Shelf Succession Research Articles

Tectonostratigraphic synthesis and structural style of the alborz mountain system in Northern Iran

Based on lithologic, structural and stratigraphic analyses of the rocks exposed in the Alborz poly-orogenic system of northern Iran, seven tectonostratigraphic and a number of metamorphic rock assemblages, including remnants of the Paleo-Tethys oceanic realm, are distinguished. The tectonostratigraphic units are: (1) an uppermost Precambrian to lower Ordovician epicontinental sequence; (2) an Ordovician to Devonian mafic magmatic assemblage; (3) a Devonian to upper Triassic continental shelf sequence; (4) an upper Triassic and lower Jurassic foreland siliciclastics succession; (5) a middle Jurassic to upper Cretaceous discontinuous epicontinental/continental shelf succession; (6) an upper Cretaceous and Cenozoic arc-type magmatic assemblage; and (7) the Tertiary and Quaternary synorogenic mostly siliciclastic molasse deposits. The metamorphic assemblages include the upper Paleozoic metamorphosed carbonates, volcanics, and siliciclastics of the Masuleh region, the upper Triassic-lower Jurassic slates and metagraywackes of the Binalood region, and the Paleozoic metavolcanics and metacarbonates of the Gorgan area (“the Gorgan schists”), which are all correlatives of the above tectonostrati-graphic units, as well as the Paleo-Tethys remnants of the Mashhad-Torbat Jam region in eastern Alborz, the Shanderman complex in western Alborz, and the Gasht complex in the vicinity of Masuleh. These rocks, structurally, form either single thrust sheets or complex duplex systems which are transported generally from north-northeast to south-southwest by numerous thrust faults. The thrust faults, which collectively constitute a composite antiformal stack, formed during the Cimmeride and Alpide orogenies.

Cementation in Oligo-Miocene non-tropical shelf limestones, Otway Basin, Australia

The Oligocene to Miocene Heytesbury Group of the Otway Basin, southeastern Australia, is a non-tropical carbonate shelf succession. This subsurface stratigraphic succession consists of bioclastic grainstones to mudstones containing bryozoa, benthonic and planktonic foraminifera, echinoderms, brachiopods, and molluscs. The basal bryozoa-dominated Clifton Formation was deposited in relatively shallow waters. The overlying Gellibrand Marl, rich in planktonic foraminifera and bryozoa, formed under deep waters. The uppermost unit, benthonic foraminifera-dominated Port Campbell Limestone, was deposited in a moderately deep, middle shelf environment. Cementation is limited and includes: (1) scalenohedral calcite (non-luminescent to bright, Mn- and Fe-poor, in inter- and intraparticle porosity); (2) blocky equant calcite (dull, Mn- and Fe-rich, in interparticle porosity); (3) syntaxial calcite overgrowths on echinoderm particles (non/bright/dull-zoned); (4) euhedral dolomite (in the interparticle pore space of the Port Campbell Limestone); (5) bladed-prismatic calcite (ferroan, in planktonic foraminifera); (6) fibrous calcite (non-luminescent, in intraparticle porosity); and (7) glauconite and iron-oxides (only in intraparticle porosity). All CaCO 3 cements are low-Mg calcites (< 4 mol% MgCO 3). The cements observed in the Heytesbury Group appear to have formed in three successive diagenetic environments: (1) early shallow burial (early Fe- and Mn-poor, non-luminescent syntaxial overgrowths and non-luminescent scalenohedral calcite, under oxidising conditions); (2) late shallow burial (bright, more Mn-rich syntaxial overgrowth zone, i.e., more than the initial non-luminescent one, and bright scalenohedral calcite, under moderately reducing pore water conditions); and (3) moderate burial (ferroan, dull blocky calcite and dull syntaxial overgrowth zone, under reducing conditions). Pressure solution was the main cement-producing mechanism, although seawater and aragonite dissolution could account for the precipitation of the early non-luminescent cements. Scarcity of early interparticle cements may have been caused by the calcite-rich character of the original sediments. The ferroan dully luminescent blocky equant calcite and syntaxial overgrowths, typically interpreted to represent products of deep burial in ancient carbonate successions, are observed in the shallow-buried Port Campbell Limestone.

Structural and tectonic evolution of the Humber Zone, western Newfoundland 2. A regional model for Acadian thrust tectonics

The Humber Zone of the western Newfoundland Appalachians represents a Cambrian‐Ordovician passive continental margin which was deformed in Taconian (mid‐Ordovician) and Acadian (Silurian‐Devonian) orogenic events. A deformation front is imaged in seismic reflection data offshore of western Newfoundland. Structures associated with this deformation front are exposed on Port au Port peninsula, where Silurian rocks are strongly deformed but Mississippian strata are flat lying, indicating that latest thrusting was Acadian. A gravity low in the Gulf of St. Lawrence corresponds to a sediment‐filled Acadian foreland basin. Previous models suggest that the on‐land shelf succession is autochthonous to parautochthonous. However, two Lithoprobe seismic reflection transects show sub‐horizontal reflections between 2 and 5 s two‐way travel time, which extend up to 85 km east of the thrust front. These are interpreted as autochthonous platform and basement. In this model, shallower reflectors and outcropping units include both allochthonous platform and basement, comprising the Acadian Port au Port allochthon. The Taconian Humber Arm allochthon was carried westward as a high structural slice during thrusting of this allochthon. No major structural discontinuity exists between Grenville age crystalline rocks of the Long Range massif and platform rocks interpreted as allochthonous in the northern seismic line. A monocline at the southern extremity of the Long Range probably represents an oblique or lateral hanging wall ramp above the basal detachment. Within the Long Range thrust zone at the western margin of the massif the Long Range thrust shows only a few kilometers of displacement. However, the Parsons Pond thrust, which we interpret to run offshore at Green Point, juxtaposes contrasting successions with different structural and thermal histories; it probably carries a much larger amount of the total displacement. The basal decollement of the Port au Port allochthon is therefore interpreted to pass beneath the southern part of the Long Range massif.

Sedimentary evolution and drowning of a passive margin shelf (Giumal Group; Zanskar Tethys Himalaya, India): palaeoenvironmental changes during final break-up of Gondwanaland

Abstract The Lower Cretaceous terrigenous shelf succession of the Zanskar Range (uppermost Spiti Shale and Giumal Group) testifies to major tectono-eustatic events which affected the Tethys Himalaya passive margin during final fragmentation of Gondwanaland. The sudden supply of sand-sized terrigenous detritus in the Lower Takh Formation, deposited in coastal to inner shelf environments probably around late Neocomian to Barremian times, documents a first stage of tectonic uplift and rejuvenation of the Indian continental block. Next, the overall deepening-upward trend recorded by the inner to middle shelf sediments of the largely Aptian Upper Takh Formation indicates relative sea-level rise, culminated with flooding, starvation and deposition of the glauconitic Labar La arenite. This tectono-eustatic event occurred probably around the Aptian-Albian boundary, and was followed abruptly by influx of abundant ‘trachytic’ volcanic detritus, recorded at the base of the Pingdon La Formation. Eruption of alkalic lavas derived from deep-seated mantle sources was confined within the Albian, when volcanic arenites continued to be deposited in inner to middle shelf settings. The local occurrence of up to very coarse-grained, coastal quartz arenites points to episodes of differential uplift and renewed erosion of the Indian Craton. Suddenly, the Zanskar clastic shelf was drowned. Widespread deposition of the Nerak glauco-phosphorite on the sediment-starved outer shelf in the early Late Albian was followed by very discontinuous sedimentation and major stratigraphic gaps at latest Albian to Cenomanian times. Largely palimpsest and condensed arenites (Oma Chu glaucophosphorite) were periodically reworked on the outermost shelf by intruding oceanic currents, which prevented deposition of micrite. Only some decimetres of pelagic oozes (base of the Fatu La Formation) could thus accumulate on the uppermost slope until the Early Turonian, when all across the Zanskar margin sufficient depth was reached to allow rapid sedimentation of foraminiferal mudstones on the upper slope (Chikkim and Fatu La Formations). A complex interplay of tectonic, eustatic and oceanographic processes concurred in the development of ‘drowning unconformities’ at the top of the Giumal clastic shelf. Rapid deepening and waning of both volcanic and quartzo-feldspathic terrigenous detritus are mainly ascribed to the global mid-Cretaceous sea-level rise and to rapid thermo-tectonic subsidence at the end of the short-lived Albian magmatic event, possibly related to a mantle plume rising beneath northern India. Intensification of the east-bound oceanic current off the north margin of India after the final break-up of Gondwanaland was responsible for continuous resuspension and minimal accumulation rates around the shelf-break. The associated coastal upwelling favoured impingement of the oxygen-minimum zone on the outer shelf, with glauco-phosphorite deposition coinciding in time with peak global transgressions and ‘anoxic events’ in the world oceans.

Reversals in the polarity of structural facing across an early ductile thrust; the Central Donegal Slide, northwest Ireland

AbstractThe Dalradian succession of central Donegal represents a tectono‐stratigraphic sequence everywhere separated into two distinct successions by the Central Donegal Slide. Each stratigraphic package consistently youngs away from this major D1structure which originated during lower greenschist facies metamorphism and corresponds with the development of locally preserved penetrative S1fabrics. The tectonic slide and associated reversal in the direction of structural facing are commonly marked by mylonites and tectonic schists, which have been subsequently folded and reactivated by kilometric‐scale sheath folds and the Ballybofey Nappe.Structural correlations with northwest Donegal suggest that the Central Donegal Slide relates to the Knockateen Slide, and represents an early thrust with top to the northwest sense of shear. The Central Donegal Slide cuts up‐section through the Middle Dalradian sub‐basin centred on southern Donegal, emplacing younger, anoxic basinal sediments and turbidites over an older, inverted, shelf succession. The location of the Central Donegal Slide adjacent to the margin of this sub‐basin suggests that the siting and form of ductile thrusts may be strongly influenced by the geometry of the pre‐existing sedimentary basin.

Correlation of the Hythe Beds Formation (Lower Greensand Group: early-mid-Aptian), southern England

The outcrop and sub-surface stratigraphy of the Hythe Beds Formation shows a coarsening-up cyclicity in the arenaceous successions of the western Weald Basin (Hants. & Surrey), and in the glauconitic/argillaceous deposits of Sussex/Kent. Correlation can be achieved through the identification of hiatus surfaces, the most distinctive of which are dark, smectite-enriched clays with belemnites and phosphate nodule beds. These beds are interpreted to be the product of transgressive conditions. Correlation may be difficult in clastic shelf successions: the use of distinctive transgressive event beds provides a model for other successions displaying similar facies.

Tectonic history of the central Humber Zone, western Newfoundland Appalachians: post-Taconian deformation in the Old Man's Pond area

In the Humber Zone of the west Newfoundland Appalachians, the Middle Ordovician Taconian orogeny led to emplacement of the Humber Arm and other allochthons above a rift and shelf succession of late Precambrian to Early Ordovician age. Later deformation reversed this stacking in the Old Man's Pond area, where shelf sediments are thrust over the Old Man's Pond Group. East-vergent folds were subsequently developed in association with a regional, west-dipping cleavage that overprints the Taconian structures and intensifies eastward. Later, a major episode of shearing along normal-sense, northwest-dipping shear zones juxtaposed the Old Man's Pond Group against metamorphosed rift-related sediments and volcanics of the Hughes Lake Slice. This extensional episode may be related to intrusive events in adjacent central Newfoundland. The shear zones are overprinted by "cross folds," west-vergent folds, crenulation cleavage, and thrusts. These late structures are inferred to be related to westward transport of the rift–shelf succession and the Grenville-age basement rocks of the Long Range massif above a detachment surface, probably in Devonian time. The structural history is difficult to reconcile with a single, Devonian, "Acadian" orogeny, but is consistent with published isotopic data from central Newfoundland that suggest continued protracted tectonic activity in the Humber Zone in Silurian time.

Late Triassic fluvial and marine shelf succession, Quebrada Doña Inés Chica, Atacama region, northern Chile

A 400-meter-thick volcanic and fine-grained clastic sedimentary succession in Quebrada Doña Ines Chica (26°07′S latitude; 69°20'W longitude) provides a record of Late Triassic deposition in the Atacama region of northern Chile. The strata are conformably overlain by fossiliferous marine limestones and sandstones of Late Triassic to Early Jurassic (Sinemurian) age which contain the oldest ichthyosaur remains known from Central and South America. The clastic succession is interpreted as coastal fluvial deposits, with the overlying limestones representing shelf deposits.

Sedimentological investigations of metamorphic clastics: the basal clastic rocks of the Brenner Mesozoic (Stubai Alps, Austria/Italy)

ABSTRACTSedimentological investigations were carried out in the basal clastic rocks of the Brenner Mesozoic in the Stubai Alps. These are Scythian in age and metamorphosed to a greenschist grade. The increase of temperature and pressure from north to south allows the metamorphic effects on Sedimentological features to be gradually traced into higher grade metamorphic areas. The use of various laboratory methods as well as profiling with lithofacies types shows the limitation of using only single methods in sediments suffering thermal and deformational overprint. Only a combined application of different methods allows the extraction of useful sedimentological information.Based on the results of these investigations, a fan delta to shelf succession is proposed for the basal clastic rocks. Petrographic analyses show the relation of the sediments to an underlying weathering horizon.

Stratigraphy and sedimentology of lower carboniferous (Dinantian) boreholes from West Co. Meath, Ireland

AbstractThe 2‐km deep Athboy Borehole (1439/2) together with the lower part of boreholes EP30 and N915 form a standard type section for strata of Dinantian (Courceyan to Asbian) age in west Co. Meath. Above a thin basal red‐bed siliciclastic sequence, the marine Courceyan shelf succession is almost 600 m thick. It comprises the Liscartan, Meath, and Moathill Formations of the Navan Group and the Slane Castle Formation of the succeeding Boyne Group. The shallow‐water limestones include micrites, oolites, and sandy bioclastic packstones and grainstones with subordinate skeletal wackestones and shales. Lateral facies changes from north to south in the Navan area suggest deepening across a shelf towards a depocentre further to the south around Trim.The deeper‐water Waulsortian Limestones of late Courceyan to Chadian age (Feltrim Formation, ca. 213 m thick) form a series of five sheet‐like mudbanks, interbedded with generally thin units of nodular crinoidal limestones and shales. The mudbanks are formed of bryozoan‐rich peloidal wackestones and lime‐mudstones with phase C and D components. Rare soft‐sediment breccias occur at the bottom and top of banks.The succeeding Fingal Group commences with a thin interval (3–20 m) of black shales, laminated packstones, and micritic limestones of Chadian age, the Tober Colleen Formation. This is followed by the Lucan Formation (Chadian to Asbian) predominantly of laminated and graded calciturbidites, laminated sandstones, cherts, and black shales, which is over 1300 m thick. Ten sedimentary units have been informally defined, based on lithofacies and facies associations. The oldest unit, the Tara Member, is characterized by proximal debris‐flow breccia deposits and nodular mudstones. A thick bioturbated micrite and shale unit (Ardmulchan Member) in the middle of the formation is overlain directly by a coarse oolitic and crinoidal grainstone unit (Beauparc Member). Near the top of the formation is a distinctive unit of coarse‐grained laminated sandstones and shales (Athboy Member). The highest rocks in the Borehole are clean thickly‐bedded limestones of the Asbian Naul Formation (&gt;90 m thick). The youngest Dinantian strata in the area, the Brigantian Loughshinny Formation, marks a return to shale‐dominant basin sedimentation.The significance of this work lies in the fact that the Athboy borehole is the longest continuously cored borehole in the Carboniferous of Ireland and provides a continuous sedimentary and biostratigraphic record for the northern part of the Dublin Basin. Foraminiferal biozones (Cf2–Cf6) have been recognized in this and in borehole N915, and Stage boundaries identified, which can be applied throughout the Basin. The sedimentary record for the Lucan Formation indicates four tectonic pulses during the Viséan, in the late Chadian/early Arundian, mid‐Arundian, Holkerian, and late Holkerian/early Asbian.

Origin of thick, first-cycle quartz arenite successions: Evidence from the 1.7 Ga Ortega Group, northern New Mexico

The 1700-Ma-old Ortega Group in northern New Mexico represents a > 1000-m-thick succession of metamorphosed quartz arenites and subordinate pelites. Published ages of detrital zircon grains from the Ortega Group indicate that the sediments were derived mainly from 1760-1700-Ma-old volcano-plutonic basement rocks and not from the Archean Wyoming Province to the north. Evidence for recycling within the Ortega Group also is lacking. Consequently, the Ortega Group quartz arenites are considered to be first-cycle sediments. The compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering. Abnormally high Al contents of pelites within the Ortega Group imply intense chemical weathering in the provenance similar to present-day low-relief tropical regions. Quartz arenites in the Ortega Group were deposited exclusively within a shallow-marine shelf environment influenced by tide, storm and wave processes. Prolonged sediment transport, especially on the tide-dominated inner shelf, in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains. Although the maturity of sediments in the Ortega Group can be accounted for, the mode of origin of > 850 m of quartz arenites remains enigmatic. Thick quartz arenite sequences of global distribution in the Phanerozoic coincide with first-order eustatic sea-level rises. World-wide occurrences of 1700-Ma-old, thick quartz arenite shelf successions suggest that the Ortega Group may have accumulated in response to a similar major eustatic sea-level rise.

Chadian faunas and flora from dyserth: Depositional environments and palaeogeographic setting of Viséan strata in northeast wales

AbstractNew floral and faunal data from the oldest Dinantian limestones (Foel Formation) in the Dyserth area, suggest that these sediments are of Chadian age, rather than the Asbian age concluded by earlier workers. The basal late Chadian limestones rest conformably on Dinantian Basement Beds of ?Chadian age or older.The initial inundation of St. George's Land occurred during Chadian times, when shallow‐water marine limestones accumulated in the Dyserth area and further to the south, together with terrestrially derived siliciclastics, containing drifted plant fragments. Periodically, a restricted hypersaline lagoonal environment was established but an open marine, neritic environment with abundant stenohaline fauna prevailed in this area.These Chadian sediments accumulated on the proximal part of a carbonate ramp and are presumed to have passed laterally downslope into deeper water basinal facies with Waulsortian buildups of the Irish Sea Basin. In the later Arundian, a carbonate ramp–to–platform transition occurred, with widespread deposition of shallow‐water carbonates. In the Asbian this platform developed a rimmed margin, with buildups forming a linear belt between platform and basin.An almost complete Chadian to Brigantian Lower Carboniferous sequence can now be recognized in North Wales. This succession is comparable with the shelf succession in south Cumbria on the northern margin of the Irish Sea Basin.

Organic geochemistry of a phosphorite to black shale transgressive succession: Wilhelmöya and Janusfjellet Formations (Rhaetian-Jurassic) in central Spitsbergen, Arctic Ocean

The phosphorite to black shale transgressive shelf succession of the Wilhelmöya and Janusfjellet Formations (Rhaetian-Jurassic) in central Spitsbergen shows a change in the content and nature of preserved organic matter from residual woody organic matter (kerogen type IV) in the basal phosphorites to land-derived material (kerogen type III) in the overlying gray shales and up to amorphous organic matter of mixed composition (kerogen type II) in the black shale facies. The phosphorites were subjected to maximum exposure during sedimentogenesis which led to the advanced decomposition and oxidation of organic matter and to early phosphate concentration within starved sandy sediments. The black shale facies is enriched in organic carbon resulting from both planktonic and terrestrial inputs into an anoxic bottom water. Re-deposition of organic matter in this facies is considered essential for phosphorus depletion relative to organic carbon. The phosphorite-black shale association in the Spitsbergen shelf differs from many examples described in the literature in that it was not deposited directly below the zone of persistent upwelling.

Tectonism and an Upper Silurian Ramp-Prodelta-Rimmed Shelf Succession from Arctic Canada: An Intracratonic Product of Caledonian Compression: ABSTRACT

Tectonism and an Upper Silurian Ramp-Prodelta-Rimmed Shelf Succession from Arctic Canada: An Intracratonic Product of Caledonian Compression: ABSTRACT

Stratigraphic and Structural Relationships of Strawn Group, Brown, Coleman, and Runnels Counties, Texas: ABSTRACT

The Strawn Group (Atokan-Missourian) is exposed in two approximately triangular areas in north-central Texas: the Brazos River valley and the Colorado River valley. The exposures in the Brazos River valley are relatively well understood and are represented by cycles of fluvial-deltaic and transgressive shallow marine facies. These exposures have been traced into the subsurface, and grade laterally into a mostly carbonate shelf succession. The Strawn Group in the Colorado River valley has not been extensively studied, but recent work is increasing their knowledge of this significant area. In the Colorado River valley, the Strawn Group is subdivided into two lithogenetic units which accumulated in at least two depositional settings. The lower Strawn Group constitutes basin-fill sediments, and represents outer shelf, slope, and submarine-fan depositional environments. The upper Strawn, although exhibiting significant differences, is essentially correlatable to the Strawn Group known in the Brazos River valley and westward into the subsurface. Subsurface mapping westward from the Colorado River valley demonstrates the general applicability of published cyclic interpretations based on surface and subsurface work in north-central Texas. However, in the Colorado River valley and westward into the subsurface, mapping reveals a system of faults presumed to be related to the horst andmore » graben structural development in the Llano uplift area. In addition, the Concho arch, a feature closely related to the Llano uplift, was a positive area which resisted subsidence and acted as a buttress to terrigenous clastic deposition. It thus became a core for a persistent algal buildup.« less

Metamorphic Limestones of the Great Glen Area: Reply

SIRS—Rock et al. (1984)-highlighted the problems of assigning the limestones of the Great Glen to the Lewisian, Moinian or Dalradian but preferred to leave them unaffiliated. Following Drever (1936, 1939, 1940) we consider these limestones to belong to the Dalradian and propose that at least the Ardgour and Glen Dessarry limestones are the diachronous “missing links” between the Vendian Middle Dalradian basinal succession to the SE and the early Cambrian shelf succession to the NW (Fig. 1). We agree that the limestones do not correlate with the Lower Dalradian both on the geochemical grounds advanced by Rock et al. (1984) and also because of the general absence of authigenic K-feldspars in the Lower Dalradian, a mineral found in both the Ardgour and the Glen Dessarry limestones (Rock et al. 1984) as well as in the anhydrite-bearing dolomitic Fucoid Beds of early Cambrian age (Bowie et al. 1966; Allison and Russell 1985). Barrow (in Flett 1907) had mapped a calc-silicate horizon in the Middle Dalradian Deeside limestone that contains alkali feldspar. These Deeside rocks are favoured by Drever (1940) as being most comparable to the Ardgour limestones. Again in the Middle Dalradian, barium-rich K-feldspar has been found in the Loch Tay Limestone (Scott, pers. com. 1984) and we have found K-feldspar in the Shira Limestone.

Transition from Arc Volcanism to Stable-Shelf and Subsequent Convergent-Margin Sedimentation in Northern New Mexico from 1.76 Ga

The Precambrian of northern New Mexico is part of an extensive 1,800 to 1,500 m.y.-old terrane extending from Colorado through northern New Mexico into central Arizona. Three lithostratigraphic sequences are present in New Mexico. The oldest consists of 1,760 to 1,720 m.y.-old metamorphosed bimodal volcanic and volcaniclastic rocks to which no basement has been recognized. This juvenile crust developed as a magmatic arc complex and represents an early period of crustal instability. Between 1,740 and 1,700 m.y., the volcanogenic sequence was intruded by voluminous, coeval granodiorites and tonalites which progressively stabilized the early crust. Unconformably overlying the volcano-plutonic terrane is a thick sequence of metamorphosed quartz arenites and subordinate argillites, the Ortega Group, which accumulated on a sTable continental shelf. The inner shelf was dominated by tidal processes with subordinate reworking by storm and fair-weather waves. Storm processes were responsible for sand deposition on the outer shelf. The Ortega shelf sloped gently to the southeast and experienced an overall transgression which culminated in drowning of the outer shelf with onlap of black basinal muds from the south. STable-shelf sedimentation resulted from prolonged thermal subsidence following cratonization of the juvenile volcanogenic crust by intrusion of granitoid plutons. The third lithostratigraphic sequence, the Marquenas Quartzite, consists of polymictic metaconglomerates and texturally-immature metasandstones deposited in a braided-alluvial environment. These terrigenous sediments were supplied from a general southerly direction, and pebble compositions indicate derivation from the underlying volcanogenic and shelf sequences. The Marquenas Quartzite signifies cannibalization of the underlying magmatic arc and shelf succession in response to deformation of the craton margin to the south. This margin defined the southern limit of the proto-North American craton, which extended from central Arizona to southern Wisconsin at ca. 1,700 m.y. ago and was destroyed between 1,660 and 1,630 m.y. ago.

The mid-Carboniferous eustatic event

Stratigraphic and paleontologic evidence from mid-Carboniferous (Namurian) basin and shelf successions in widely scattered parts of the world indicates that a major eustatic event occurred about 330 Ma. The event began with a regression that is recorded in most shelf sequences, the regression was followed by a brief transgression about 328 Ma, and the event ended with a transgression that flooded large shelf areas about 325 Ma. The Mississippian-Pennsylvanian unconformity in North America is a well-known product of this event, but equally prominent and contemporaneous unconformity surfaces are also present in Europe, North Africa, and elsewhere. The event is believed to have caused numerous extinctions, and it resulted in marked fluctuations in faunal diversity. 94 references, 2 figures.

Mesothemic cyclicity in the mid-Carboniferous of the Ozark shelf region?

Mesothems are cyclic, chronozonally defined depositional intervals in basins, with thin, unconformity-bounded shelf equivalents. If eustatically controlled, as has been suggested for British Namurian mesothems, mesothemic cycles should be recognizable in distant but contemporaneous intervals. This appears to be the case in the mid-Carboniferous of the Ozark shelf region, where the record of thin, shallow-shelf deposits, punctuated with lacunae, is compatible with the Namurian mesothemic model. In the Ozark shelf succession, equivalents of only 9 of the 19 ammonoid zones of the British Namurian are represented. According to the mesothemic model, parts of a few additional zones may be discovered on the Ozark shelf, but a complete sequence will be represented only in the Ouachita Basin to the south.

Origin of a Lower Pennsylvanian Depositional Cycle

ABSTRACT The Marble Falls Group, a Lower Pennsylvanian limestone exposed at the margins of the Llano uplift in central Texas, was deposited on a carbonate shelf bordered by the Fort Worth basin. Outcrops near the basinward margin of the shelf reveal a depositional cycle containing a regressive sequence, a low-relief unconformity, and a transgressive sequence. Minor cycles within the transgressive and regressive sequences include inner shelf successions of facies capped by thin intervals of the outer shelf facies. Vertical progressions of inner shelf facies, algal bank to calcarenite bank to tidal bar-tidal flat, were clearly the product of regression. Thin capping intervals of the outer shelf facies contain open marine clay and carbonaceous matter mixed with siliceous sponge spicules, micrite and pellets. Incursions of the outer shelf facies onto inner shelf facies are judged to be a product of transgression. The regressive sequence exhibits a striking down-to-basin shift of progressively younger minor cycles, interpreted to indicate net regression interrupted by minor transgressions. The transgressive sequence exhibits similar cyclic tendencies and is interpreted to represent net transgression interrupted by minor regressive episodes. There is striking similarity between the interrupted patterns of transgression and regression in the Marble Falls cycle, and the discontinuous nature of ice sheet advances and retreats in Pleistocene climatic cycles. Estimates of relative sea level changes necessary to account for Lower Pennsylvanian facies patterns are in the same range as those resulting from Pleistocene climatic cycles. After carefully considering all cycle-generating mechanisms it is concluded that the Marble Falls cycle was most likely produced by glacio-eustatic changes.