evaporites. An alternative explanation for the variations in the mineralogy of these deposits invokes differences in the degree of dolomitization during the evaporation of seawater. The apparent near constancy of the K{sup +} content of seawater during the Phanerozoic supports this hypothesis. It seems likely that during periods of rapid seafloor spreading the rate of seawater cycling is greater than today, that sea level is higher, that very large carbonate platforms are more abundant, and that changes in seawater chemistry together with increases in dolomitization during the evaporation of seawatermore » lead to MgSO{sub 4}-free potash deposits. The relative importance of these several effects can probably be determined with certainty only when we have analyses of relatively unaltered seawater from the several Phanerozoic periods. 27 refs., 1 fig., 1 tab.« less

Rotational deformation during Palaeogene thrusting and basin closure in eastern central Greece: palaeomagnetic evidence from Mesozoic carbonates

SUMMARY Palaeomagnetic studies in Greece have concentrated upon Neogene sequences, with Mesozoic rocks of mainland Greece receiving little attention. The Neogene data base is now sufficiently extensive, however, to allow meaningful interpretation of older units. This paper presents new palaeomagnetic data from Mesozoic and Tertiary sequences of eastern central Greece, which document a pre-Neogene phase cf rotational deformation. Mesozoic carbonates have been sampled within the classic Pelagonian Zone in the Argolis Peninsula, Beotia and Evvia. Pelagonian sequences record the development, drowning and subsequent re-establishment of a carbonate platform separating two strands of the Neotethys Ocean (the Pindos and Vardar basins). Most platformal facies are tdo weakly magnetized for palaeomagnetic analysis. However, pelagic horizons consisting of pink condensed limestones have been deposited periodically in response to platform subsidence. These Middle-Late Triassic, Jurassic and Upper Cretaceous sequences yield high-quality palaeomagnetic data, which reveal a consistent clockwise rotation of about 94. The constancy of rotation angle deduced from the Mesozoic rocks suggests that the total Tertiary rotation is uniform throughout the area studied. This implies that observed variations in declination within the Neogene sequences result from sampling of rocks of different ages deposited during a period of rapid overall rotation. Removal of the largest Neogene rotation (45) from the Mesozoic data set reveals a pre-Middle Miocene, post-Late Cretaceous clockwise rotation of the southern Pelagonian Zone of 50. The major tectonic event during this time period was the closure of the Pindos Ocean during the Eocene. A large carbonate platform within the Pindos basin, now preserved in the Parnassos Zone, presented a major obstacle to the advancing Pelagonian nappes during SW-directed thrusting (present coordinates). Impingement of the nappes on to the Parnassos platform resulted in pinning and subsequent clockwise rotation of thrust units around the south-eastern margin of the platform. The southernmost tip of the Pelagonian Zone (southern Argolis) was detached along a pre-existing line of weakness along the present Migdhalitsa Graben, and experienced a different rotation history. Additional palaeomagnetic data from the Pindos nappes in the eastern Peloponnesos also provide evidence for significant rotation of thrust sheets during Eocene basin closure. The Pindos sequences formed by pelagic sedimentation within the Pindos Ocean. An overall anti-clockwise rotation with respect to the underlying Gavrovo-Tripolitza autochthon is implied by the data. This probably reflects oblique emplacement of the Pindos thrust sheets on to the adjacent platform. Variations in rotation angle probably result from variations in footwall topography.

Numerical Models of Fluid Flow in Carbonate Platforms: Implications for Dolomitization

ABSTRACT The role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models. A two-dimensional, finite-difference code (Bethke 1985) was used to simulate single-phase flow of variable-salinity fluids driven by compaction, convection, and topography in carbonate platforms and adjacent basinal sediments. A simple hydrostratigraphic model consisting of a platform-reef complex surrounded by basin-filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence. Most pore fluids in the shale migrate to the surface at velocities less than 0.1 mm/yr and basin-derived fluids are focused into the reef from only a limited distance of 10 km. Relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform-reef complex. Magnesium mass-balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization. Two-layer hydrostratigraphic models involving variable-salinity fluids show that evaporation of seawater results in downward and seaward fluid migration; the magnitude of flow is a function of fluid density. Even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments, albeit at rates of less than 1 cm/yr. Paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface-derived waters evaporated past gypsum saturation is a viable dolomitization mechanism. Calculations utilizing rates of Mg mass flux show that dolomitization of large carbonate platforms (>10 km3) by fluids below halite saturation requires millions of years of reflux-driven flow. /P> Thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow. The thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75.0 and 13.0 mm/yr. respectively. The potential for dolomitization is high because a large reservoir of Mg (seawater) is tapped, elevated temperatures drive fluids further into the stability field for dolomite, and kinetic inhibition of dolomite nucleation and growth is decreased. Simulations of high-frequency glacio-eustatic sea-level oscillations suggest that a dynamic hydrologic regime may be established. A sea-level fall of 2 m lasting for 10,000 yr can drive lateral flow at rates up to 0.15 m/yr. Dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea-level fluctuations.

Submarine Cementation of Subsurface Pliocene Carbonates from the Interior of Great Bahama Bank

ABSTRACT Thick intervals of early Pliocene subtidal carbonate sediments in the subsurface of northwestern Great Bahama Bank were lithified by submarine cement. This cementation, subsequently altered by meteoric diagenesis, occurred over a broad expanse of the platform interior. It shows that submarine cements can lithify thick packstone and grainstone intervals in the interior of large shallow-water carbonate shelves and platforms given open circulation, moderate to slow sedimentation rates, and minimal accumulation of fine sediments. A submarine origin for these cements is inferred from direct and circumstantial evidence, including: (1) regional depositional and diagenetic setting; (2) fabric and habit similar to modern submarine cements; (3) alteration of cements from mineralogically unstable precursors indicated by (a) abundant inclusions, (b) relict precursor fabric, and commonly (e) yellowish coloration, (d) undulose crystal extinction, and (d) selective leaching of specific generations of cement or cement types; (4) sutured mid-pore crystal contacts; (5) precipitation coeval with accumulation of internal marine sediment; and (6) association with organisms requiring a firm substrate for attachment. These considerations may help other workers recognize submarine cements in ancient carbonate rocks.

Upper Cretaceous Stratigraphy of the Island of Brac within the Geodynamic Evolution of the Adriatic Carbonate Platform

Though news coming out of Yugoslavia these days is generally bad, this unusual book is a welcome exception. The work was stimulated by a project develop a new geologic map of Croatia; the result is a highly readable book, with the authors' senses of history, curiosity, and humor readily evident throughout. The purpose, as stated in the Introduction, is to rehabilitate the concept of (p. 128) from dull, descriptive cataloging of names of units an approach stratigraphy that is dynamic, interpretive and set in a global context. The authors recognized that their contribution could not be fully utilized by both local and international audiences unless their results were published in Croatian and in English. The book should be useful for geologists in at least four categories: (1) the work provides a synthesis of Upper Cretaceous stratigraphy of a sizeable portion of the Adriatic carbonate platform and will be a valuable resource for specialists; (2) the Croatian text will be an important educational tool in Yugoslavia, presenting both lithologic and paleontologic details and a philosophical approach stratigraphic research; (3) the English text provides the international audience with a demonstration of stratigraphy as an exciting, dynamic, and interpretive science; and therefore (4) will be useful as a thoughtprovoking reading in advanced stratigraphy courses. The book includes 160 pages of text (20 x 29 cm format) with 22 figures including three foldouts, and 20 plates. Figures, with both Croatian and English captions, are located in the 102 pages of Croatian text. The English text covers 32 pages. The first two sections, the Introduction and the Short Review of Previous Stratigraphic Research on the Cretaceous of Brac, are relatively brief and nearly comparable in length and content in both Croatian and English. Section 3, Description of the Lithostratigraphic Units, and the discussion sections that follow, are about 40 percent as long in English as in Croatian. Detailed arguments concerning history and choice of formation names are only available in Croatian. Furthermore, consistent with the educational and philosophical aim of the Croatian text, more extensive general discussions of sedimentological and paleoecological topics are included there than in English. In Section 3, lithofacies and biofacies of each of the six Upper Cretaceous formations outcropping on the island are described; brief descriptions of these units and the authors' interpretations of them are presented below. Biofacies descriptions and interpretations emphasize foraminifera, mollusks, algae, and cyanobacteria. Biostratigraphic interpretations are based upon foraminifera in cases where differences in age interpretations have arisen between diagnostic foraminifera and rudists. The authors utilize both modern and Cretaceous carbonate facies models, as well as ecological studies of modern foraminifera and mollusks, in developing paleoenvironmental interpretations of facies within formations. During the Upper Cretaceous, what is now the Island of Brac was located in the interior of the large Adriatic carbonate platform. The oldest of the Upper Cretaceous units on Brac is the Cenomanian age Milna Formation, which is on the order of 370 m in thickness. The unit is characterized by alternation of three predominant facies: grain-supported packstone/grainstone, peloidal-skeletal wackestone, and micritic and cryptalgal laminites. The paleoenviromental interpretation is restricted, muddy platform-interior setting as indicated by intertidal laminites and shallow subtidal Chondrodonta oyster beds and radiolitid biostromes, and by frequent storm beds. The predominantly mud-supported limestones of the Sveti Duh Formation, illustrated as 140 m in thickness, conformably overlie the Milna Fm. Where the transition is gradual, intercalations of micrite repeatedly alternate with cryptalgal laminites in the top 10 m of the Milna and the boundary is arbitrarily placed after the last occurrence of laminite. Sveti Duh mudstones contain calcispheres and small planktonic foraminifera, as well as other fossil debris in varying proportions. In some cases, concentrations of fossil debris form bioclastic calcarenites that appear be allochthonous gravity-displaced turbidites. The Sveti Duh Fm is interpreted represent hemipelagic deposition associated with the Latest Cenomanian-Early Turonian global eustatic sea level rise. Unfortunately the authors do not address two interesting aspects of this unit. They do not speculate on how hemipelagics might alternate with laminites, interpreted as intertidal. Neither do they suggest an origin for the gravitydisplaced shallow-water bioclastics. However, these are among the few such anomalies that they do not consider, which is partly why the book is so provocative. The third major unit, the Gornji

Stratigraphic Response of a Carbonate Platform to Relative Sea Level Changes: Broken Ridge, Southeast Indian Ocean (1)

In contrast to terrigenous margins, the stratigraphic response of carbonate platforms to relative sea level changes is influenced by carbonate productivity, dissolution, and diagenesis. Using seismic reflection and Ocean Drilling Program (ODP) drilling data from Broken Ridge, we assess the importance of these processes in controlling the stratigraphic response of a carbonate platform to relative sea level changes. Prior to middle Eocene rifting, Broken Ridge and Kerguelen-Heard Plateau constituted a large carbonate platform. ODP drilling data from the Kerguelen-Heard Plateau (Leg 120) and Broken Ridge (Leg 121) reveal that the carbonate platform had two major episodes of tectonic uplift. The early Maastrichtian uplift was confined to the southern section of the Kerguelen- eard Plateau (Sites 747, 748) and gently tilted the platform toward the north. The middle Eocene rift-induced uplift affected both Broken Ridge and the northern section of the Kerguelen-Heard Plateau (Sites 752-755). Watergun seismic reflection and drilling data indicate the following stratigraphic response of the platform to relative sea level changes. (1) Prograding clinoforms consisting of carbonate sediments were deposited during a long-term relative sea level rise. Submergence of the uplifted parts of the platform (southern Kerguelen-Heard Plateau) increased the area available for productivity allowing the carbonate sediment to prograde northward into the basin during the rise. (2) Onlapping sequences consisting of carbonate detritus were deposited in the basin during the middle Eocene relative sea level fall. The carbonate strata on Broken Ridge were uplifted to at least wavebase and eroded. The detritus was transported northward toward the basin and onlaps the preexisting strata. The approx mate volume balance between the sediment eroded from the crest of Broken Ridge and the onlapping sediments deposited along the northern slope suggests that mechanical erosion and transportation of carbonate sediment into the deep sea were important processes controlling the stratigraphy at Broken Ridge during the relative sea level fall. We propose that the stratagraphic response of Broken Ridge to the middle Eocene relative sea level fall is consistent with stratigraphic predictions based on terrigenous margins. However, during a relative sea level rise, biogenic carbonate production may allow carbonate sediment to prograde basinward despite the fact that water depths are increasing. Stratigraphic models based on terrigenous margins do not account for the increase in sediment supply along carbonate margins during a relative sea level rise, and thus require minor modification in order to predict the development of the prograding clinoforms.

Sedimentological investigations of the Multicoloured 'series' (Eleonore Bay Group, Late Precambrian) in the Scoresby Land – Andrée Land region, North-East Greenland

The approximately 1200 m thick Multicoloured 'series' (bed-groups 7–13) and adjacent units of the upper Eleonore Bay Group have been the subject of detailed sedimentological investigation during 1989 in the area between northern Scoresby Land and Andree Land in North-East Greenland. The Multicoloured 'series' consists of alternating units, 100–300 m thick, of carbonate and siliciclastic deposits which are uniformly developed throughout the central fjord zone. It represents the transitional stage from a pure siliciclastic shelf environment to a carbonate platform environment. During its development initial carbonate sedimentation was repeatedly terminated by drowning and siliciclastic deposition, until a large carbonate platform represented by the overlying Limestone-Dolomite 'series' was firmly established.

Tectonic and Ecologic Controls on Formation and Drowning of Isolated Carbonate Platforms: ABSTRACT

During the Early Jurassic, a large carbonate platform - the Calcare Massiccio platform - developed on the Italian continental crust. The subsequent opening of the Ligurian Ocean resulted in crustal thinning and subsidence of the part of the Italian Peninsula that is now the Northern Apennines. A complex network of intersecting normal faults fragmented the large carbonate platform, producing a group of small, isolated platforms separated by interconnected basins up to 1 km deep. The subsequent drowning of these isolated platforms had a profound effect on the later history of the basin and accounts for the important stratigraphic differences between the Northern Apennines and the adjacent Central Apennines. The sedimentary record indicates two stages of drowning: incipient and complete. After the isolated platforms formed, they entered a state of incipient drowning that lasted until the end of the Jurassic or the beginning of the Cretaceous, at which time they were completely drowned. The stage of incipient drowning was characterized by water depths within the euphotic zone and low rates of sediment production and accumulation. During this stage, a change in the ecology or environment could have returned the rates of carbonate production and accumulation to high enough levels to prevent more » drowning; but these rates remained unusually low, allowing the isolated platforms to drown completely. The prolonged state of incipient drowning indicates that crustal subsidence was not the sole cause of the drowning; the important causal factors are those that disabled the normally efficient system of carbonate production and accumulation. These causal factors can be roughly divided into tectonic factors such as the system of extensional faults related to the opening of the Ligurian Ocean, and ecologic factors such as the general scarcity of reef-building organisms during the Lower Jurassic, when these platforms were created. « less

Paleogeographic significance of a Permian sedimentary megamictite in the Central Belt of the northern Sierra Nevada

The Central Belt of the northern Sierra Nevada at Oroville Lake, California, includes large outcrops of Permian limestone as clasts surrounded by siliceous diamictite. Limestone ranges from pebbles to megaboulders as much as 100 m in size, and five clasts contain Wolf-campian to Leonardian corals and fusulinids. The diamictite which encloses limestone clasts has a pebbly mud texture and a chaotic fabric. Paleozoic megafossils occur in the mud matrix of the diamictite, and mixing of limestone clasts and diamictite was a Permian event. The diamictite and enclosed limestone megaboulders, collectively termed a “megamictite,” probably represent a base-of-slope sedimentary deposit. Limestone boulders and megaboulders are tabular stratigraphic fragments which detached from lithified carbonate escarpments and moved downslope by sliding along their flat surfaces. Lithologies of limestone clasts indicate derivation from a shallow carbonate platform. Corals and fusulinids within limestone clasts include endemic taxa found in the McCloud Limestone of the Eastern Klamath Mountains, and they represent a Permian biogeographic province distinct from those of western terranes of the Klamath Mountains and the North American craton. A large carbonate platform in the eastern Klamath area is proposed as the source of limestone clasts in the Central Belt at Oroville Lake.

Carbonate Platform Accretion by Cementation of Ribbons of Dune Ridge Sediments, Yucatan, Mexico: ABSTRACT

The north coast of the Yucatan Peninsula, Mexico, consists of an almost continuous dune ridge backed by a swamp/estuary system. Conditions peculiar to a platform of almost pure carbonate rock result in delivery to the dune ridge of large quantities of fresh to brackish ground water saturated with respect to calcium carbonate. This ground water, which passes beneath the swamp/estuary system, arrives at the dune ridge charged with CO/sub 2/ and with a head higher than mean sea level. The result, over practically the entire north coast of the peninsula, is that CO/sub 2/ is released from ground water as it arrives at the dune ridge. Concomitantly, CaCO/sub 3/ becomes supersaturated and precipitates in pore spaces in the beach sand to produce a 300-km ribbon of impermeable rock between layers of unconsolidated carbonate sand of unstable mineralogy. This rock layer separates a surface lens of salt water about 3 m thick from the deeper lens of fresh water. Regional conditions require that, deeper still, lies a classic salt water intrusion. They submit that the system they report here may be a common, hitherto unrecognized feature important in the stabilization of large carbonate platforms. One interesting characteristic of this system ismore » that it should result in protected lenses of carbonate sand of unstable mineralogy, which can become lithified at a later time and in a different chemical and physical environment. They welcome suggestions of possible analogs in the geologic record.« less

Initiation, Evolution, and Destruction of an Early Paleozoic Carbonate Shelf, Eastern North Greenland

In the early Cambrian of eastern North Greenland an initial peritidal carbonate platform developed following a transgression across a Precambrian craton. Subsequently, this was replaced by a terrigenous clastic shelf formed by shelf tilting, drowning offshore regions, and relatively uplifting shoreface areas so that clastics prograded shelfwards. In the late early Cambrian abrupt shelf drowning cut the supply of clastic material, and condensed phosphatic and glauconitic hardgrounds developed. Drowning may have been associated with initial shelf tilting, which proceeded to uplift the eastern shelf and caused substantial middle and late Cambrian erosion and produced a northwesterly prograding carbonate ramp wedge. A widespread early Ordovician transgression was followed by a large peritidal carbonate platform, which during the middle and late Ordovician evolved into a rimmed carbonate platform with occasional drowned interior. In the late Ordovician-early Silurian a peritidal carbonate platform developed, possibly by build-out from the rim and over the subtidal facies under the influence of a eustatic regression. Subsequently, the early Silurian shelf is comparable to a ramp as a composite series of sand shoals and drowned segments developed. Finally, the carbonate shelf was destroyed as it foundered into deep water, and terrigenous clastic turbidites covered it. Pinnacle reefs developed briefly at the beginning of subsidence.