Search

Interactive comment on Clim. Past Discuss., https://doi.org/10.5194/cp-2019-3, 2019.

Carbonate Facies, Carbon Isotopes, Greenhouse, Aptian, Adritc Platform

Upper Callovian to Oxfordian Muddy Supersequence of Southern Adriatic Platform, Croatia

Sequence Stratigraphy and Oxygen-Carbon Isotope Signal from Albian Platform-Interior Carbonates, Southern Croatia

Sequence development influenced by intermittent cooling events in Aptian greenhouse, Adriatic Platform, Croatia

Aptian carbonate sequences of a cooler phase within Cretaceous greenhouse, Adriatic platform, Croatia

Coarse, fabric destructive post-depositional dolomites, Late Jurassic-Early Cretaceous Adriatic platform, Croatia: origin by mesohaline reflux

Sequence and Carbon Isotope Stratigraphy from the Aptian carbonate platform interior, southern Croatia

Facies stacking pattern, sequence stratigraphy and carbon-isotope signature of Albian platform-interior carbonates, southern Croatia

Cyclostratigraphy of Upper Albian tropical platform-interior carbonate facies, Korcula Island, Croatia

Sedimentary and carbon isotope record from the Aptian carbonate platform, Croatia

Albian cyclic peritidal facies of Adriatic platform interior in southern Croatia

Bioturbated-carbonate-capped parasequences: a key to short term climate/sea level change on carbonate platforms controlled by climate and plant and animal bioturbation

Late Valanginian to Early Barremian peritidal facies of the Adriatic platform, Croatia: Evidence of subaerial exposure

Sequence Stratigraphy, Carbon Isotope Signature, and Dolomitization of a Late Jurassic Greenhouse Platform, Croatia

Late Valanginian to Late Hauterivian Peritidal and Subaerial Exposure Facies, Adriatic Platform, Croatia

Carbonate platform signature of greenhouse worlds

Marine platform dolomites, Late Jurassic Early Cretaceous Bahama type platform, Croatia

Small scale parasquences characterize carbonate platforms from the Late Archean to Holocene. These cycles are bound by marine flooding surfaces. On shallow platforms shallowing upward regressive, peritidal parasequences are common. Thin transgressive lags form during transgressions when deepening water (sedimentation less than accommodation) may be tied to submarine erosion, bypass and lag time/or depth, followed by accumulation of regressive subtidal to peritidal sediments under stable or falling relative sea level, and possible subaerial erosion updip. Cycles are capped by barren muds, tidal flat facies and subaerial exposure surfaces, and evaporites in arid settings. Rapid deepening to deeper subtidal depths, followed by upward shallowing to storm or fair weather wave base as sea level falls on outboard of low and mid-latitude platforms in deeper water, commonly causes regressive coarsening up parasequences of carbonate muds, interbedded muds and sands, grainy tops, capped by marine erosion surfaces. Transgressive and transgressive-regressive parasequences are common on some platform interiors (e.g. Triassic). Here sea level rise just outpaces sedimentation rate, and low energy and insufficient bioturbation prevent removal of the deepening upward section so a marked transgressive record is preserved, often capped by a paleosol if sea level fall is rapid enough or the peritidal cap is stripped by erosion. On low-latitude greenhouse platforms this transgressive part of a cycle may also contain beaches of ooids, sublittoral sand sheets, and even hypersaline ponds of the platform interior. On deeper parts of low-latitude platforms, deepening and fining upwards parasequences may form on the deep ramp or within intrashelf basins, near storm wave-base. Cycles typically start with a sharp basal contact, overlain by high energy grainstone grading up through packstone, wackestone to mudstone, capped by the next truncation surface. These are deeper ramp, symmetrical transgressive-regressive cycles whose regressive cap has been stripped by marine erosion when sea level is lowered. Deepening upwards, but upwards coarsening parasequences occur in mid-latitude epeiric ramps within low energy embayments and estuaries bordering swell wave dominated shelves. The cycles have low energy muddy lower parts and deepen up into high energy open marine skeletal grainstones deposited near the wave abrasion depth.

This study explores Aptian aged platform-interior carbonate facies exposed on the Peljesac Penninsula and the islands of Korcula, Hvar, and Mljet in Southern Croatia. The Lower Aptian succession is characterized by non-cyclic, thick-bedded to massive subtidal facies of lime mudstone and skeletal-intraclastic lime mudstone and wackestone with rare benthic foraminifera, calcareous algae, microbial encrusters (Bacinella), bivalve fragments, as well as subordinate pelagic crinoids (Saccocoma) and planktic foraminifers (Hedbergella). The shallowest facies are skeletal-peloid wacke-packstones and grainstones with rudists ; these are calculated to occur in about 0-3 meters water depth. The cyclicity of the facies increases with proximity to the Early-Late Aptian boundary. The Upper Aptian is more cyclic, with parasequences consisting of basal skeletal mudstone-wackstone overlain by peloid-intraclast-skeletal packstone and grainstone, and barren mudstone, fenestral lime mudstone or microbial laminite. The emersion breccia horizons and/or residual clay horizons as parasequence caps are very common ; up to seven horizons are recorded in the lower, and six in the upper part of Upper Aptian.Maximum water depths of facies are estimated at 5 meters based on the range of stratigraphic distances of facies below base of laminate cap, breccias, or top of barren lime mudstone. The facies stacking patterns suggest an important environmental change during Aptian, with Early Aptian transgression, coeval with drowning of numerous Tethyan carbonate platforms, and pronounced Late Aptian regression marking a significant biological crisis in the peri-Adriatic region. The published data show that accumulation rates for the southern part of the Adriatic platform were 4 cm/k.y. on average, reaching a minimum value of less than 1 cm/k.y. in the Aptian. These low accommodation and accumulation rates likely caused amalgamated subtidal facies in the Early Aptian, and periodic emergence and well-developed paleosols in the Late Aptian. The thickness variations between the columns may be due to syndepositional tectonics combined with sea level oscillations.

Late Jurassic carbonates, Adriatic platform, Croatia are being studied to evaluate their paleoclimate signal. The Kimmeridgian supersequence has 2 major sequences bounded by breccia. The lower sequence contains 2 high frequency sequences separated by a breccia. Parasequences are 0.5 to 3 m thick, dominated by lump-oncoid wackestone-mudstone capped by lime mudstone and microbial laminite ; packstone-grainstones are rare. In the lower Kimmeridgian sequence, lump-skeletal wackstone and mudstone cap most parasequences ; microbial laminite caps parasequences in late highstands of the Kimmeridgian sequences. The Tithonian supersequence has 4 smaller scale sequences. The lower Tithonian is dominated by subtidal parasequences and the upper Tithonian by peritidal and oolitic parasequences. Parasequences are 1 to 5 m thick units of basal subtidal skeletal-oncoidal wackestone-mudstone, up into intraclast-peloid packstone grainstone. In more interior areas, bases of parasequences have transgressive ooid grainstone ; regressive oolitic units are rare. Lime mudstone caps most parasequences near the platform margin, while elsewhere microbial fenestral laminites cap cycles. Clayey breccias occur at sequence boundaries in the interior of the platform. The cyclic facies suggests greenhouse low amplitude sea level changes within the precessional band, crudely based on the duration of the Tithonian and number of cycles. Carbon and oxygen isotopes were obtained from carbonate mud matrix of the Lower Tithonian dasyclad-oncoid mudstone-wackestone (shallow lagoon) ; one set at 5 m intervals from a 200 m thick section closer to the platform margin, and the other from 1 m intervals from a 200 m thick section in the more inboard part of the platform. Isotope values PDB of dolomite are compatible with semi-arid tidal flat origin with δ18O values 2 to 4 ‰ heavier than the calcite mudstones that have average values of marine calcite (0.0 to -3.0 ‰). Many of the small positive excursions in C isotopes appear to correlate with the high relative sea level stands, suggesting that carbon is being sequestered during high frequency high stands of sea level (100 k.y.?), or that organic productivity in the photic zone is high during the high frequency highstands, perhaps related to the large area of platform that is shallowly submerged ; effects of diagenesis are also being evaluated.

The 240-meter-thick Late Valanginian to Late Hauterivian roadcut section on Mljet Island, Croatia was deposited in a tectonically stable Adriatic carbonate platform interior. The Valanginian is dominated by massive to very-thick-bedded cream, coarse sucrosic dolomudstone. This subtidal facies locally contains scattered gastropod molds and/or interbeds of gastropod-mold dolowackestone-mudstone to floatstone ; non-dolomitized gastropod-algal wackestone-mudstone is very rare. Peritidal laminites are rare ; fine-scale faint lamination in coarse sucrosic dolomite is fenestral and planar. Shallowing-upward dolomudstone-to-laminite parasequences are infrequently capped by few-centimeter-thick reworked intraclast, paleosol-like layers. The Hauterivian is dominated by well developed subtidal cycles with basal rooted horizons and mainly fenestral laminite caps suggesting humid subtidal to emergent conditions. The most common facies is peloid lime mudstone-wackestone to floatstone with branching, up to 1-cm-wide root burrows ; some horizons are heavily root-disrupted with green dolomitized shale fills, or completely dolomitized and represented by olive-gray, root-disrupted sucrosic dolomudstone. This facies also contains gastropods, clams, benthic foraminifera and calcareous algae, as well as pockets of peloid packstone ; clams and gastropods are locally leached and molds are filled with green shale. Root-disrupted horizons sporadically exhibit highly irregular and karstified tops with incipient breccia composed of granule-to-pebble-size angular clasts in lime mudstone matrix or dolomitized green shaly interstices. Laminated facies (upper intertidal to supratidal) commonly overlies rooted horizons and is distinguishable by fenestral peloid packstone and mudstone, frequently with sharp tops. Locally, fenestral laminites are transgressive ; they overlie rooted horizons with karstified tops and grade up into massive, rooted mudstone-wackestone. Transgressive, deepening parts of subtidal cycles are less frequently characterized by transgressive lags of microbial-lump and pisoid wackestone-packstone to floatstone, and thin dark-colored, broken-and-rehealed coarse ooid rudstone.

The Albian carbonate succession (approximately 400 m thick) of Adriatic Platform in southern Croatia, although predominantly peritidal, locally contains basal several meters thick sequence of deeper lagoon laminated limestones that were deposited during a regional deepening of the platform concomitant to oceanic anoxic event (OAE) 1B. Limestones are thin-bedded, platy, and planar laminated pellet packstone and lime mudstone, locally fine dolomite with petroliferous odor ; these rapidly pass up into meter-scale cycles dominated by fenestral caps suggesting relatively humid conditions. The cyclic peritidal carbonates that are markedly different from those described above make up the rest of the Albian. These meter-scale cycles consist of, from bottom to top: (1) Lime mudstone, locally heavily burrowed, with peloidal fills and scattered clam and gastropod fragments ; locally, this member is represented by coarse dolomite with rare oncoids and clam/gastropod moulds ; (2) pellet-skeletal wackestone with oncoids, calcareous algae and benthic foraminifera ; (3) peloid-skeletal-intraclast wacke-packstone with abundant calcareous algae (Salpingoporella) and benthic foraminifera, and scattered rudists and gastropods ; (4) intraclast-peloid-skeletal grainstone ; (5) gastropod and rudist floatstone to rudstone with lime mudstone, skeletal-peloid, or course dolomite matrix ; locally, oncoid-lump and pisoid rudstone ; (6) barren lime mudstone with tubular fenestrae and greenish infills, rare gastropod moulds ; (7) fenestral laminite ; platy and microbial laminite are subordinate ; (8) intra/lithoclast floatstone-rudstone with black and/or tan lime fenestral mudstone clasts or pebbles in a green clay, tubular fenestral wacke-mudstone, or dolomitized matrix. Cycles may also have basal thin transgressive lag of intraclast-oncoid floatstone in muddy matrix that grades up into skeletal bioturbated mudstone with gastropod fragments. Cycle tops are commonly microcarstic with mud-filled green vugs of leached mollusks, or with clay mm-seams.

A composite section through the Adriatic platform, Croatia from the Late Jurassic Kimmeridgian to the Early Cretaceous Albian stage provides a high-resolution climate and sea level record spanning fifty million years. The Kimmeridgian was dominated by warm humid conditions (local brackish alga Chara) marked by platform flooding interrupted by two cooling events, sea level fall and breccia development. The Tithonian warms into full-blown greenhouse conditions. The platform became increasingly semi-arid with meter-scale oolitic cycles capped by microbial laminites. Four cooling events punctuate the succession with the last one being in the Early Creataceous Berriasian (multiple subaerial breccia horizons). The Berriasian is cyclic with fenestral laminite or restricted, unfossiliferous lime mudstone capping most parasequences. The Valanginian is dominated by coarse dolomitized shallow subtidal carbonates and rare peritidal laminites. Transition from Valanginian dolomite to basal Hauterivian limestone may be synchronous with oceanic anoxic event (OAE) off the platform. The Hauterivian is dominated by well developed subtidal cycles with rooted horizons and mainly fenestral laminites suggesting humid subtidal to emergent conditions. Multiple emergence horizons at the top-Hauterivian and base-Barremian are associated with microkarsting and paleocave development. The Barremian contains abundant meter-scale cycles suggesting greenhouse conditions, becoming increasingly semi-arid upward (cycle caps become more stromatolitic and less fenestral compared to the lower part). Multiple short term cooling events are marked by black-pebble emergence horizons associated with sea level drops. The Early Aptian is marked by significant warming and shallow flooding of the platform and deposition of non-cyclic shallow carbonates. The Late Aptian is marked by multiple emergence horizons suggesting multiple cooling events and long term sea level fall. The Albian is marked by a regional deepening of the platform (OAE-1B) and deposition of deeper lagoon laminated limestones. This rapidly passes up into meter-scale cycles dominated by fenestral caps suggesting relatively humid conditions.

The Jurassic-Cretaceous Adriatic Platform provides a window into tropical shallow-marine carbonate deposition in the western Tethys. The highly cyclic Barremian sediments are superbly exposed on the islands and mainland of southern Croatia . Five logged sections, ranging from 20 to 100 m thick, display superbly developed peritidal parasequences ; the extensive Barremian microbial laminite facies likely contain the best preserved tidal flat facies compared to any Lower Cretaceous interval of the Adriatic Platform. The Barremian consists of 2 supersequences. The sequence boundary zone 1 in the Lower Barremian consists of up to 3 breccias. Supersequence 1 has at least 2 high frequency sequences. Sequence boundary zone 2 contains one to 2 breccias. Supersequence 2 has four high frequency sequences (HFS). Each HFS consists of shallowing upward succession with dominantly subtidal lower part, and laminated to fenestral upper part. Individual parasequences within each sequence are composed of shallow subtidal (lime mudstone and peloid-intraclast-skeletal packstone-grainstone) to intertidal (microbial laminites and fenestral lime mudstone) and subaerial exposure facies (greenish clay sheets or breccias with green clayey matrix). Presence of platform-wide subaerial exposure surfaces capping some cycles suggest relative sea level falls affected the platform over large areas. Most parasequences were formed by high-frequency sea level fluctuations perhaps related to orbital forcing ; possible durations of the interval and its component parasequences are being evaluated to see if these are compatible with a precessional vs longer term obliquity or eccentricity signals. Some parasequences probably also formed by autocyclic processes and local tectonics associated with the impending (Aptian) collision between the Adriatic microplate and Eurasia. Rapid thickness variations and facies changes over several tens of kilometers indicate that the synsedimentary differential subsidence (possibly over fault blocks) was a strong influence, with a superimposed sea-level fluctuations generating the sequences and component cycles.

The Late Jurassic-Early Cretaceous has been considered to lie in the Mesozoic “cool” mode of Frakes et al. (1992), as evidenced by oxygen isotope signals (VEIZER et al. 2000) and by the pCO2 plots GEOCARB III of BERNER & KOTHAVALA (2001). At the long-term scale, these climate proxies are in agreement, but at shorter time scales, there is some conflict both in degree or warming or cooling and timing of the events (VEIZER et al. 2000 ; ROYER et al., 2004). This study uses data from the Adriatic platform of Croatia to evaluate whether the approximately 5 to 6 m.y. long Late Jurassic Tithonian, has facies compatible with a Mesozoic “cool” mode (FRAKES et al 1992). Well exposed Mesozoic sections of the Bahama-like Adriatic Platform reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform interior (over 770 m thick, approximately 5 to 6 m.y. duration). Accommodation plots show four 3rd-order relative sea-level cycles, the third being the largest both in duration and magnitude (HUSINEC & READ 2007). 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 (shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Maximum water depths (estimated from distance below tidal flat units capping parasequences) generally were less than four meters, and facies show strongly overlapping depth ranges suggesting facies mosaics. Facies in the overall transgressive, Lower Tithonian sections are arranged in 1 to 4.5 meter thick parasequences dominated by subtidal facies, some of which are capped by very shallow water grainstone-packstone or restricted lime mudstone ; laminated tidal caps only become common toward the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well developed laminite caps. Parasequence durations appear to be within the precessional band and eccentricity bundling is weak. Modeling suggests that precessional sea level changes were small (several meters) as were eccentricity sea level changes, supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall “cool” mode of the Middle Jurassic to Early Cretaceous.

The Adriatic Platform provides a window into carbonate deposition spanning the Jurassic-Cretaceous transition in the western Tethys. The sediments exposed on the islands of southern Croatia are highly cyclic. The Barremian section is characterized by extensive microbial laminite facies that is by far the best preserved tidal flat facies compared to any Lower Cretaceous interval of the Adriatic platform. Three logged sections, each between 150 and 200 m thick, display superbly developed peritidal parasequences. The parasequences are composed of shallow subtidal (lime mudstone and peloid-intraclast-skeletal packstone-grainstone) to intertidal (microbial laminites and fenestral lime mudstone) and subaerial exposure facies (greenish clay sheets or breccia with greenish clay or mud matrix). The majority of the cycles may have been formed by high-frequency changes in sea level perhaps related to orbital forcing ; possible durations of the interval and its component parasequences are being evaluated to see if these are compatible with a precessional vs longer term obliquity or eccentricity signals. Some parasequences probably also formed by autocyclic processes and local tectonics. Presence of platform-wide subaerial exposure surfaces capping some cycles suggest a relative sea level falls affected the platform over wide areas.

This study explores Aptian aged platform-interior carbonate facies exposed on the islands of Korcula, Hvar, and Mljet, Croatia. The Lower Aptian succession is characterized by thick-bedded to massive predominantly non-cyclic lime mudstone and skeletal-intraclastic lime mudstone and wackestone with benthic foraminifera, calcareous algae and microbial encrusters (Bacinella), as well as subordinate pelagic crinoids (Saccocoma) and planktic foraminifers (Hedbergella). The Upper Aptian is distinguished by a cyclic alternation of facies with typically basal skeletal mudstone-wackstone overlain by peloid-intraclast-skeletal packstone and grainstone, and barren mudstone, fenestral lime mudstone or microbial laminite. Many cycles are capped by greenish residual clay sheets and partially dolomitized breccias/conglomerates. The facies stacking patterns suggest an important environmental change during Aptian, with Early Aptian transgression, coeval with drowning of numerous Tethyan carbonate platforms, and pronounced Late Aptian regression marking a significant biological crisis in the peri-Adriatic region. The published data show that accumulation rates for the southern part of the Adriatic platform were 4 cm/k.y. on average, reaching a minimum value of less than 1 cm/k.y. in the Aptian. These low accommodation and accumulation rates likely caused amalgamated subtidal facies in the Early Aptian, and periodic emergence and well-developed paleosols in the Late Aptian.

Well exposed Late Jurassic sections (over 750 m thick, approximately 5 to 6 m.y. duration) of the Bahama-sized Adriatic platform along the Dalmatian coast, southern Croatia, have well developed parasequences dominated by shallow subtidal to peritidal facies. These include dasyclad-oncoid mudstone-wackestone-floatstone (shallow lagoon), skeletal-peloid wackestone-packstone (very 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). Oversized, whole, broken and recoated radial ooids that form distinctive units resemble ooids of modern, low energy lake and marginal marine ponds. Their common position at bases of parasequences indicates formation during initial inundation of supratidal flats, along the shores of shallow hypersaline ponds and restricted lagoons of the platform interior. The abundance of these distinctive ooid facies in the Tithonian of Tethys may be due to arid climate, high calcite saturation state of platform waters, and the flat-topped, platform-interior morphology. The Tithonian parasequences appear to be less than 20 k.y. average duration and likely were formed by small sea-level changes driven by precessional forcing. During long-term (2.5 m.y.) sea-level rise, predominantly subtidal cycles were deposited, whereas peritidal cycles developed on the long-term (2.5 m.y.) sea-level fall. Evidence of an eccentricity signal in the parasequence bundling is relatively weak and produced parasequence sets whose component facies become successively more restricted upward. Paleosols are conspicuously absent from the Tithonian, suggesting that relative sea-level falls driven by precession were suppressed by high accommodation rates (12 to 15 cm/k.y.). The parasequence stacking patterns strongly suggest that the Tithonian likely was a time of hot global greenhouse during the postulated Middle Jurassic to Early Cretaceous "cool" mode.