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 ABSTRACT Keywords: radial ooids, low energy, plat- (Fig. 1). This huge Mesozoic, Bahamas-like form-interior parasequences, Late Jurassic, platform is characterized by a 6-km-thick pile of Dark-gray oolitic units characterized by Adriatic Platform. predominantly shallow-water carbonate depos- oversized ooids with primary radial cal- its, punctuated by periods of subaerial exposure, cite fabrics occur in the interior of the Late INTRODUCTION paleokarst and bauxites, and by several pelagic Jurassic Late Tithonian, Adriatic Platform, a (incipient drowning) episodes (e.g., Veli´c et al., large Mesozoic, Tethyan isolated platform in Distinctive oolite units on the Late Jurassic 2002; Jelaska, 2003; Vlahovi´c et al., 2005). The Croatia. They differ from open-marine, plat- (Tithonian) Adriatic Platform, Croatia (Tišljar, Lastovo Island section is ~25 km inboard from form-margin ooid grainstones in their dark 1985), are characterized by oversized ooids the platform margin (Grandi´c et al., 1999) and color, cerebroid outlines, broken and recoated with radial primary calcite fabrics. Many have grains, abundant inclusions, highly restricted cerebroid outlines, are broken and recoated, and biota, and lack of cross-stratifi cation. They are poorly sorted. The oolite units have super- have been interpreted as being of vadose ori- imposed vadose fabrics, lack marine biotas, and 10°E AUSTRIA 20°E gin (“vadoids”) at tops of upward-shallowing do not have high-energy sedimentary structures. A HUNGARY parasequences. However, detailed sections They also have been reported from the Italian 45°N CROATIA 45°N show that most oolitic units occur at bases of and French Jurassic (Simone, 1974; Adams and ADRIATIC ?? PLATFORM precessional parasequences, overlying ero- MacKenzie, 1998). ? sional surfaces on fenestral carbonates. The These oolitic units lack the characteristic fea- Adriatic??????? Sea? oolitic units are similar to quiet-water ooids tures of high-energy marine oolites. They have N ITALY that form today in low-energy settings. They been considered to be vadose caps on regressive developed in an arid climate during initial carbonate cycles resulting from upward shallow- transgression of supratidal fl ats, along low- ing, culminating in emergence. The ooids were energy shores seaward of tidal fl ats, and along termed “vadoids” by Tišljar (1985). However, 10°E 20°E the margins of restricted lagoons and inter- detailed measured sections show that many of 17°E tidal ponds. Superimposed fenestral fabrics, these distinctive oolitic units occur in the bases B PELJEŠAC meniscus micrite cements, and grain break- of parasequences in the oolite-rich sections. The 43°N ˘ age occurred as they aggraded to high-tide evidence suggests that they developed along Blato KORCULA level and were subjected to wetting and dry- low-energy shorelines and in intertidal ponds ing, thermal expansion and contraction, and and lagoons established on previously emergent Adriatic Sea Study area wind transport. They migrated landward with hypersaline fl ats during transgressions driven transgression, forming extensive sheets, and by precessional forcing. We suggest that their were overlain by subtidal lagoonal facies that geologic distribution refl ects the juxtaposition 42°45´N LSLLSL 42°45´N LASTOVO shallow up into fenestral carbonates. These of calcite seas, high supersaturation states, arid 0 5 10 20 km N distinctive facies may have been overlooked climate, and presence of fl at-topped platforms in 16°30´E 17°E in the geological record, or their geological a greenhouse world. distribution requires juxtaposition of calcite Figure 1. (A) Map of south-central Europe seas, high-calcite supersaturation states, arid SETTING showing location of Adriatic Platform (mod- climate, and presence of fl at-topped carbon- ifi ed from Grandi´c et al., 1999; Veli´c et al., ate platforms in a greenhouse world. The Jurassic (Tithonian) oolitic carbonates are 2002). Rectangle shows area enlarged in B. well exposed on Lastovo Island in southern Cro- (B) Location of the studied section on Las- †E-mails: [email protected]; [email protected]. atia, on the southern part of the Adriatic Platform tovo Island (LSL). GSA Bulletin; May/June 2006; v. 118; no. 5/6; p. 550–556; doi: 10.1130/B25864.1; 5 fi gures. 550 For permission to copy, contact [email protected] © 2006 Geological Society of America LATE JURASSIC LOW-ENERGY RADIAL CALCITE OOIDS exposes an undisturbed ~750-m-thick succes- basal transgressive oolite, skeletal mudstone- ing of ooids, even though aragonitic mollusk and sion of Upper Jurassic–Tithonian shallow-water wackestone (rare), skeletal-intraclastic pack- algal fragments that form nuclei of ooids and carbonate sediments of the platform interior, stone-grainstone, regressive oolite (less com- aragonitic grains in enclosing beds were leached the upper part of which is oolitic. The intense mon), and unfossiliferous mudstone capped by and fi lled by sparry calcite. This is compatible dolomitization further inboard makes it diffi - fenestral grainy and muddy carbonate. with the calcite seas of the Late Jurassic–Creta- cult to determine how far into the platform the Parasequences are grouped into parasequence ceous (Wilkinson et al., 1985), in which calcite oolitic facies extends. The Tithonian limestones sets commonly 8–14 m thick. Sets consist of precipitation was promoted because of oceanic are underlain by Upper Kimmeridgian–Lower several parasequences. Lower parts of some sets Mg/Ca ratios below two (Stanley and Hardie, Tithonian shallow subtidal and peritidal (fenes- commonly contain thin units of the deeper sub- 1998). The Mg/Ca ratio of the oceans, and hence tral) limestones with the dasycladacean alga tidal facies and thick oolitic carbonates; upper aragonite versus calcite seas, largely refl ects pro- Clypeina jurassica (Korolija et al., 1977; Sokaˇc parts of sets commonly contain peritidal fenes- cesses controlled by spreading rates and global et al., 1984). They are overlain by earliest Cre- tral carbonates. mid-ocean-ridge volumes. taceous (Berriasian) peritidal limestones with The oolitic units (Fig. 4) occur as 0.1–1.5-m-, paleosols (Husinec, 2002; Fig. 2). and rarely, 4-m-thick, massive beds of grain- Evidence against Open-Marine Origin for The oolitic carbonates described here from stone and packstone. The oolitic units range Oolite Units Croatia resemble similar facies that occur in from light-gray beds of fi ne- to medium-sand- Italy and as far west as France (Simone, 1974; size, commonly superfi cial ooids, to darker, Modern high-energy oolitic sands on isolated Adams and MacKenzie 1998). brown-to-gray, large coarse-sand-size to gran- platforms commonly are located within a few ule-sized (1–3 mm) units. They also contain kilometers, and rarely up to 15 km, from the mar- DESCRIPTION AND OCCURRENCE OF whole and fragmented-and-recoated radial gin (Harris and Kowalik, 1994). The oolitic units OOLITIC FACIES ooids (Fig. 4A–D; “vadoids” of Tišljar, 1985), in the studied section, located 25 km inboard or bimodal mixtures of fi ne and coarse ooids; from the platform margin, show few features The typical distribution of these oolitic units other grains include grapestone-like aggregates typical of high-energy marine ooid sands. They is shown in Figures 2 and 3. Oolitic facies pre- of ooids bound by marine cement and thin generally lack marine burrow structures, ripple dominate in the topmost 160 m of the Upper oolitic coatings (Fig. 4F–G), minor peloids, cross-lamination, herringbone cross-stratifi ca- Tithonian of the study area (Fig. 2), although and a restricted biota of gastropods and dasy- tion, or foreset bedding associated with migra- individual oolite beds occur a few tens of meters cladacean algae. The ooids commonly have cer- tion of submarine dunes. In addition, they have both below and above this interval. They are ebroid outlines (Fig. 4E) with well-developed superimposed vadose diagenetic fabrics (fenes- interbedded with relatively restricted marine radial structure (Figs. 4A and 4D–E), and lesser- tral fabrics, crystal silts, and meniscus cements) facies and fenestral carbonates. They commonly developed concentric laminae of micritized car- and contain numerous broken and recoated ooids occur in parasequences (Fig. 3) that consist of bonate (Fig. 4F). The cortex contains numerous (which are rare in radial oolitic units of high- dark micron-sized inclusions of possible organic energy, open-marine subtidal origin). They also material(?). Coatings show little evidence of have very restricted biotas, which include the abrasion of radial crystal terminations (Fig. 4E) dasycladacean algae (Campbelliella) and small nor is there much rounding of broken ooids gastropods, both of which groups are common in Peritidal (Fig. 4D). Some units have superimposed fenes- hypersaline settings today (Logan et al., 1974). parasequences tral fabric, and many ooids typically are bound The coarser oolitic units are dark gray (probably CRET. EARLY with paleosols together by meniscus cements of micritic car- due to incorporation of numerous dark organic Berriasian bonate (Figs. 4B–C) and later, fi ne- to coarse- inclusions in the ooid cortex). In contrast, most Ooid