Paleoenvironmental controls on Skolithos serratus: A case study from the Lower Cretaceous of the Sanfranciscana Basin

The purpose of this paper is to highlight ideas from a Hedberg Conference concerning detection of unconformities and associated porosity in carbonate strata. Many types of information, including cores and outcrops, seismic data, eustatic sea level curves, wireline logs, biostratigraphic data, stable isotope trends, cycle stacking patterns, and tectonic and basin evolution models, can be used to predict and/or detect subaerial unconformities. All methods of detecting subaerial exposure with these types of information have limits and pitfalls. Detailed analysis of the stratigraphy, sedimentology, and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure, though an integrated approach using all a ailable information is recommended. The relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted. Subsurface porosity is the product of many factors operating during deposition, subaerial exposure, and burial. Several concepts should be recognized when evaluating porosity associated with subaerial unconformities. (1) Carbonate diagenesis during subaerial exposure rearranges pore networks, but usually does not increase the total porosity. Near-surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone. In many cases, total porosity in carbonates under unconformities is actually reduced during subaerial exposure. (2) Permeability is likely to change more than porosity during subaerial exposure, increasing in some c ses but decreasing in others. (3) Pore systems evolve with time during subaerial exposure. Short periods of subaerial exposure (10,000-400,000 yr) are often associated with greater porosity than long intervals of subaerial exposure (1-20 million yr). Prolonged subaerial exposure may change permeability less than porosity because high-permeability, karst-related conduits can form quickly and persist for millions of years. (4) Many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction, cementation, and stratal collapse reduce porosity during burial. (5) Unconformity-related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial. (6) Vugs, caverns, and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure. (7) Lithologic changes at unconformities, like shales overlying carbonates, can influence the flow of subsurface fluids during End_Page 857------------------------------ deeper burial, resulting in deep-burial dissolution localized along unconformities. If subaerial exposure and exposure surfaces are integrated into a sequence stratigraphic, paleogeographic, and climatic framework, systematic and hopefully predictable patterns will emerge. However, we do not fully understand the complex interplay between the factors critical to creating and preserving porosity in carbonates. Additional research is needed to quantify critical processes and products so we can reliably predict porosity associated with subaerial exposure during exploration in frontier basins, and on a smaller scale, during reservoir analysis.

This paper presents a stratigraphic and depositional systems review to the Phanerozoic cover of the Sao Francisco Craton, defined as the Sanfranciscana Basin. The Phanerozoic cover is composed mainly of sedimentary continental rocks and minor explosive volcanic rocks. The stratigraphy and sedimentology of the Phanerozoic successions are summarized as follows: Santa Fe Group (Carboniferous-Permian) -divided into Floresta and Tabuleiro formations. This glaciogenic sequence represents the gondwanan glaciation record in the Sanfranciscana Basin. These sediments are preserved in valleys excavated in the basement and crop out in most of the basin . Areado Group (Early Cretaceous) -constituted by the Abaete, Quirico and Tres Barras formations, characterized by lateral and vertical interfingerings. The Abaete Formation was deposited by braided stream and the in southern area by alluvial fans; the Quirico Formation records a lacustrine sedimentation. and the Tres Barras Formation, was deposited in the fluvial, fluviodeltaic and aeolian environments. The Areado Group is thick in southern portion of the basin (200 meters); it is less than 60 meters and discontinous in the central-northern areas. Mata da Corda Group (Late Cretaceous) -composed by the Patos and Capacete formations. The Patos Formation is composed by alkaline volcanic rocks. The Capacete Formation represents the distai epiclastic sediments with important aeolian sand contribution. It is present only in the southern sector of the basin. Urucuia Group (Late Cretaceous) - composed of sandstones, divided into Posse (with Facies l e 2) and Serra das Araras formations, respectively interpreted as dry field dune deposits, braided stream of channelized deposition and braided stream deposited by sheet flows. It is present in the entire basin. In the southern portion of the basin, it is recovered by volcaniclastic sediments, and in the northern area it becomes the most important unit. Chapadao Formation (Quaternary), represents the recent sandy, unconsolidated, covers of talus, residual or alluvium origin. Provenance studies show the following transportation vectors and source areas: Santa Fe Group - NE to S W transport, with source areas in Northern Espinhaco Range and in the Bambui Group; Areado Group - transport from the adjacent elevated blocks, showing important axial flow; Urucuia Group - transport from NEE to SWW, with sources in the northest Sao Francisco Craton and the Capacete Formation - showing two directions of transport: volcanic rocks from South to North and aeolian sand from NE to SW.

The Buntsandstein in the Northern Vosges (France) originates mainly in an inland braidplain fluvial environment which passes in the upper part of the sequence into deltaic milieu in the coastal plain along the border of the sea, with the continental environment finally being drowned with the transgression of the shallow sea. The fluvial sedimentation is characterized by the presence of two facies throughout the Buntsandstein : channel facies and overbank plain facies. The channel facies comprises sandy and conglomeratic deposits forming within active streams by strong currents, whereas the overbank plain facies is built up of silty-clayey sandstones or silt/clay originating in stagnant water in abandoned watercourses, ponds, pools and puddles. The significance of particularly the floodplain sediments is subjected to considerable changes throughout the Buntsandstein sequence. There are all stages of transition between overbank plain deposits being only preserved in ghost-like facies as reworked clasts due to effective secondary removal of primarily occasionally formed suspension fines, and an abundance of autochthonous floodplain sediments in the depositional record resulting from favourable conditions of primary origin and secondary preservation. Reworked ventifacts within fluvial channel sediments testify to subordinate aeolian influences in the alluvial plain, with reasonable reworking, however, having removed all in situ traces of wind activity. Declining aridity of palaeoclimate towards the top is indicated by the appearance of violet horizon palaeosols in the Zone-Limite-Violette and the Couches intermediaires being accompanied by Brockelbank carbonate breccias originating from concentration of reworked fragments of pedogenic carbonate nodules. Biogenic traces are in the lower part of the sequence mainly present as Planolites burrows in the finer-grained sediments. Palaeosalinities as revealed from boron contents indicate progressively increasing supersaturation of stagnant waters with time. The fluvial environment persists up to the lower part of the Gres a Voltzia where the progression of the sea towards the west gives rise to a close intertonguing of fluvial and marine influences in a deltaic setting. Lenticular sandstone bodies are laid down as stream mouth bars at the end of the distributary channels and as river bars in the watercourses during both normal and flood discharge. Silty-clayey sediments settle out in stagnant water in restricted ponds, pools and puddles as well as in extensive veneers of shallow water in the overbank plain between the streams. Carbonate-bearing deposits originate in the coastal littoral mud flat, marsh seam, beach belt and tidal flat. The Gres a Voltzia has the greatest palaeoenvironmental and palaeoecological significance in the Buntsandstein of the Northern Vosges due to the occurrence of a wealth of extraordinarily well-preserved plant and animal fossils (having been recovered by Louis Grauvogel during almost 50 years and since abt. 25 years by Jean-Claude Gall). The rich suite of faunal and floral elements includes aquatic invertebrates, terrestrial animals and continental plants. The aquatic invertebrate fauna lives in fresh lakes and brackish ponds in the overbank plain and in brackish lagoons in the coastal seam as well as in hypersaline and euhaline marginal marine waters. The terrestrial plants colonize both dry and wet substrates, and the continental fauna consists of mainly arthropods, amphibians and reptiles inhabiting the levee zones of standing and flowing waters and strolling across the desiccated flats. The marine euryhaline association of invertebrates is with time replaced by a stenohaline community, and the deltaic plain of the Gres a Voltzia is finally inundated by a pellicular transgression representing the first stage of the Muschelkalk sea setting an end to Buntsandstein continental deposition.

The interpretation of cyclic successions of the Middle and Upper Triassic of the Northern and Southern Alps

Constitutive Equations for Engineering Mateials, Vol. Elasticity and Modeling

Two continuous cores (Unda and Clino) drilled during the initial phase of the Bahamas Drilling Project on top of the western Great Bahama Bank (GBB) penetrated proximal portions of prograding seismic sequences.As such, these cores provide the shallow-water record of sea-level changes and fluid flow of the Bahamas Transect that was completed with the deeper water sites of Ocean Drilling Program (ODP) Leg 166 in the Straits of Florida.The record of several hierarchies of sea-level fluctuations is identified in the lithology and log signature of two core borings (Unda and Clino), and the nature of fluids responsible for diagenetic alteration is interpreted from formation waters and the stable isotope signal of the sediments and rocks.Facies successions document that several hierarchies of changes in relative sea level are responsible for pulses of progradation.These pulses are seen on seismic data as seismic sequences and in the cores as depositional successions.On the platform, the boundaries of the depositional successions are indicated by subaerial exposure, changes in facies, and diagenetic overprint.On the slope, the sequence boundaries are marked by major discontinuity surfaces within the depositional successions consisting mainly of fine-grained skeletal and nonskeletal sediments.These discontinuity surfaces are characterized by hardgrounds that are overlain by 7-and 28-m-thick, coarser grained packages containing sand-sized blackened lithoclasts, planktonic foraminifers and minor amounts of platform-derived grains.The coarser grained intervals are interpreted as deposits during relative sea-level lowstands, while the fine-grained sediments are interpreted as highstand deposits.Higher order sea-level changes are recorded in the rocks and in the geophysical logs.On the platform top, these changes are recorded in shallowing-upward cycles bounded by exposure horizons.On the slopes, higher order sea-level changes are recognized by facies variations, whereby intervals of coarser grained sediments in the periplatform ooze indicate sea-level falls.The change in sedimentation rate and hydrology during these intervals results in the formation of firmgrounds.The intervals are well recognized as sharp peaks on the gamma-ray and velocity logs.The lower permeability on top of these intervals is likely to separate the fluid flow into several levels within each sequence and influence later patterns of diagenesis.The next higher order of cyclicity is represented by alternations (0.3-1 m) of coarser and finer grained beds within the coarse-grained intervals.Because of their relatively thin nature and low contrast in rock properties, these high-frequency cycles are not recorded in the logs.The slope portions of Unda and Clino yield several age diagnostic foraminifers and nannofossil marker species.Although low in abundance, these microfossils are good indicators of depositional age and provide the base for age determination.By combining micropaleontology, strontium-isotope stratigraphy, and magnetostratigraphy, a chronostratigraphy is obtainable in the prograding margin of GBB.The chronostratigraphy helps assess the stratigraphic evolution of the Great Bahama Bank margin, the timing of sea-level changes observed in the sedimentary record, and the synchroneity of the seismic sequence boundaries.The correlation of the rock with the seismic record not only confirms some of the major assumptions of the sequence stratigraphic concept but also showed some of the limitations when using sequence stratigraphy as a dating tool.The assumption that seismic reflections follow depositional surfaces (i.e., time lines) is indicated by two lines of evidence.The combination of changes in composition and diagenesis produces the necessary impedance contrasts for the imaging depositional unit boundaries as seismic reflections.In addition, chronostratigraphic dating shows no crossing of seismic reflections with time lines.The inability to recognize condensed or expanded sections causes problems when dating sequences solely by correlation to the global cycle chart.For example, the condensed interval in the early Pliocene falls below seismic resolution, whereas the thick Quaternary sequences are of different duration than the ones shown on the global chart of Haq et al. (1987).Geochemical data from the rocks and fluid samples taken during the drilling provide abundant evidence of the existence of fluid flow within the margin of Great Bahama Bank.Despite pervasive alteration and marine cementation, pore water analyses in the upper 100 m show an essentially isochemical profile suggesting the active flushing of seawater through this interval.Below 100 m in both cores, there are large increases in the concentration of nonconservative elements such as Sr, indicating dissolution and cementation processes.

Sedimentology of the Late Quaternary Wadi Hasa Marl Formation of Central Jordan: a record of climate variability

Fluvial geomorphic elements in modern sedimentary basins and their potential preservation in the rock record: A review

The oldest record of a tyreophoran track in Gondwana: Geological implications of subaerial exposure in the lower part of the Lajas Formation at the Covunco section (Neuquén Basin), Patagonia, Argentina

Planetary Geology and Astrobiology research in South America

Braidplain, floodplain and playa lake, alluvial-fan, aeolian and palaeosol facies composing a diversified lithogenetical sequence in the permian and triassic of South Devon (England)

Some carbonate grainstones have distinctive fitted fabrics that form due to dissolution in the vadose zone that are only rarely recognized or correctly interpreted. These grainstones have flattened and concavo-convex grain contacts where the grains fit together like puzzle pieces and are commonly lined with early marine or meteoric cement. Examples of these fitted fabric grainstones have been identified in carbonates from the Archean to the Holocene and likely occur in shallow marine, eolian and lacustrine carbonate grainstones throughout the geologic record. Fitting occurs due to dissolution at grain contacts by meteoric or mixed marine-meteoric fluids that over time flattens the grain contacts. Although these grainstones may at first appear to be compacted, burial compaction can be ruled out because there is commonly no sign of pressure solution, the early cement that forms at the surface clearly postdates the fitting of the grains and is unaltered by the fitting process, and because these fabrics are found in Pleistocene and Holocene grainstones that have never been buried. Because this feature forms during periods of subaerial exposure, it can help to identify cryptic exposure surfaces and sequence boundaries.

Meter-scale shallowing-upward cycles are recorded in many carbonate successions around the world. It is often difficult to recognize whether they represent autocycles, formed through intrinsic controls, or allocycles, resulting from orbital forcing or tectonic movements, or both. Here, we review the criteria used in the identification of the two types of cyclicity and apply them to two newly described lower Pliensbachian outcrops in the Traras Mountains, northwestern Algeria. Throughout the investigation of six sections, the deposits are suggested to have formed in intertidal–supratidal to shallow subtidal environments on a tropical ramp in the Western Tethys. In this area, shallowing-upward small-scale peritidal and subtidal cycles have been shown to be, and are assumed to be, ordered. The carbon isotope data mirror the recorded cycles and indicate different lengths of subaerial exposures. These cycles, in a developed within synrift setting, have been interpreted as produced mainly by autocyclic processes, but interacting with allocyclic factors. Peritidal cycles are thought to be generated by progradation of intertidal and supratidal flats into lagoonal sediments, while subtidal cycles are interpreted to have been controlled by lateral migration of shoals. The impact of the minor fluctuations of eustatic sea level is weakly marked, and only long subaerial exposure can reveal the contribution of these fluctuations to the formation of the recorded cycles. Tectonic movements resulting from spreading of the Tethys are interpreted to have controlled cycle distribution and thickness at a regional scale. However, synsedimentary tectonic features are rare in the studied area; this suggests that sediment transport would control the thickness and duration of cycles instead of the rate at which accommodation was created.

Coarse fluvial conglomerates containing numerous cobbles and boulders occur in various formations within the Lower Triassic continental red beds of Middle Europe. The rudites mainly originate as longitudinal gravel bars in highly-braided river systems with narrowly-spaced and straight to slightly sinuous channels. The high-energy stream sedimentation and the frequent and rapid lateral shifting of the watercourses control origin, distribution and preservation of finer-grained substratum deposits, topstratum sediments, aeolian dune sands and palaeosols within the alluvial plain which are governed by both internal and external mechanisms. The main processes operating within the alluvial plain which result in stacking of coarse conglomerates from successive cyclothems to multistorey complexes are primary-depositional restriction and suppression of formation and secondary-erosional destruction of finer-grained channel sediments, floodplain deposits, aeolian dune sands and palaeosols. The internal processes leading to vertical superimposition and lateral amalgamation of polygenetic conglomerate complexes are elements working under control of various external mechanisms. These are mainly slow subsidence of the basin allowing effective vertical and lateral erosion to take place during sidewards shifting of channels as well as supply of large amounts of coarse detritus from the source areas by tectonic uplift keeping pace with or even slightly exceeding erosional downcutting of relief, leading to inhibition of advance of the evolution of fluvial style beyond the initial stages of the depositional history. In the early stages of infilling of marginal parts of the basin, the partially considerable relief of the pre-Triassic basement provides locally shelter for parts of the floodplains by blocking off winds and deflection of alluvial channels. Depending on distribution of source areas, configuration of the basin and palaeocurrent pattern, wedge-shaped or sheet-like conglomerate bodies are formed. Wedge-shaped conglomerate bodies often testify to lateral-marginal supply of coarse gravel additionally to longitudinal-axial delivery of fine gravel and sand within elongated basins as a consequence of the activity of several source areas with different relief. Sheet-like conglomerate bodies mainly document transport of gravel into the basin following downcutting of relief in the main source area. Distally declining flow velocity within the river systems and progressive lowering of the source area relief with time lead to both horizontally and vertically decreasing size and abundance of gravel. Diminishing diameter and amount of clasts gives horizontally rise to lateral zoning of fluvial depositional environment and results vertically in an evolution of alluvial sedimentary milieu which both lead to transition of coarse fluvial conglomerates into finer-grained alluvial sediments in time and space. The possibilities of formation and preservation of particularly aeolian dune sands and calcrete palaeosols in the alluvial plain are illustrated by six depositional models comprising end members and significant intermediate stages of a continuous spectrum of an evolution of fluvial sedimentary environment. The chances of origin of products of subaerial exposure and of their subsequent incorporation into the depositional record progressively ameliorate with declining braiding of the river systems. The evaluation of the distribution of the six sedimentary models in the investigated case studies indicates that highly-braided stream complexes that are almost totally unsuitable for accumulation and preservation of aeolian sands or palaeosols occur in the lower part of most of the conglomerate sequences, but evolutionary changes towards less interwoven channel networks in the upper parts of the sequences in many cases lead to appearance of restricted or even more widely distributed products of subaerial exposure in the upper parts of the successions.

Integrated facies and subsidence analysis enable the identification and quantification of controlling factors on carbonate platform evolution. Palaeo biologic evolution as intrinsic factors, subsidence and accommodation change as extrinsic factors are the most important controlling factors on carbonate platform growth on a platform (and basin)-wide scale. Other factors like auto-cyclic processes, windward-leeward effects, nutrient supply, wave expositions and tidal currents are also important but play a minor, localised role. Whereas subsidence and accommodation change can easily be deduced from the sedimentary record and be quantitatively assessed, the other factors are much more difficult to identify lest quantify due to overprinting and poor preservation potential. Facies analysis and the comparison of the slope- and reef-facies from Latemar (Upper Anisian/Lower Ladinian, Dolomites) and Concarena (Upper Ladinian/Lower Carnian, Lombardic Alps) stresses the importance and links between subsidence, sedimentation, facies assemblages and cementation throughout the development of Triassic carbonate platforms. High rates of carbonate accumulation/accommodation increase combined with low topography of the reefal margin prevent massive early cementation at Latemar; low rates during the last stages of the Concarena platform together with a walled reef promote massive early cementation. Combined thermal, numerical basin reverse and sequence stratigraphic forward modelling constrained burial and subsidence histories of the Rosengarten and Latemar platforms. Vitrinite reflectance and apatite fission track data both argue for a shallow maximum burial. Present day topography was overlain by a maximum thickness of 1100m of overburden. There are no indications for a Neogene flysch or molasse-type setting as inferred from other Alpine basins. The subsidence pulse at the verge of the Anisian (Reitzi-zone) is recorded at Latemar and Rosengarten with similar total subsidence rates of 800-850m/Ma. Both platforms successfully kept up with this rapid increase in accommodation space. Additionally, sequence stratigraphic forward modelling of the Rosengarten platform showed that minimum vertical accumulation rates of up to 1000m/Ma are necessary in order to reproduce observed present day geometries. The carbonate production rates of both platforms reached values of (sub)recent carbonate platforms. Despite the resemblance of the Latemar reef to other early Anisian reefs of the Dolomites (encrusting microproblematica dominating over framebuilding organisms) and the low diversity of faunal elements with respect to slightly younger reefs (e.g. Concarena, Ladinian/Carnian), this adds further evidence to the observation of a full recovery of the carbonate factory already at the Late Anisian (lower Middle Triassic).