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The Williston Basin retained a high-resolution record of global δ13C fluctuations during the ~13 Myr duration Upper Ordovician-lower Silurian, and is a possible missing link in correlations between Laurentian margins and its interior. This study integrates δ13C and conodont biostratigraphy to yield a high-resolution, continuous composite spanning from the Upper Ordovician Aphelognathus grandis Zone into the lower Silurian D. staurognathoides Zone. From oldest to youngest, six global δ13C excursions are identified: (1) Whitewater (=Moe) and (2) Elkhorn (=Paroveja) in the Amorphognathus divergens Zone ; (3) LHICE (KaH?) in the Aphelognathus shatzeri Zone ; (4) HICE in the Ozarkodina hassi Zone ; (5) early Aeronian within the Distomodus kentuckyensis – Pranognathaus tenuis zones and (6) late Aeronian within the Psuedolonchodina expansa – D. staurognathoides zones. This δ13C record helps to alleviate some of the uncertainties in determining the precise order, duration, and timing of events influencing the biotic, climatic, oceanic, and sea-level evolution of the Upper Ordovician-lower Silurian successions across Laurentia.

Most Upper Jurassic studies of astronomical forcing have focused on deeper-water sections which are relatively continuous. An Upper Jurassic (Kimmeridgian) section on the greenhouse Adriatic Carbonate Platform, Croatia, was studied to determine if astronomical forcing can be recognized in a 5.8 ± 0.1 Myr duration, disconformity-prone shallow platform succession. The succession consists of metre-scale subtidal parasequences intermixed with peritidal parasequences, and intermittent subaerial breccias at sequence boundaries. Ages were constrained by biostratigraphy and δ13C chemostratigraphy, and most sequence boundaries appear to match those of the coastal onlap curve of Haq (2018). Logged sections were converted into depth–rank time series and parasequence–thickness time series. Accumulation rates were statistically evaluated for the rank series against an astronomical-forcing model, and compared with long-term accumulation rates (thickness divided by time). The statistical rates were used to select the ca 100 kyr eccentricity cycle to tune the series. Spectral analysis showed peaks at ca 400 kyr (superbundles) and ca 100 kyr (bundles), along with obliquity (38 kyr and 27 kyr) and precessional (18−22 kyr) cycles (parasequences). The Kimmeridgian sequences are ca 400 kyr, ca 800 kyr and ca 1.1 Myr duration. Sequence scale (0.4 to 1.2 Myr) stratigraphic completeness based on statistical accumulation rates versus long-term rates is ca 60%. This study estimates ca 1 Myr missing time in parasequences stacked into superbundles and 1.6 Myr in four major sequence boundaries. Given that the Kimmeridgian was the hottest time of the Middle and Late Jurassic, aquifer eustasy may have influenced the timing of sequence boundaries, although documented late Kimmeridgian cooling could have triggered a glacio-eustatic component.

Reservoir zonation is one of the fundamental processes conducted on permeability-porosity core data to classify a reservoir thickness based on its productivity potential. In all the methods currently used, human decision plays a prominent role at every stage, from basic reservoir rock clustering to final flow zone determination. This study aims to reduce the often biased human decision-making role at each stage of the reservoir zonation by utilizing simple mathematical algorithms, and enhance the reservoir rock clustering approach. To do so, we propose the application of the elbow algorithm as a straightforward yet effective method to optimize the cluster numbers, and the Pruned Exact Linear Time (PELT) algorithm to effectively determine the zones with identical fluid flow potential (in this paper regarded as Hydraulic Flow Unit – HFU). Both the elbow and PELT algorithms are employed to effectively classify reservoir thickness based on the commonly used flow zone indicator (FZI), reservoir quality index (RQI), and flow unit speed (FUS). The efficiency of the suggested mathematical algorithms was tested against the results of the sedimentological and petrophysical study of the Upper Dalan (Khuff) gas reservoir, the Persian Gulf. Three lithofacies associations (sabkha and tidal flat, lagoon, and shoal) with 11 distinctive lithofacies plus crystalline dolomite have been identified and the major diagenetic processes influencing their petrophysical characteristics have been assessed. Based on the automated approach, the studied Upper Dalan succession can be subdivided into five HFUs with consistent poroperm values and distinct lithofacies and diagenetic modifications: barrier unit (HFU 1; permeability (k) < 0.1 mD), baffle unit (HFU2-3; mean k < 0.1 mD), normal unit (HFU4; k = 10–100 mD, mean k 17 mD), permeable unit (HFU5-6; k = 10–100 mD), and highly-permeable unit (HFU7; k >100 mD). The study suggests that the automated approach may be an effective alternative to a conventional reservoir zonation of similar mixed carbonate-evaporite reservoirs.

The Upper Dalan (Khuff-equivalent) Formation constitutes the principal reservoir of the giant gas fields in the Persian Gulf Superbasin. A sedimentological and sequence stratigraphic analysis was conducted on selected cores from the South Pars, Kish, and Lavan gas fields, offshore Iran, to evaluate a recently proposed method for automatic reservoir zonation, and discuss the predictability of such defined zones. The succession consists of nine evaporite-carbonate lithofacies grouped into three shallow-marine facies associations (shoal, lagoon, and tidal flat) that were deposited on a low-gradient homoclinal ramp. Lithofacies are stacked into two complete long-term (3rd-order?) transgressive-regressive depositional sequences. Sequence boundaries were defined by facies stacking patterns and presence of evaporites and meteoric diagenetic features. The reservoir quality was improved by both early-stage dolomitization and dissolution, whereas pervasive pore-filling anhydrite cementation, compaction, and late-stage over-dolomitization reduced the reservoir quality. Whereas dolomitization overall slightly affected porosity, it significantly increased the permeability in mud-dominated lithofacies. Fibrous and bladed calcite rim cements, as well as micritization of the grain-dominated lagoon and shoal lithofacies, prevented porosity reduction during the early- and late-stage burial by building a stronger framework. Core-plug porosity and permeability measurements were used to calculate the Winland R35, Reservoir Quality Index (RQI), and Flow-Zone Indicator (FZI) values. A novel, fully automated approach, was used to effectively identify the hydraulic flow units (HFUs). The HFUs are sedimentologically distinct units with characteristic combinations of the original rock texture and the subsequent diagenetic overprint, and their subsurface position within the sequence stratigraphic framework may be predicted.

Lower to Middle Eocene organic-rich deep-water limestones of the ∼335-m-thick (1100 ft) Radwany (Thebes-equivalent) Formation represent a source rock and a potential reservoir unit at the October Oil Field in the Gulf of Suez. However, in spite of recent exploration advances, the pore system and diagenetic history of the formation are still poorly understood. This study aims to discriminate porosity types and their vertical distribution, assess the diagenetic processes controlling porosity evolution, and evaluate the unit's overall reservoir potential. To achieve these goals, we utilized an integrated petrographic, mineralogical, geochemical and petrophysical dataset from four offshore wells. At October Oil Field, the Radwany Formation is informally divided into three intervals (from the base up: C, B and A) which are composed of planktonic foraminiferal wackestone, wackestone-packstone and packstone microfacies. Pores include both fabric selective (interparticle, intraparticle and moldic) and non-fabric selective (fracture) types. Visible porosity reaches up to 11.6%, and the highest porosity values are associated with interparticle pores in foraminiferal packstone in interval C. The two overlying intervals have lower porosities but have TOC values of up to 6 wt %. The Nuclear Magnetic Resonance (NMR) analysis yields low amplitudes of short T2 components (max. 52 ms), which suggests the presence of small pores (intraparticle, moldic, fracture, and fissure). Based on the electrical quality index (EQI) rock classification, nine groups were identified, suggesting a high heterogeneity of the formation. The highest EQI values positively correlate with high porosity (visible and estimated) in the lower part of Interval C which is dominated by interparticle/intraparticle porosity, thus reflecting the low tortuosity values of this microfacies. Dominantly low EQI values in Intervals A and B indicate low porosity efficiency and higher tortuosity. Carbonate sediments of the Radwany Formation are interpreted to have undergone a complex series of diagenetic processes. These processes modified the primary pore system either by enhancing the reservoir properties (dissolution and fracturing), or by reducing or destroying porosity (cementation, mechanical and chemical compaction, and minor pyritization, chertification, and asphaltene precipitation). The study highlights the unconventional reservoir potential of the middle and upper parts of the Radwany Formation (intervals B and A) at October Oil Field, and the conventional reservoir potential of interval C. It emphasizes the complexity of diagenetic controls on the porosity evolution in deep-water limestones of the Gulf of Suez, and may provide valuable insight into similar deep-water carbonate systems in rift basin reservoirs elsewhere.

The Adriatic Platform, Croatia is a large isolated carbonate platform that developed under a warm greenhouse climate interspersed with cooler episodes in western Tethys. However, the δ13C record and high-resolution sequence stratigraphy of the Middle and Upper Jurassic portion generally is poorly known. The ~1900 m thick (30 Myr duration) mainly Middle to Upper Jurassic succession of the platform was studied bed-by-bed to track the fluctuations of δ13C, facies stacking into bundles, superbundles, and sequences and potential sea-level changes within ongoing studies of evidence of astronomical forcing of the units. The study interval consists of cyclic subtidal intervals (upper Toarcian-lower Aalenian, lower Callovian, Oxfordian, lower Kimmeridgian, and lower Tithonian) interspersed with highly cyclic peritidal units (upper Bajocian-lower Bathonian, upper Kimmeridgian, and upper Tithonian-basal Berriasian). Ages were constrained by biostratigraphy and δ13C chemostratigraphy. The absolute δ13C values obtained from the shallow-marine bulk carbonate matrix are similar to those from coeval pelagic carbonates and exhibit similar trends, are characterized by overall little data spread (

The Late Ordovician late Katian was a period of generally warm, transitional greenhouse to icehouse climate with sea-level oscillations dominated by moderate-amplitude eustasy. This study aims to improve the understanding of the climatic and sea-level fluctuations prior to the end-Ordovician glaciation, using a comprehensive core, wireline log, and thin-section dataset spanning the topmost Red River-Stony Mountain Formation within the subsurface of North Dakota, Williston Basin, North America. Eleven facies are identified, interpreted, and grouped into four facies associations, including salina to peritidal evaporative, restricted-marine subtidal, open-marine subtidal, and open-marine deeper water. Stratigraphic trends, basin-wide facies variations, and correlation of individual surfaces for ~380 km across the study area, suggest that the facies were deposited under different climatic and sea-level regimes as part of the long-term Stony Mountain sequence. The sequence (37–52 m) is bounded by anhydrites (basin center) and their correlative thin exposure-related breccias (updip proximal basin), and contains a set of high-frequency sequences (HFS; 2–26 m) that can be traced across the study area. The lowstand systems tract (LST) anhydrites formed in a shallow and well‑oxygenated setting under an arid climate during relatively short periods of the basin restriction. The sea-level rise during the early transgressive system tract (TST) resulted in a higher evapotranspiration rate and an increasingly humid climate that peaked during the late TST with the input of siliciclastics off the Transcontinental Arch. The increased precipitation and runoff resulted in quasiestuarine circulation. Return to semi-arid conditions characterizes the highstand systems tract (HST), with increasingly more restricted, shallow-marine and oxygenated setting with antiestuarine circulation. The work provides insight into the interaction of siliciclastic input, climate, and sea-level changes on depositional dynamics and sequence architecture of a mixed carbonate-siliciclastic-evaporite succession that formed in a tropical epicontinental basin setting prior to the end-Ordovician glaciation.

The Upper Permian Dalan Formation is a supergiant gas reservoir in the Persian Gulf Basin. Reservoir quality is controlled by the original facies type, dolomitization, and anhydrite cementation. This study focuses on the texture (petrographic analysis), bulk stable-isotope values, and trace element composition in order to identify the original mineralogy of carbonate grains and cements, and to assess the synsedimentary and diagenetic processes, including the timing and setting of dolomitization. The limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone, and neomorphism and low-Mg calcite cementation (blocky and drusy crystals, syntaxial overgrowths) in the active freshwater phreatic zone. The relatively pristine δ18O and δ13C values, low Mn and relatively high Sr and Na contents indicate that this meteoric diagenesis was relatively limited. The remaining porous space was cemented by blocky calcite later during burial. Four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites: dolomicrite, dolomicrosparite, dolosparite, and saddle dolomite. Dolomicrite formed syndepositionally from the evaporated Permian seawater or by reflux of dense evaporated brines during shallow burial. The second major episode of dolomitization (dolomicrosparite and dolosparite) was driven by reflux of warmer and more saline fluids at greater depth, which affected both previously non-dolomitized and dolomitized sediments. The final diagenetic stabilization during deep burial is associated with extensive pressure solution, fracturing, vein-filling of calcite and dolomite cements, and crystallization of saddle dolomite.

Despite the vast research on various aspects of its stratigraphy, relatively little is known about the source-rock potential of the Upper Jurassic Adriatic platform, southern Croatia. Here a case palynological study has been provided to determine the kerogen type, quality, and thermal maturity, and assess its relative hydrocarbon generative potential. Palynofacies characteristics indicate a low-energy, shallow-marine oxygen-depleted setting. The deposited mud-supported carbonates contain organic matter that is almost entirely composed of a fluorescent amorphous organic matter of planktonic and bacterial origin. A high phycomata/dinocyst ratio suggests relative hydrographic stability in an environment favorable for motile-stage dinoflagellate community with reduced production of dinocysts. The lack of anaerobic degradation and very good preservation of lipoid palynomorphs indicates dysoxic conditions ; the latter also is suggested by a low proportion of algal-phytoplanktic carbohydrates. The lack of oxygen-deficient zone within the water column, coupled with well-preserved lipoid sediment components, suggests that the boundary between the oxygen-rich and oxygen-poor levels was likely located at/near the sediment-water interface. The palyno-organic facies analysis suggests that the organic matter corresponds to the type II (possibly I/II) kerogen (VRo ∼0.3–0.4%) that was protected from degradation before, during and after sedimentation. Well-preserved lipoid components suggest a good hydrocarbon-generative potential, but the low TOC values (

Patch reefs and their surrounding skeletal–peloidal–intraclast grainstone facies are very abundant on modern (sub-) tropical carbonate platforms. Compared to the barrier and fringing reef facies, the processes influencing composition, mineralogy, and stable-isotope record of peri-reefal sediment have received much less attention, despite a potentially significant volume of such sediment in many Phanerozoic carbonate platforms. To better understand the evolution and sedimentary record of patch-reef settings, this study investigates the patch-reef biota (scuba diving surveys) and composition (petrography), mineralogy (X-ray diffraction) and stable-isotope record (mass spectrometry) of peri-reefal sediments at East Beach on the northeastern, windward margin of San Salvador Island, the Bahamas. The macroalgae-dominated ecology is assessed, and a decadal decline in stony coral coverage with a shift to more opportunistic, stress-tolerant species is documented. The most common stony corals on these shallow-water (3–5 m) patch reefs are Diploria strigosa, Porites astreoides, and P. porites, and Halimeda lacrimosa dominates among green algae. Peri-reefal sediment is pure carbonate sand composed of roughly equal volumes of aragonite and high-Mg calcite (HMC), with mean MgCO3 content in HMC of 15.2%. The bulk-sediment isotope record shows very limited variation (δ13C between + 2.3‰ and + 3.2‰, and δ18O between − 0.8‰ and − 0.6‰) and this is consistent with similar modern settings. A lack of very fine particles and an abundance of intraclasts suggest that the environment is strongly influenced by bio- and physical erosion. Fast-growing algae have likely reached an abundance beyond the grazing capacity, which has drastically reduced the space for coral settlement and recruitment, and thus has compromised the patch-reef resilience in this part of the Bahamian Archipelago.

The long (700 m) Hauterivian to Albian Adriatic Platform section mainly from Mljet Island, Croatia underwent little post-Mesozoic burial or little later diagenesis. Consequently its smoothed δ13C and δ18O record from calcite lime mudstone matrix provides one of the most continuous stable-isotope curves from an Early Cretaceous platform. This record captures the carbon isotope excursions (CIEs) and oceanic anoxic events (OAE1a, b, c) evident in published hemipelagic sections, with the added advantage of providing information on the shallow platform response to the OAEs. The platform facies appear to have been little affected by the OAEs, except for OAE1a when deposition of organic-rich laminated limestones occurred in local downwarps.

The Lower Triassic Kangan Formation of the Persian Gulf Superbasin forms one of the largest reservoirs in the South Pars/North Field, the world's largest gas field. The upper Khuff-equivalent, gas-hosting carbonate-evaporite succession was formed on a gently-sloping, homoclinal carbonate ramp in a warm, tropical aragonite sea under a hot-and-arid greenhouse climate. This study explores the impact of vertical variations in depositional facies and diagenetic features on the reservoir quality to improve the prediction of the subsurface facies and hydraulic flow unit distribution. Detailed core logging and petrographic analysis resulted in the recognition of twelve distinct carbonate-evaporite facies that are grouped into four major facies associations, from shallowest to deepest: supratidal to restricted subtidal, lagoon, carbonate shoal, and shallow mid-ramp. The facies associations are stacked into two long-term sequences KS2 (58 m) and KS1 (77 m) bounded by anhydrites and/or unconformities; three higher frequency sequences (KS1a, KS1b, and KS1c) can be identified within the KS1. Based on the petrophysical properties, sedimentary facies characteristics and their diagenetic modifications, five hydraulic flow units (HFUs) are identified, with the best porosity-permeability values and flow- and storage capacities in the late transgressive (TSTs) and the earliest highstand systems tracts (HSTs) of the two long-term sequences. The reservoir quality and the distribution of hydraulic flow units are a product of interactions between primary mineralogy, depositional facies and their stacking, and the early- and late-diagenetic alterations. The reservoir quality decreases in landward direction, being the lowest in the mud-supported textures of the lagoon and supratidal to restricted subtidal facies associations. The seaward improvement in reservoir quality reflects a change to grain-supported textures that formed along the tide-and wave-agitated ooid-bioclastic shoals, whereas the best reservoir quality characterizes the shallow mid-ramp open-marine facies association, where secondary porosity was improved by dissolution and dolomitization. Both the early and late diagenetic dolomitization overall improved the reservoir quality, with the dolomitization pathways controlled by the porosity and permeability heterogeneity in the original depositional facies. The results may be useful in reservoir modeling, recognition of the productive zones, and the further development of the South Pars/North Dome Field.

The upper Katian (Upper Ordovician) Red River Formation is a carbonate-evaporite supersequence that formed in the epicratonic Williston Basin of western North America. This study focuses on the upper part of the formation in the southern, US part of the basin. The studied succession is made up of three transgressive-regressive sequences (RR1, RR2, and RR3), each ∼0.5–∼1 m.y. in duration and composed of relatively few meter-scale high-frequency cycles (parasequences). Sequence boundaries are characterized by intraclastic breccia-laminated dolomite successions that have been truncated and capped by evaporite deposits. Subaqueous anhydrites within each sequence represent evaporite Lowstand Systems Tracts (LSTs) that formed during relative sea-level lows that lead to the precipitation of gypsum (later recrystallized into anhydrite) in a quiet, salina-type environment. Transgressive (TST) and early Highstand Systems Tracts (HST) are composed of subtidal facies of predominantly skeletal mudstone to wackestone-packstone with abundant burrow mottling. Late HSTs are predominantly composed of peritidal, porous laminated dolomite. The study illustrates how the sequence stratigraphic framework of carbonate-evaporite deposits from the Williston Basin relates to the Late Ordovician glacio-eustasy, transitional climates, and low basin subsidence in an arid setting.

Secular variations in stable carbon isotope values of marine carbonates are widely used to correlate successions that lack high-resolution index fossils. Various environmental processes, however, commonly may affect and alter the primary marine carbon isotope signal in shallow epicratonic basins. This study focuses on the marine carbon isotope record from the carbonate-evaporite succession of the upper Katian (Upper Ordovician) Red River Formation of the shallow epicratonic Williston Basin, USA. It documents the carbon isotope signal between the two major Ordovician positive shifts in δ13C, the early Katian Guttenberg, and the Hirnantian excursions. Eight δ13C stages are identified based on positive excursions, shifts from positive to negative values, and relatively uniform 13Ccarb values. A correlation between carbon-isotope trends and the relative sea-level changes based on gross facies stacking patterns shows no clear relationship. Based on the available biostratigraphy and δ13C trends, the studied Williston Basin curves are tied to the isotope curves from the North American Midcontinent, Québec (Anticosti Island), and Estonia, which confirms the Late Katian age (Aphelognathus divergens Conodont Zone) of the upper Red River Formation. The differences in the δ13C overall trend and absolute values, coupled with the petrographic and cathodoluminescence evidence, suggest that carbon-isotope record has been affected by the syndepositional environmental processes in the shallow and periodically isolated Williston Basin, and stabilized by later burial diagenesis under reducing conditions and the presence of isotopically more negative fluids.

A new benthic foraminifer is described as Siphodinarella costata n. gen., n. sp. from Coniacian shallow-water platform-interior carbonates of Slovenia and Croatia. The new foraminifer is found in skeletal wackestone in association with small benthic foraminifera, thaumatoporellaceans, and calcimicrobes (Decastronema, Girvanella-type tubes). The existence of an internal siphon in Siphodinarella n. gen. is interpreted as an entosolenian tube and discussed in terms of its generic and suprageneric importance.

The Jurassic-Cretaceous dolomites, Adriatic platform, Croatia resulted from climate-influenced post-depositional reflux dolomitization (as opposed to synsedimentary peritidal- and deeper burial dolomitization), and subsequent stabilization within a Mesozoic, “calcite sea” isolated platform. The dolomites are stratiform (10 to 200 m thick), fabric destructive (20 to over 500 μm crystals), nonluminescent, and zoned with respect to Ca. Bulk dolomites have low Mn (10 to 30 ppm), moderate Sr (60 to over 200 ppm), positive δ13C and δ18O values, only moderate ordering (0.25 to 0.6) and single-phase fluid inclusions (temperatures +3‰ VPDB and Sr>100 ppm) composes most Jurassic and 40% of Cretaceous dolomites, making up turbid dolomite cores and initial clear dolomite rims. A first generation of low-Ca dolomite (LCD-1 ; 50 to 53 mol% Ca ; δ18O +1 to >+3‰ VPDB ; 100-180 ppm Sr) forms cement and variably replaces (stabilizes) earlier HCD cores. HCD and LCD-1 formed in refluxing marine-dominated pore waters under semi-arid climate (55 mol%) phases. A second generation of much younger, low-Ca dolomite (LCD-2 ; fracture-associated, more negative δ18O from -1.4 to +1‰ VPDB, Sr

Early Paleozoic calcareous algae are potentially useful for stratigraphic correlation but remain underutilized, likely due to presence of graptolites, conodonts, brachiopods, and other fossils that are commonly used in high-resolution biostratigraphy. This study focuses on the siphonous green algae within a 2-to-24-m-thick B interval of the Red River Formation, North Dakota, where the abundance of green algae suggests an important paleoenvironmental control; the algae also had a major role in carbonate production during that narrow stratigraphic interval. The bryopsidalean genus Dimorphosiphon Hoeg is abundant in algal wacke-packstone facies interpreted as shallow subtidal deposits. One hundred and twenty-two individual Dimorphosiphon thalli were identified and studied in detail in randomly oriented thin sections; measurements indicate that Williston Basin specimens belong to the species D. talbotorum Boyd, previously reported exclusively from the Bighorn Mountains, Wyoming. Dimorphosiphon is found in Upper Ordovician low-latitude, warm-water shelf carbonates of Kazakhstania, Baltica, and Laurentia; commonly, it is a major component of sediment. Several species of Dimorphosiphon appeared simultaneously in different and remote parts of the Paleotethys and Iapetus Oceans, suggesting a geologically instantaneous dispersal of the genus. Dimorphosiphon talbotorum, the focus of this study, has only been reported from western North America where it occurs within strata corresponding to the upper Katian Aphelognathus divergens Subzone of the Aphelognathus ordovicicus conodont Zone. Given its abundance, ease of identification, and short stratigraphic range, D. talbotorum potentially is very useful for regional correlation, facies, and paleobiogeographic studies of Upper Ordovician Richmondian shallow-marine strata of western North America.

Some parasequences in the interior of the Early Cretaceous isolated carbonate platform, Croatia, which formed under a greenhouse climate, lack the typical regressive tidal flat laminites. Instead, these bioturbated emergent parasequences have burrowed and rooted upper parts, capped with thin greenish paleo­ sols, and are reminiscent of Triassic “Lofer cycles.” We propose that the bioturbated cycle tops reflect coastal salinities that are normal marine, brackish, or metahaline, whereas the transgressive (rare) and abundant regressive tidal flat laminites formed under hypersaline coastal waters. Thus, these features may be used to track gross salinity changes of coastal waters on the platform through time. In addi­ tion, we suggest ways to evaluate whether the salinity changes relate to hypersaline embay­ ments on the platform or changes in the cli­ mate at the parasequence scale. We propose that the bioturbated emer­ gent parasequences could have formed under semiarid conditions, seaward of subtle , shallow embayments on the platform. Con­ temporaneously, in the bayheads, micro­ bial laminites developed—these would require time­equivalent updip and downdip laminate­capped and bioturbated emer­ gence–capped parasequences. However, the para sequences could also have been con­ trolled by climate changes. The laminite­ capped parasequences could have resulted from salinity increase with shallowing of the platform interior under semiarid condi­ tions (stable climate). In contrast, the bio­ turbated emergence–capped parasequences could have formed during times of more humid climate punctuating the overall semi­ arid climate. The humid phase would have favored brackish to normal marine salinities in the coastal zone, along with macrophytes, and intense animal and plant bioturbation, which would suppress development of lami­ nite caps. Thus, these parasequences provide a means of tracking gross salinity of coastal waters, and if climate induced, then they have implications for fluctuating climates in greenhouse worlds.

The Upper Permian Upper Dalan Formation contains one of the largest gas reservoirs in the world. The formation consists of carbonates with some evaporite intercalations that developed on a gently sloping homoclinal carbonate ramp facing the Late Permian Paleo-Tethys Ocean. This study focuses on the Kish Gas Field (Zagros offshore basin situated between Iran and Qatar), and is based on a 222-m-thick continuous core. Based on the integration of core- and wireline-log data coupled with petrographic analyses of 580 thin sections, three major depositional environments (facies belts) with 11 carbonate microfacies are identified. These include (1) sabkha to tidal flat (laminated to massive anhydrite, dolomudstone with anhydrite nodules, dolomudstone, and intraclastic dolowackestone), (2) lagoon and leeward shoals (bioclastic wackestone/dolowackestone to packstone, and peloid dolopackstone and peloid–bioclastic dolopackstone), and (3) mobile (windward) sand shoal (ooid–peloid dolograinstone, ooid dolograinstone, ooid–intraclast dolograinstone, ooid–bioclast dolograinstone–packstone, and coarse bioclast–intraclast dolograinstone). Diagenetic evolution of the Upper Dalan Formation is associated with evaporative marine, shallow-water normal-marine, meteoric, and burial diagenetic environments. Common diagenetic effects include dolomite and calcite cementation, mechanical and chemical compaction, dissolution, dolomitization, and evaporative (anhydrite) mineralization. Reservoir quality is strongly affected by variations in the original rock fabrics and subsequent diagenetic alterations. The most common pore types include interparticle, moldic, and connected vug (fracture and cavernous). The interparticle porosity–permeability relationship for the studied facies suggests that the reservoir quality is not affected by different crystal sizes and most samples plot in the low porosity and low to high permeability field, or display Lucia class 1 or 2 petrophysical relationships. The study shows that the pervasive pore-filling anhydrite mineralization lead to a significant decrease in porosity and permeability; poikilotopic anhydrite cement reduced matrix porosity, but the pore size was less affected.

Superb sections of Tithonian to Cenomanian carbonates of the Adriatic (Dinaric) platform are exposed on the islands of southern Croatia. A succession approximately 1,800 m thick consists exclusively of shallow-water marine carbonates (limestone, dolomitized limestone, dolomite, and intraformational breccia), formed in a protected and tectonically stable part of the platform interior. Several phases of exposure and incipient drowning are recorded in the platform interior. Four are crucial for understanding the Late Jurassic to mid-Cretaceous evolution of the wider peri-Adriatic area: (1) latest Jurassic–earliest Cretaceous sea-level fall, (2) Aptian drowning, followed by (3) Late Aptian platform exposure, and (4) Late Albian–Early Cenomanian sea-level fall. Deciphering these complex events from the vertical and lateral facies distribution has led to an evaluation of facies dynamics and construction of a relative sea-level curve for the study area. This curve shows that long-term transgression during the Early Tithonian, Hauterivian, Early Aptian, and Early Albian, resulted in generally thicker beds deposited in subtidal environments of lagoons or shoals. Regression was characterized by shallowing-upward peritidal parasequences, with well-developed tidal-flat laminites commonly capped by emersion breccia and/or residual clay sheets (Early Berriasian, Barremian, Late Aptian, Late Albian). The southern part of the Dinarides was tectonically quiet during the Tithonian through Aptian; sea-level oscillations appear to have been the primary control on facies stacking. Some correlation exists between local sea-level fluctuations and the published global eustasy charts for the Tithonian through Aptian. A significant departure is recognized at the Albian–Cenomanian transition, suggesting that it was influenced by tectonics associated with the disintegration of the Adriatic (Dinaric) platform.

The Lower Cretaceous shallow-marine succession of Mljet Island in Croatia records the geological history of the southern part of the Adriatic Platform during its last tectonically quiet period, prior to the Late Cretaceous collision processes between the Adria Microplate and the Eurasian Plate. We studied the Early Cretaceous biostratigraphy of benthic foraminifera and calcareous algae in order to establish a precise, combined benthic biozonation for the Adriatic, which in turn facilitates a better understanding of the global distribution of these microorganisms during the Early Cretaceous. Thirty-four species from 26 genera of benthic foraminifera and 32 species of calcareous algae, distributed among 11 genera, were recovered from the Lower Cretaceous shallow-water carbonate rocks of Mljet Island in southern Croatia. Nine biostratigraphical units, eight of which are based upon the benthic foraminifera and calcareous algae, and one upon the microencruster Bacinella irregularis, have been distinguished within this interval: the Clypeina parasolkani-Humiella catenaeformis Interval Zone (Berriasian–earliest Valanginian), Epimastopora cekici-Pseudoclypeina? neocomiensis Interval Zone (Late Valanginian), Montsalevia salevensis Taxon-range Subzone (Late Valanginian), Clypeina? solkani Abundance Zone (late Early and Late Hauterivian), Salpingoporella melitae-Salpingoporella muehlbergii Interval Zone (Barremian), Bacinella irregularis Assemblage Zone (Early Aptian), Salpingoporella dinarica Abundance Zone (Late Aptian), Orbitolina (Mesorbitolina) texana-“Valdanchella” dercourti Interval Zone (Early Albian) and “Valdanchella” dercourti Taxon-range Zone (Late Albian). The stratigraphical position of the main genera and species of benthic foraminifera and calcareous algae within the Berriasian–Albian interval is discussed, and the established biozones are correlated within the Adriatic Platform domain. The Early Cretaceous diversification of benthic foraminifera in the area investigated can be shown to follow the sea-level curves for that period, and the major foraminiferal turnovers coincide with global sea-level rise and fall. During relative sea-level rises, coupled with reduced oceanic circulation and expansion of nutrient-poor, shallow tropical waters, these organisms were able to diversify into various euphotic habitats, particularly within shallow subtidal environments of the platform interior. Regressive episodes resulted in the reduction of oligotrophic habitats and a decrease in species richness.

Dark-gray oolitic units characterized by oversized ooids with primary radial calcite fabrics occur in the interior of the Late Jurassic Late Tithonian, Adriatic Platform, a large Mesozoic, Tethyan isolated platform in Croatia. They differ from open-marine, platform-margin ooid grainstones in their dark color, cerebroid outlines, broken and recoated grains, abundant inclusions, highly restricted biota, and lack of cross-stratifiThey have been interpreted as being of vadose origin (“vadoids”) at tops of upward-shallowing parasequences. However, detailed sections show that most oolitic units occur at bases of precessional parasequences, overlying erosional surfaces on fenestral carbonates. The oolitic units are similar to quiet-water ooids that form today in low-energy settings. They developed in an arid climate during initial transgression of supratidal fl ats, along lowenergy shores seaward of tidal fl ats, and along the margins of restricted lagoons and intertidal ponds. Superimposed fenestral fabrics, meniscus micrite cements, and grain breakage occurred as they aggraded to high-tide level and were subjected to wetting and drying, thermal expansion and contraction, and wind transport. They migrated landward with transgression, forming extensive sheets, and were overlain by subtidal lagoonal facies that shallow up into fenestral carbonates. These distinctive facies may have been overlooked in the geological record, or their geological distribution requires juxtaposition of calcite seas, high-calcite supersaturation states, arid climate, and presence of fl at-topped carbonate platforms in a greenhouse world.

A continuous, ∼350 m thick succession of cyclic, predominantly muddy upper Callovian-lower Kimmeridgian shallow-marine carbonates was logged in southern Croatia to define the sequence stratigraphy. The basal, late Callovian part contains three sequences (AdCal-x to AdCal-z) dominated by parasequences consisting of intraclast-skeletal-peloid packstone-grainstone capped by fenestral-laminated carbonates. The bulk of the Oxfordian (eight sequences: AdOx-1 to AdOx-8) and lower Kimmeridgian (AdKimm-1) is represented by low energy and muddy lithofacies. The parasequences consist of basal skeletal-microbial lump wackemudstone capped by barren lime mudstone. Lack of tidal flat facies caps to parasequences indicates incomplete shallowing to intertidal depths, and suggests that some parasequences are amalgamated. Given the duration of Oxfordian (∼8 m.y.), these likely represent third-order cycles with an average duration of ∼1 m.y. (suggestive of long-term obliquity forcing). The total number of Oxfordian parasequences (57, ∼100 k.y. duration) is suggestive of short-term eccentricity forcing. Samples for δ13C analysis were collected every meter from bulk carbonate matrix. The carbon-isotope values vary from −2.3 to +4.0‰ PDB, and show two significant isotope excursions in the Oxfordian The isotopic signature shows relatively constant values (0.8–1.8 ‰PDB) for the late Callovian, followed by a negative excursion to 0‰ at the beginning of Oxfordian, and a stepwise increase to 2.7‰. The subsequent negative excursion (-2.3‰) is followed by a stepwise increase to 3.8‰ in the middle of Oxfordian, slight decrease with values remaining between 1.5‰ and 3.3‰, and finally by a stepwise increase with a positive excursion to 4.1‰ in the late Oxfordian. Stepwise positive shifts in carbon-isotope values in the middle and late Oxfordian are both associated with shallowing upward parts of sequences AdOx-3 and AdOx-6, respectively. The remainder of the curve shows generally decreasing C-isotope values into the lower Kimmerdgian. The studied succession supports overall late Callovian cooling (marked by tidal flats) followed by mid-Oxfordian maximum flooding (widespread subtidal facies), with parasequence development suggestive of short-term eccentricity forcing and superimposed long-term obliquity forcing. It is interesting that the dominantly subtidal Oxfordian of the Adriatic platform corresponds to time of intrabasin source rock formation in the Middle East.

Clypeina lagustensis n.sp. has been found in the Lower Tithonian deposits of the Lastovo Island (Dalmatia, Croatia). It is visually similar, obviously related and in some sections appearing almost identical, to Clypeina jurassica FAVRE, from which it differs by visible swellings and thinning of the central cavity, more pronounced distance between neighbouring whorls of fertile branches, and shape and structure of the interverticillate thallus parts, characterized by having well developed, hairy, sterile branches. These, after emerging from the exit pore, divide into several bundles which form a common turf with a calcareous envelope in the proximal part.

Palorbitolina bearing samples were obtained from three localities on the islands of Cres and Lošinj in the northern Adriatic. Although a relatively small number of specimens are studied (less than 60), the size of the embryonic chamber and test diameters of Palorbitolina lenticularis indicate a Lower Aptian age. This is confirmed for the Adriatic carbonate platform by the presence of Lower Aptian index taxa. Both embryonic chamber and test diameter variation are pronounced. No change has been observed concerning the relationship between stratigraphic horizon and the embryonic chamber diameter. A proportional relationship between the size of the embryonic chamber and the test is determined. These data show that the predominately mixed clastic-carbonate environments of the Mediterranean were colonized by palorbitolinas before the Adriatic carbonate platform, as has been suggested by Velić & Sokač (1978). Although an essentially westwards water circulation during the late Barremian and early Aptian probably aided the colonization of the Tethyan realm by palorbitolinas, it can neither explain the simultaneous existence of palorbitolinas in different and remote parts of the Tethys nor their dispersal, which must have been very rapid.

The island of Cres is located in the northern part of the Adriatic Sea. The island is built up of predominantly Cretaceous carbonates deposited in north-western part of extensive and long-lasting Adriatic Carbonate Platform. Owing to the influence of synsedimentary tectonics supported by eustatic changes during the latest Albian/Early Cenomanian, different sedimentary environments were established: from shallow intraplatform basin and related slope, across basin margin to protected shallow-platform. During the Early to Middle Cenomanian rudist communities (ichthyosarcolitid/caprinid/radiolitid) flourished along a relatively high-energy intraplatform basin margin. Fair amounts of coarse-grained bioclasts, derived almost exclusively from broken rudist shells, were deposited over a marginal depocenter. Contemporaneously, pithonellid wackestone-packstones containing microbioclasts and planktonic foraminifera were deposited basinward while marginal bioclastic sediments and limestone blocks of the basin margin origin were sporadically deposited within the basin. The opening of the Cres intraplatform basin was aborted and the basin was finally filled up during the Late Cenomanian. Since the Cres intraplatform basin was established at the beginning of the Cenomanian it probably represented the initiation phase in the north-western extension of the later Adriatic Trough development.

The mid Cretaceous shallow-water limestones on the islands of Cres and Losinj contain abundant and diversified orbitolinid foraminifera. A biostratigraphy, based on the orbitolinid taxa and their chronostratigraphic interpretation, is proposed for the Aptian, Albian and Cenomanian deposits in the area investigated. Four orbitolinid biozones have been recognized: (1) Palorbitolina lenticularis Taxon-Range Zone in the Lower Aptian; (2) Orbitolina ( Mesorbitolina ) texana Assemblage Zone in the Lower Albian, the body of strata being characterized by a distinctive assemblage of Orbitolina ( Mesorbitolina ) texana , O. ( M .) subconcava , O . ( M .) parva and O. ( M .) pervia ; (3) ‘Valdanchella’ dercourti Taxon-Range Zone in the basal Upper Albian; and (4) Orbitolina ( Conicorbitolina ) conica Abundance Zone in the Lower–Middle Cenomanian, a body of strata in which the abundance of the species Orbitolina ( Conicorbitolina ) conica is significantly greater than is usual in the adjacent parts of the section, regardless of either association or range. The established biozones on the islands of Cres and Losinj are correlated with orbitolinid biozonations and orbitolinid stratigraphic distributions in the adjacent areas, with emphasis on the Karst Dinarides.

Depositional sequences capped by peritidal carbonates and breccias on the Aptian Adriatic carbonate platform, Croatia, were studied to evaluate evidence for glacioeustasy within an age framework constrained by carbon-isotope chemostratigraphy. Sequence Ad1 (17–60-m [56–197-ft] thick; uppermost Barremian–lower Aptian) is dominated by shallow subtidal parasequences. Sequence Ad2 (7–13-m [23–43-ft] thick; lower Aptian–lowermost upper Aptian) contains oceanic anoxic event (OAE) 1a, associated with lagoonal laminated carbonates. Sequence Ad3 (3–8-m [10–26-ft] thick) probably is lower upper Aptian and likely is separated by a major hiatus from sequence Ad4 (8–20-m [26–66-ft] thick; uppermost Aptian), which spans OAE1b. Both Ad2 and Ad3 are dominated by peritidal parasequences updip in the lower transgressive systems tract and upper highstand systems tract and by subtidal parasequences elsewhere, whereas sequence Ad4 is dominated by shallow subtidal parasequences. Low accommodation rates (4.0–6.0 cm [1.6–2.4 in.] in the earliest Aptian, decreasing to approximately 1.0 cm/k.y. [0.3 in./k.y.] later) promoted widespread breccia development during relative sea level falls, aided by tectonic warping. The sequence-capping breccias, eccentricity-dominated cyclicity, restriction of peritidal facies to late highstands, and coeval off-shelf oxygen-isotope records all suggest that sea level falls occurred during times of cooling and had a significant glacioeustatic component. These intermittent cooler periods and continental ice buildup punctuated the Aptian greenhouse climate.

Carbonate platforms are sensitive recorders of environmental change through geologic time. Climatically induced changes in sea level or changes in subsidence are expressed through migration of sediment belts that are recorded in the accumulating sedimentary succession. Changes in nutrient levels are recorded in shifts in the biotic community and, if eutrophic levels are reached, platforms may drown due to breakdown in carbonate production (Follmi et al., 1994). Our group has been focusing on the ∼60 Ma Late Jurassic–Early Cretaceous period, which was originally considered to have had uniform greenhouse conditions (Fischer, 1982). Available climate proxy data, however, such as oxygen isotope data (δ18O) from deep sea cores, the paleontologic data (e.g., Frakes et al., 1992), and the 87Sr/86Sr and carbon isotope data of Jurassic and Cretaceous carbonates (Jenkyns and Wilson, 1999), indicate that there were major cooling and warming events that likely affected global ice volume, sea level, and the accumulating sedimentary and microfossil record of carbonate platforms. Numerous carbonate platforms were developed within the tropical–subtropical belt of the circumequatorial Late Jurassic–Early Cretaceous Tethys Ocean. Many of these platforms have comparable shapes, sizes, facies, subsidence rates, and geologic structure to the present-day Bahamas Banks, a commonly used modern-day analogue (e.g., Bosellini, 2002, and references therein). In addition, the outcropping Tethyan platforms provide important clues to the poorly known subsurface Mesozoic carbonates that underpin the modern Bahamas platform. The Tethyan platforms were characterized by high rates of sedimentation (from 100 m/myr; D'Argenio et al., 1999) and exhibit meter-scale shallowing-upward cycles or parasequences generated during high frequency, small sea-level fluctuations within the Milankovitch band (Strasser, 1991; Schulz and Schafer-Neth, 1997; Lehmann et al., 1999; Strasser et al., 1999; Immenhauser et al., 2004; Husinec and …

The Mesozoic, mid-Cretaceous (Barremian to Cenomanian) deposits of southern Croatia comprise a succession of shallow tropical-water, inner-platform deposits that formed on a Bahama-type isolated Adriatic carbonate platform in the Dinarides. This succession is dominated by benthic foraminifers and dasycladalean algae, and is exposed in a nearly continuous outcrop section on the islands and along the coast of southern Croatia. It has been studied in terms of sedimentary facies, paleoecology, and biostratigraphy. The present study documents that several species of benthic foraminifers (cuneolinids, orbitolinids, alveolinids) and dasycladalean algae (Salpingoporella) have exceptional age-diagnostic value for mid-Cretaceous biostratigraphy. These are abundant, and they have a widespread distribution and a restricted stratigraphic range. They evolved rapidly and became extinct suddenly. The mid-Cretaceous benthic associations, including a total of 106 species and 57 genera of benthic foraminifers and 48 species and 20 genera of dasycladalean algae, were analyzed to establish the principal diversity patterns at (sub)stage level of resolution. The Early Aptian marked the foraminiferal diversity maximum, whereas significant diversity drops are recorded in the Late Aptian and Early Cenomanian. The foraminiferal distribution within the oligotrophic habitats of the platform interior was controlled primarily by relative sea-level oscillations, variations in oceanic circulation rate, and nutrient availability in surface waters. There is a positive correlation between episodes of increased diversification and the regional relative sea-level rises, whereas regressive episodes resulted in reduction of oligotrophic habitats and decreased species richness. The dasycladaleans were the most diversified during the tidal-flat-dominated Barremian, and from that peak diversity decreased. through the Early Aptian. A significant diversity drop occurred in the Late Aptian, and it was contemporaneous with the maximum abundance of Salpingoporella dinarica. The mid-Cretaceous dasycladaleans never fully recovered from the Early Aptian platform deepening event, and their post-Aptian diversity pattern implies dependence on factors other than relative sea level and associated changes in habitats.

Fischer plots are plots of accommodation (derived by calculating cumulative departure from mean cycle thickness) versus cycle number or stratigraphic distance (proxies for time), for cyclic carbonate platforms. Although many workers have derived programs to do this, there are currently no published, easily accessible programs that utilize Excel. In this paper, we present an Excel-based spreadsheet program for Fischer plots, illustrate how the data are input, and how the resulting plots may be interpreted. The plots can be used to derive periods of increased accommodation, shown on the plots as a rising limb (which commonly matches times of more open marine, subtidal parasequence development). Times of decreased accommodation, shown on the plots as a falling limb, generally are coincident with thin, shallow, peritidal parasequences.

Well-exposed Mesozoic sections of the Bahama-like Adriatic Platform along the Dalmatian coast (southern Croatia) reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform-interior (over 750 m thick, ca 5–6 Myr duration). Facies within parasequences include dasyclad-oncoid mudstone-wackestone-floatstone and skeletal-peloid wackestone-packstone (shallow lagoon), intraclast-peloid packstone and grainstone (shoal), radial-ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Parasequences in the overall transgressive Lower Tithonian sections are 1– 4AE5 m thick, and dominated by subtidal facies, some of which are capped by very shallow-water grainstone-packstone or restricted lime mudstone; laminated tidal caps become common only towards the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well-developed laminate caps. Maximum water depths of facies within parasequences (estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences) were generally

The shallow marine carbonate deposits on the island of Cres, overlyingdeeper-water Cenomanian–Turonian limestones, are characterizedby an assemblage of rudists, benthic foraminifera, and associatedmicrofossils. The paucispecific character of the fossil association suggestsdeposition in shallow areas of a carbonate platform, with lowcurrent-energies and restricted circulation. Similar assemblages indicatingsimilar palaeoenvironments, are common in the Upper Cretaceousdeposits of the Adriatic Carbonate Platform and adjacent areas.The assemblage of rudists (hippuritids) and microfossils indicatethe Turonian to (?)Coniacian age of the investigated carbonate succession.The biostratigraphic importance of the so-called “primitive”hippuritids within the micropalaeontologically poorly defined biostratigraphyof deposits of this age, is accentuated.