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ABSTRACT

Dark-gray oolitic units characterized by oversized ooids with primary radial cal-cite 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, plat-form-margin ooid grainstones in their dark color, cerebroid outlines, broken and recoated grains, abundant inclusions, highly restricted biota, and lack of cross-stratifi cation. They have been interpreted as being of vadose ori-gin (“vadoids”) at tops of upward-shallowing parasequences. However, detailed sections show that most oolitic units occur at bases of precessional parasequences, overlying ero-sional 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 low-energy shores seaward of tidal fl ats, and along the margins of restricted lagoons and inter-tidal ponds. Superimposed fenestral fabrics, meniscus micrite cements, and grain break-age occurred as they aggraded to high-tide level and were subjected to wetting and dry-ing, 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 carbon-ate platforms in a greenhouse world.

Keywords: radial ooids, low energy, plat-form-interior parasequences, Late Jurassic, Adriatic Platform.

INTRODUCTION

Distinctive oolite units on the Late Jurassic (Tithonian) Adriatic Platform, Croatia (Tišljar, 1985), are characterized by oversized ooids with radial primary calcite fabrics. Many have cerebroid outlines, are broken and recoated, and are poorly sorted. The oolite units have super-imposed vadose fabrics, lack marine biotas, and do not have high-energy sedimentary structures. They also have been reported from the Italian and French Jurassic (Simone, 1974; Adams and MacKenzie, 1998).

These oolitic units lack the characteristic fea-tures of high-energy marine oolites. They have been considered to be vadose caps on regressive carbonate cycles resulting from upward shallow-ing, culminating in emergence. The ooids were termed “vadoids” by Tišljar (1985). However, detailed measured sections show that many of these distinctive oolitic units occur in the bases of parasequences in the oolite-rich sections. The evidence suggests that they developed along low-energy shorelines and in intertidal ponds and lagoons established on previously emergent hypersaline fl ats during transgressions driven by precessional forcing. We suggest that their geologic distribution refl ects the juxtaposition of calcite seas, high supersaturation states, arid climate, and presence of fl at-topped platforms in a greenhouse world.

SETTING

The Jurassic (Tithonian) oolitic carbonates are well exposed on Lastovo Island in southern Cro-atia, on the southern part of the Adriatic Platform

(Fig. 1). This huge Mesozoic, Bahamas-like platform is characterized by a 6-km-thick pile of predominantly shallow-water carbonate depos-its, punctuated by periods of subaerial exposure, paleokarst and bauxites, and by several pelagic (incipient drowning) episodes (e.g., Velic et al., 2002; Jelaska, 2003; Vlahovic et al., 2005). The Lastovo Island section is ~25 km inboard from the platform margin (Grandic et al., 1999) and

Transgressive oversized radial ooid facies in the Late Jurassic Adriatic Platform interior: Low-energy precipitates from highly supersaturated

hypersaline waters

Antun Husinec†

Croatian Geological Survey, Sachsova 2, HR-10000 Zagreb, Croatia

J. Fred Read†

Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, Virginia 24061, USA

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†E-mails: ahusinec@vt.edu; jread@vt.edu.

Figure 1. (A) Map of south-central Europe showing location of Adriatic Platform (mod-ifi ed from Grandic et al., 1999; Velic et al., 2002). Rectangle shows area enlarged in B. (B) Location of the studied section on Las-tovo Island (LSL).

GSA Bulletin; May/June 2006; v. 118; no. 5/6; p. 550–556; doi: 10.1130/B25864.1; 5 fi gures.

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exposes an undisturbed ~750-m-thick succes-sion of Upper Jurassic–Tithonian shallow-water carbonate sediments of the platform interior, the upper part of which is oolitic. The intense dolomitization further inboard makes it diffi -cult to determine how far into the platform the oolitic facies extends. The Tithonian limestones are underlain by Upper Kimmeridgian–Lower Tithonian shallow subtidal and peritidal (fenes-tral) limestones with the dasycladacean alga Clypeina jurassica (Korolija et al., 1977; Sokac et al., 1984). They are overlain by earliest Cre-taceous (Berriasian) peritidal limestones with paleosols (Husinec, 2002; Fig. 2).

The oolitic carbonates described here from Croatia resemble similar facies that occur in Italy and as far west as France (Simone, 1974; Adams and MacKenzie 1998).

DESCRIPTION AND OCCURRENCE OF OOLITIC FACIES

The typical distribution of these oolitic units is shown in Figures 2 and 3. Oolitic facies pre-dominate in the topmost 160 m of the Upper Tithonian of the study area (Fig. 2), although individual oolite beds occur a few tens of meters both below and above this interval. They are interbedded with relatively restricted marine facies and fenestral carbonates. They commonly occur in parasequences (Fig. 3) that consist of

basal transgressive oolite, skeletal mudstone-wackestone (rare), skeletal-intraclastic pack-stone-grainstone, regressive oolite (less com-mon), and unfossiliferous mudstone capped by fenestral grainy and muddy carbonate.

Parasequences are grouped into parasequence sets commonly 8–14 m thick. Sets consist of several parasequences. Lower parts of some sets commonly contain thin units of the deeper sub-tidal facies and thick oolitic carbonates; upper parts of sets commonly contain peritidal fenes-tral carbonates.

The oolitic units (Fig. 4) occur as 0.1–1.5-m-, and rarely, 4-m-thick, massive beds of grain-stone and packstone. The oolitic units range from light-gray beds of fi ne- to medium-sand-size, commonly superfi cial ooids, to darker, brown-to-gray, large coarse-sand-size to gran-ule-sized (1–3 mm) units. They also contain whole and fragmented-and-recoated radial ooids (Fig. 4A–D; “vadoids” of Tišljar, 1985), or bimodal mixtures of fi ne and coarse ooids; other grains include grapestone-like aggregates of ooids bound by marine cement and thin oolitic coatings (Fig. 4F–G), minor peloids, and a restricted biota of gastropods and dasy-cladacean algae. The ooids commonly have cer-ebroid outlines (Fig. 4E) with well-developed radial structure (Figs. 4A and 4D–E), and lesser-developed concentric laminae of micritized car-bonate (Fig. 4F). The cortex contains numerous dark micron-sized inclusions of possible organic material(?). Coatings show little evidence of abrasion of radial crystal terminations (Fig. 4E) nor is there much rounding of broken ooids (Fig. 4D). Some units have superimposed fenes-tral fabric, and many ooids typically are bound together by meniscus cements of micritic car-bonate (Figs. 4B–C) and later, fi ne- to coarse-grained, equant calcite cement. Broken and recoated ooids become common near the tops of some oolitic units (Tišljar, 1985). Hardgrounds that are planar at the outcrop scale are irregular at the microscopic scale and contain incipient meniscus cement (Fig. 4H). Although aragonite fossils in the oolitic carbonates are leached and fi lled with fi ne- to coarse-grained equant calcite cement, ooids are not leached.

ORIGIN OF THE UPPER JURASSIC ABERRANT OOLITE UNITS

Mineralogy

In contrast to modern, as well as Early and Middle Jurassic radial ooids, which commonly are aragonitic (Halley, 1977; Loreau and Purser, 1973; Tišljar, 1983), the Late Jurassic ooids in this paper are composed of primary calcite. This is indicated by the lack of any evidence of leach-

ing of ooids, even though aragonitic mollusk and algal fragments that form nuclei of ooids and aragonitic grains in enclosing beds were leached and fi lled by sparry calcite. This is compatible with the calcite seas of the Late Jurassic–Creta-ceous (Wilkinson et al., 1985), in which calcite precipitation was promoted because of oceanic Mg/Ca ratios below two (Stanley and Hardie, 1998). The Mg/Ca ratio of the oceans, and hence aragonite versus calcite seas, largely refl ects pro-cesses controlled by spreading rates and global mid-ocean-ridge volumes.

Evidence against Open-Marine Origin for Oolite Units

Modern high-energy oolitic sands on isolated platforms commonly are located within a few kilometers, and rarely up to 15 km, from the mar-gin (Harris and Kowalik, 1994). The oolitic units in the studied section, located 25 km inboard from the platform margin, show few features typical of high-energy marine ooid sands. They generally lack marine burrow structures, ripple cross-lamination, herringbone cross-stratifi ca-tion, or foreset bedding associated with migra-tion of submarine dunes. In addition, they have superimposed vadose diagenetic fabrics (fenes-tral fabrics, crystal silts, and meniscus cements) and contain numerous broken and recoated ooids (which are rare in radial oolitic units of high-energy, open-marine subtidal origin). They also have very restricted biotas, which include the dasycladacean algae (Campbelliella) and small gastropods, both of which groups are common in hypersaline settings today (Logan et al., 1974). The coarser oolitic units are dark gray (probably due to incorporation of numerous dark organic inclusions in the ooid cortex). In contrast, most modern and many ancient marine oolites are light cream, refl ecting intense wave and current energy that results in oxidation of any organics. Thus, the evidence indicates these oolitic units are not typical, high-energy oolitic sand bodies (cf. Ball, 1967; Bathurst, 1973; Harris, 1984).

Transgressive versus Regressive Setting of Vadose Ooids

Parasequences containing the oolitic units are likely to be precessional cycles, given their 2.5 m average thicknesses and the 15 cm/k.y. accumulation rate for the Tithonian (Husinec and Read, 2005). The parasequence sets of 12 m or so, composed of 4–6 cycles, may be short-term eccentricity cycles. The well-developed tidal-fl at caps and the associated shallow-water facies strongly suggest global greenhouse con-ditions and low-amplitude sea-level changes (cf. Read, 1998; Husinec and Read, 2005).

LA

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Figure 2. Generalized stratigraphic column showing stratigraphic location of Upper Tithonian oolitic limestones within platform interior succession.

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Tišljar (1985) interpreted the oolitic facies to be a result of vadose diagenesis following shal-lowing from subtidal lagoonal carbonates up into subtidal–lower intertidal microbial fenestral carbonates. With further emergence, the sedi-ments were partly eroded, and “vadoids” were formed in the vadose and subaerial zone. Tišljar (1985) suggested that broken and recoated vadoids were carried in from the neighboring vadose zone. Thus, the oolitic units were inter-preted to be vadose caps to shallowing-upward cycles (Tišljar, 1985).

We agree that some of the oolitic units are regressive, especially those in upper parts of parasequences, beneath tidal-fl at laminites. Some of these regressive oolite units may have developed along low-energy shores in front of prograding tidal fl ats, or in intertidal-supratidal ponds (cf. Loreau and Purser, 1973). However, oolitic units at bases of parasequences overlie fenestral carbonates (commonly with erosional contacts), and are transgressive deposits. It would be diffi cult to develop these extensive oolitic units by simple shallowing upward, culminating in deposition of oolite in vadose supratidal settings (Fig. 5). The erosional sur-faces capping the fenestral carbonates beneath the oolitic units were supratidal surfaces that developed during regression, and thus renewed marine ooid deposition would require transgres-sion and refl ooding of fl ats, discounting eolian deposits (Fig. 5). Thus, rather than capping upward-shallowing parasequences, many of the oolite units in the bases of parasequences likely are transgressive. Such a transgressive setting also would have provided accommodation for the thicker oolite units of 1–4 m to develop. This is not to imply that all Late Jurassic ooids else-where on the platform were formed in this way.

Low-Energy versus Marginal-Marine Origin

Marine, high-energy radial ooids are well sorted, spherical, and have a cortex composed of

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large ooid

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Figure 3. (A) Uppermost Tithonian section from Lastovo Island (section LSL; Fig. 1) showing facies stacking, parasequences, and parasequence sets. Upward-deepening units are shown by upward-narrowing tri-angle; upward-shallowing units are shown by upward-widening triangle. (B) Details of portion of measured section showing loca-tion of oolitic units within parasequences. (C) Idealized succession of facies within an Upper Tithonian parasequence. M—lime mudstone, W—wackestone, P—packstone, G—grainstone, F—fl oatstone, R—rudstone.

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A B

f

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E F

G H

Figure 4. Thin-section photomicrographs of typical Upper Jurassic (Tithonian) ooid grainstone composed of large, whole and fragmented-and-recoated radial ooids, Lastovo Island, Croatia. Bar scale is 1 mm. (A) Typical ooid grainstone facies. (B) Large fenestral pore (f) developed within ooid grainstone. Meniscus cements shown by arrows. (C) Ooids bound together by micritic meniscus cements (arrows). (D) Broken radial ooids showing little or no abrasion, coated with radial envelopes. (E) Ooid (center) characterized by cerebroid outlines with well-developed radial structure, and lesser-developed concentric laminae of micritized carbonate. (F and G) Small clusters of whole and fragmented ooids forming grapestone-like aggregates coated with radial calcite laminae. (H) Hardground and contact of skeletal (Campbelliella) wackestone and ooid grainstone.

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concentric laminae with radial fabric alternating with concentric “micritized” laminae, resulting from periods of abrasion and micritization alter-nating with periods of ooid accretion (Strasser, 1986; Davies and Martin, 1976). Thus, high-energy marine ooids are quite distinct from the Tithonian ooids described here.

Small radial ooids form on low-energy sand fl ats along the seaward fringes of the sabkhas in the Arabian Gulf, whereas larger radial ooids form along the margins of restricted hypersa-line lagoons, ponds, and minor depressions behind beach spits. These larger ooids are up to 1–3 mm, in contrast to most marine ooids, and are subjected only to small waves and moder-ate energy, due to protection by adjacent spits and barriers (Loreau and Purser, 1973). The large size of the ooids is favored by a low pro-duction rate of nuclei (Sumner and Grotzinger, 1993), as might be expected in restricted hyper-saline lagoon and pond settings during gradual transgression, because there is little produc-tion of skeletal fragments, fecal pellets, and eroded grains that would provide nuclei. Large ooids also would be favored by high growth rates (Sumner and Grotzinger, 1993), which would occur in hypersaline ponds and restricted lagoons being replenished by tidal waters in an evaporative semi-arid setting.

We propose that, rather than being purely vadose in origin (Tišljar, 1985), the radial ooids were formed in low-energy subaqueous settings (Strasser, 1986). The Tithonian ooids are strik-ingly similar to modern radial ooids that are forming in low-energy settings in lakes (Eard-ley, 1938; Halley, 1977; Popp and Wilkinson, 1983), and low-energy marginal-marine settings in the Middle East (Loreau and Purser, 1973; Friedman et al., 1973) and Texas (Rusnak, 1960; Freeman, 1962). In the Upper Jurassic deposits, quiet-water depositional environments are sup-ported by lack of abrasion of broken ooids, and cerebroid outlines of many ooids. The poor sort-ing suggests an absence of high-velocity cur-rents. Primary calcite radial ooids do not always form in quiet water, but many do (Strasser, 1986; Popp and Wilkinson, 1983).

Transgressive oolite formation on the Juras-sic platform would have occurred when shal-low hypersaline, intertidal to very shallow sub-tidal, low-energy, oolitic ponds and lagoons were established as the fl ats were inundated. It is noteworthy that the thicker vadose pisolite complexes of the Permian of West Texas are postulated to have formed within transgres-sive parts of sequences (Tinker, 1998). On the Jurassic platform, organics produced by cyano-bacteria in the ponds may have been incorpo-rated in the ooids to form the dark coloration. Once the fl ats started to be fl ooded, oolite

precipitation would have commenced almost immediately along the shore of shallow ponds and restricted lagoons (Fig. 5). Oolitic envi-ronments fringing the supratidal fl ats and bor-dering intertidal ponds and restricted lagoons would have migrated laterally as well as land-ward with transgression to form sheet-like ooid units. As transgression continued and waters became less supersaturated, the tidal-supratidal ponds would have coalesced and merged with the regional lagoonal setting, in which pellet-intraclast-skeletal packstone-grainstone would have formed on shallow subtidal sand fl ats. This deepening into shallow subtidal depths would have been facilitated by a lag between the oolitic units and the overlying subtidal nonoolitic facies (Fig. 5).

Why do the transgressive oolite units of the Jurassic differ markedly from the transgres-sive relict, oolitic units that fl oor large interior portions of the Bahama Platform (Winland and Matthews, 1974)? The Bahamian units are now dominantly grapestone, formed by micritization, aggregation due to organic and inorganic binding, and cementation of concen-trically laminated aragonite ooids. Much of the original ooid structure has been destroyed in the Bahamian examples, because they were not covered by later, shallower water deposits, which would have protected them from fab-ric-destructive marine diagenetic processes at the sediment-water interface. We suggest that the Jurassic ooids were protected from marine micritization by rapid burial, promoted by the high long-term accumulation rates (up to 15 cm/k.y.).

Origin of the Vadose Fabrics

How can we reconcile this transgressive set-ting for the bulk of the oolite units and their association with vadose features? Radial cal-cite ooids that formed on wave- and tide-swept open-marine subtidal shoals in calcite seas rarely show broken ooids. This suggests that it is not the radial fabric that is the main control of breakage of ooids. Broken ooids are abundant in carbonate eolianites (Hunter, 1993), which suggests that expansion and contraction due to heating and cooling in the subaerial environ-ment (Tišljar, 1985) and grain-to-grain contact during eolian transport likely causes the break-age of the ooids. Thus, the broken ooids imply that either the ooids accreted to supratidal lev-els along pond shores, or that the ponds peri-odically dried out. Periodic refl ooding of these ponds and lagoons would have allowed recoat-ing of the fragmented ooids. Eolian transport of some of the ooids is evidenced by their wide-spread dispersion on the platform, where they occur as isolated grains in almost every facies, including low-energy mudstones (cf. Eardley, 1938; Abegg et al., 2001).

The Jurassic oolitic units lack the classic hypersaline vadose tepees and aragonite and high-Mg calcite supratidal crusts of pendant radial and micritic cement, such as those form-ing in the Arabian Gulf and the seepage face in Lake McLeod (Scholle and Kinsman, 1974; Handford et al., 1984). Nor do they show the well-developed teepees and vadose pisolites of concentrically laminated cryptocrystalline carbonate, as in the Permian of West Texas

erosionalsurface

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Figure 5. One-dimensional model illustrating how transgressive oolite parasequences may form under low-amplitude precessionally driven sea-level oscillations. Following emergence of tidal-fl at laminites, transgression initiated renewed fl ooding of the tidal-fl at surface and ooid formation in hypersaline low-energy marginal-marine settings in tidal vadose zone. Subsequent deepening occurred following cessation of ooid deposition. Oolitic units became buried beneath prograding shallow-marine sediments. Less common, regressive oolites (not shown) would develop in front of, and be buried by, prograding tidal-fl at facies

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(Esteban and Pray, 1977). The Tithonian parase-quence tops also do not show evidence of cali-che (calcrete) formation, which would develop with long-term exposure (cf. Read, 1974).

The relative scarcity of such vadose crusts and fabrics in the Tithonian, and the deposi-tional setting of modern radial, oversized ooids suggest that the Tithonian examples formed in low-energy, very shallow subtidal to intertidal, subaqueous settings that were periodically sub-jected to vadose processes. The shallow ponds and restricted lagoons containing the Tithonian ooids might have periodically dried out, allow-ing tidal-vadose fenestral fabrics and meniscus cements to develop. Periodic drying of restricted lagoons and intertidal ponds could have been due to strong seasonal, offshore winds that sup-pressed tidal infl ow for days or weeks, as occurs in Shark Bay (cf. Logan et al., 1974). Lower-ing of saline groundwater tables also could have been due to periodic climate change and increased aridity. Additionally, rapid upbuilding of the oolite surface to supratidal levels due to localized high sedimentation rate would have placed the oolitic units within the tidal-vadose zone, resulting in the vadose overprint. This would have subjected the ooids to weathering processes, breakage, and eolian transport. It also is possible that each transgression driven by pre-cessional forcing was not a single uniform rise, but might have been interrupted by higher-fre-quency sub-Milankovich sea-level fl uctuations, such as those proposed for the Middle Triassic Latemar (Zühlke et al., 2003) and the Holo-cene (Bond and Lotti, 1995). This would have resulted in periodic emergence and vadose dia-genesis of the accumulating oolitic units.

Climatic and Saturation State Controls

The Late Jurassic throughout western Tethys was characterized by a relatively dry climate, which is marked by the extensive evaporites of the Middle East (Murris, 1980). This dry cli-mate likely was a key element in developing these distinctive oolitic facies, which extend as far west as France (Adams and MacKenzie, 1998). The low rainfall allowed the develop-ment of hypersaline ponds that must have been highly supersaturated with respect to calcite but in which low Mg/Ca ratios prevented aragonite precipitation (Stanley and Hardie, 1998). The lack of widespread evaporites on the Adriatic Platform (Veseli, 1999) suggests that the climate was not suffi ciently arid or that the platform was not suffi ciently restricted to promote widespread deposition of gypsum or anhydrite.

The widespread deposition of calcite ooids in the Adriatic Platform interior in the Late Jurassic may have been strongly infl uenced

by high calcite supersaturation of the platform waters and global ocean in the Late Jurassic, due to relatively low Mg/Ca ratios of the ocean (Wilkinson and Given, 1986; Stanley and Har-die, 1998; Kiessling et al., 2003). Wilkinson et al. (1985) showed that the Late Jurassic had the highest abundance of oolite since the Carbon-iferous. Overall, the Jurassic–Early Cretaceous was considered a time of mild cyanobacterial calcifi cation by Riding (1992), compared to the intense calcifi cation of the Triassic, and the extremely mild cyanobacterial calcifi cation of the Late Cretaceous–Cenozoic. However, microbial calcifi cation and microbial oncoids in subtidal facies reached their peak in the Late Jurassic and are extremely rare today, sug-gesting that oceans had high carbonate super-saturation in the Late Jurassic (Kiessling et al., 2003). Precipitation perhaps was restricted to calcite even under high supersaturation by the low Mg/Ca content of the oceans (Stanley and Hardie, 1998).

If these oolitic units required just an arid cli-mate, high calcite supersaturation, and peritidal settings to develop, why aren’t they more abun-dant in the Phanerozoic geologic record? Have these types of ooid facies been overlooked in the Phanerozoic, or are they relatively rare? Large deposits of giant ooids occur in several areas in the Neoproterozoic (Sumner and Grotzinger, 1993; Grotzinger and James, 2000). Low nucle-ation rate and low fl ux of nuclei, high supersatu-ration, increased storminess due to prevalence of ramps rather than rimmed margins, and possibly stormier climates have been invoked for these giant ooids. The Jurassic oolitic facies described in this paper appear to be distinct from the Neo-proterozoic giant ooids, and low nucleation rate and low fl ux of ooids, coupled with postulated high supersaturation, appear to be common to both. However, the postulated relatively low-energy setting of the Tithonian examples differs from the Neoproterozoic cases. If these distinc-tive facies that typify the Tithonian examples in this paper have not been overlooked, then their scarcity might be due to a special combination of circumstances, including the fl at-topped iso-lated platform setting, low-energy, greenhouse conditions, and arid settings that promote cal-cite supersaturation as platform waters become hypersaline (but below the salinities required for widespread calcium sulfate precipitation), with low Mg/Ca ratios of the oceans at the time precluding aragonite precipitation. The geo-logic community might examine stratigraphic sections within carbonate platform interiors for similar massive, non-cross-bedded oolitic facies at bases of parasequences for oversized and broken radial ooids to determine if this facies is indeed rare, or has just been overlooked.

Implications for Picking Parasequences

If we are correct, the transgressive rather than regressive position of many of the oolitic units has implications for picking parasequences. In many platform-margin settings, the oolitic units overlie lower-energy, fi ner-grained subtidal muddy facies, and thus cap upward-coarsening parasequences (Harris, 1984). However, in more interior locations of platforms, as in this paper, the oolitic units commonly form transgressive coarse-grained bases of parasequences that fi ne up into laminated and fenestral muddy carbon-ate caps. Such grainy transgressive units occur in the interior of the Cambrian-Ordovician plat-form margin, U.S. Appalachians, where grainy, locally oolitic facies form bases of upward-fi n-ing parasequences (Demicco 1985; Koerschner and Read, 1989) and in some of the Tithonian-Berriasian cycles described from the French Jura Mountains by Strasser (1994). Farther inboard on broad platforms, such grainy facies are rarely developed, and parasequences are dominated by muddy facies from base to top.

CONCLUSIONS

Oversized, whole, broken and recoated radial ooids that form distinctive units in the Tithonian (Late Jurassic) of the Adriatic Platform, Croa-tia, were formerly interpreted as having formed in the vadose zone at tops of regressive parase-quences. Based on their striking resemblance to modern low-energy lake and marginal-marine pond ooids, and their common position at bases of parasequences, we reinterpret these as hav-ing formed dominantly during initial inundation of supratidal fl ats, along the shores of shallow hypersaline ponds and restricted lagoons of the platform interior. Their abundance in the Titho-nian of Tethys may be due to an arid greenhouse climate and associated small precessional sea-level oscillations, the high calcite saturation state of the waters, and the fl at-topped, platform-interior morphology. Some oolitic carbonates at bases of parasequences within platform interiors elsewhere may have formed in a similar manner, especially on fl at-topped platforms in arid set-tings under greenhouse conditions.

ACKNOWLEDGMENTS

This paper represents part of a postdoctoral research project (grant no. 68428172) sponsored by the Ful-bright Program, U.S. State Department. We thank I. Vlahovic, T. Grgasovic, L. Fucek, and D. Palenik, who assisted in section logging during the work on the Geologic Map of Croatia (scale 1:50,000), spon-sored by the Ministry of Science, Education and Sport of Croatia. J.F. Read was supported by National Science Foundation grant EAR-0341753. We thank D. McNeill, M. Harris, and D. Sumner for their help

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in revising the manuscript. We also thank J. Tišljar for comments on an early draft.

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