Submarine Hydrothermal Weathering, Global Eustasy, and Carbonate Polymorphism in Phanerozoic Marine Oolites 1

SUBMARINE HYDROTHERMAL WEATHERING, GLOBAL EUSTASY, AND CARBONATE POLYMORPHISM IN PHANEROZOIC MARINE OOLITES 1

BRUCE H. WILKINSON, ROBERT M. OWEN, AND ALAN R. CARROLL Department of Geological Sciences The University of Michigan Ann Arbor. Michigan 48109-1063

ABSTRACT: The age and fabric of 347 Phanerozoic marine oolites have been compiled in order to evaluate secular variations in the original mineralogy of these physicochemicaUyprecipitated sedimentary carbonates. Variation in the abundance of oolite correlates with positions of global sea level; ooids were common during transgressions and regressions but were rare during lowstands and highstands. The rarity of oolite during continental emergence (early Cambrian, Permo-Triassic, late Tertiary-Quaternary) probably relates to a diminished areal extent of shallow-carbonate environments. Causes for a similar paucity during continental submergence (Silurian-Devonian, late Cretaceous) are more problematic; higher atmospheric CO2 and lower hydrospheric CO3- concentrations during highstands may have inhibited abiotic carbonate precipitation. Ooids coeval with highstands display a preponderance of fabrics evincing a primary calcite mineralogy; ooids formed during lowstands are more diverse but record dominance of aragonite. Variations in conical mineralogy relate more closely to flooded continental areas than to sea-level elevations, supporting the suggestion of MacKenzie and Pigott ( 1981) that variations in hydrospheric carbonate concentrations were more important m determining Phanerozoic ooid mineralogy than were magnesium-calcium concemrations. These data on oolite abundance and mineralogy demonstrate that abiotic Phanerozoic marine carbonates have been responsive to secular changes in atmospheric-hydrosphenc chemistry and global eustasy, and suggest that these changes are directly related to past rates of submarine weathering and sea-floor hydrothermal activity.

INTRODUCTION dition, Holocene radial high-magnesium calcite ooids are Since the pioneering study by Sorby (1879), it has been now known from a few settings and serve as modem recognized that coatings of oolitic allochems exhibit a analogues for ancient radial calcite grains. It is also now variety of fabrics and compositions in modem and an- apparent that the relative abundance of aragonite and cient marine and nonmarine units. Modem marine cor- calcitic ooids has varied cyclically throughout the Pha- rices are almost exclusively arragonite with constituent nerozoic with aragonite dominant during times of general acicular crystallites oriented parallel to grain exteriors. In continental emergence (such as we are experiencing today) ancient marine limestones, low-magnesium calcite cor- and calcite dominant during times of general continental tices exhibit one of two general fabrics; dominantly, they submergence. This relationship was first proposed by are acicular crystallites oriented normal to grain exteriors, MacKenzie and Pigott (1981) and is generally supported but equant interlocking crystals also occur (e.g., James by fabric data (MacKenzie and Pigott 1981; Sandberg and Klappa 1984). Although Sorby (18 79) recognized that 1983) and trace element data (Kahle 1965; Brand and these two fabrics, radial and sparry, related to different Veizer 1983) on ooid cortices. diagenetic histories, this distinction was ignored by sub- Those chemical processes which link the abundance sequent workers; because modern cortices are predomi- and mineralogy of such abiotic carbonate components to nantly metastable aragonite whereas ancient cortices are gross positions of global sea level are poorly understood; predominantly radial low-magnesium calcite, many in- such variation, however, must relate to one or more of dividuals concluded that ancient radial ooids arose through the parameters that affect oceanic carbonate equilibria some sort ofdiagenetic reerystallization of tangential ara- such as, but not restricted to, hydrospheric MR/Ca activ- gonite precursors. ities (Sandberg 1975; Wilkinson 1979) or atmospheric This misconception persisted for over a century until pCO2s (MacKenzie and Pigott 198 l; Sandberg 1983). Here Sandberg (1975) demonstrated that calcitized aragonite we examine the hypothesis that physical and chemical consists of interlocking crystals ofequant, anhedral spar. changes which occur in association with seafloor hydro- Ancient radial ooids, therefore, must exhibit fabrics in thermal activity are a primary causal mechanism behind calcitic cortices. In light of the fact that calcite ooids were the observed relationships between carbonate mineralogy virtually unknown from modem shallow-marine settings, and global eustasy. In the following we 1) document the Sandberg (1975) further concluded that atmospheric-hy- abundance of calcareous ooids in the Phanerozoic marine drospheric chemistry must be different today than it was rock record, 2) document the range of fabrics exhibited when radial calcite ooids perdominated in shallow-ma- by these grains, 3) evaluate cortical fabrics with respect rine settings. Subsequent studies have led to a general to their original mineralogies, 4) quantify relationships acceptance of this notion of nonuniformity of primary between primary cortical mineralogies and positions of cortical compositions through Phanerozoic time. In ad- sea level throughout the Phanerozoic, and 5) discuss these relationships insofar as they relate to long-term variations in atmospheric-hydrospheric chemistry resulting from I Manuscript received 27 March 1984; revised 27 June 1984. hydrothermal activity.

JOURNALOF SEDIMENTARYPETROLOGY, VOL. 55, No. 2, MARCH, 1985, P. 0171--0183 Copyright © 1985, The Society of Economic Paleontologists and Mineralogists 0022-4472/85/0055-017 I/$03.00 172 BR UCE H. WILKINSON, ROBERT 34. 0 WEN, AND ALAN R. CARROLL

DATA BASE Data on oolite occurrences and fabrics were tabulated //°" i from a variety of sources. Thin-section collections at the 40 r~..,g ? University of Michigan at Ann Arbor and at the Uni- N= 347 /1 : k versity of Texas at Austin were supplemented with sam- , / \ ples loaned or donated by many colleagues in academia and industry. Major geological journals which commonly contain papers on sedimentary geology were perused. In -J ~ - / ~ • .6 0 ~ ~ o _ addition, GEOREF data files yielded 1,060 references of P 20 Q. ~ i- publications in which OOI, OOL, or OOM were the first three letters in a title word or abstracted key word. These comprise all indexed North American publications since Io 1960 and publications exclusive of North America since 1966. .2 From these sources, 347 oolites were tabulated with respect to their age (by epoch), locality, and stratigraphic occurrence (by formation); siliceous, ferruginous, and do- 500 300 loo lomitic oolites were excluded from this tabulation. In order to combine redundant reports of oolitic strata which Fro. l.--Abundaa~ of oolite (bars) in Phanerozoic limestone sequences based on 347 reports and samples. Positions of global sea level have been the focus of particular attention owing to their in this and subsequent figures (dashed line) from Vail et al. (1977). Note economic importance (e.g., the petroliferous Jurassic that oolite was common during transgressions and regressions but was Smackover Formation of the Gulf Coast), to their sci- rare during continental emergence and submergence. entific importance (e.g., the Pleistocene Miami Oolite of Florida), or to their proximity to academic institutions with strong programs in carbonate geology (e.g., the Mis- submergence; ooids are rare in middle Ordovician to low- sissippian Ste. Genevieve Limestone of the Midwest), er Mississippian, and in upper Cretaceous to middle Ter- these 347 oolites were then lumped by formation and tiary sequences (Fig. 1). Although it might be argued that state (or province) into 215 "occurrences." Although such variation simply records the present relative vol- somewhat arbitrary, this groBping places all data into umes of shallow marine carbonate, available data on the stratigraphic-geographic units of roughly comparable size. lithology, thickness, and aerial extent of Phanerozoic units Finally, among the 215 occurrences, fabric data were suggest that such an explanation is inadequate. Of the 347 available for 179; these serve as the data base for eval- data summarized in Figure 1,254 (73%) represent North uating original mineralogies. American units. Variations in their abundance are not obviously different than that for the global data set (Fig. 1). Comparison of the North American data with data on OOID ABUNDANCES the volume and lithology of North America Phanerozoic sediments summarized in Cook and Bully (1977) indi- With respect to abundance, Phanerozoic oolites exhibit cates that low abundances during emergent modes do, in a striking quadramodal distribution, with peak abun- fact, reflect a significantly smaller volume of shallow- dances during the late Cambrian, late Mississippian, late marine carbonates laid down during these time periods. Jurassic, and Holocene (Fig. 1). Peaks occur at these times Sequences deposited during submergent modes, on the regardless of the abundant criteria considered, whether other hand, comprise relatively large volumes of epicra- total reports and samples (as in Fig. 1), total occurrences, tonic marine carbonate. Upper Ordovician, Silurian, De- or total occurrences per unit time. Similar peak occur- vonian, and upper Cretaceous sequences are notable in rences are also apparent in data from MacKenzie and this regard in that they contain many thick, areally ex- Pigott (19 81) and Sandberg ( 19 8 3 ). Abundances range tensive, shallow-marine units that largely lack oolitic fa- from no known upper Ordovician oolites to 6.5 occur- cies. It appears that during times of maximum continental rences per million years in Holocene units. This Holocene submergence, some combination of physical and/or value is undoubtedly disproportionately high due to em- chemical factors gave rise to conditions which largely phasis on modern carbonate environments. Cambrian, precluded the accretion of carbonate cortices. Mississippian, and Jurassic peaks, however, correspond to 1.4, 1.2, and 0.9 occurrences per million years, respectively, suggesting that age is not directly related to OOID FABRIC AND MINERALOGY oolite abundance. Oolite abundances do, however, appear to relate to The original mineralogy of calcium carbonate com- absolute positions of global sea level. Periods of general ponents in marine settings is almost exclusively either continental emergence, such as during the earliest Paleo- calcite or aragonite. Because aragonite is unstable with zoic and the Permo-Triassic, correspond to low abun- respect to low-magnesium calcite, aragonite components dances (Fig. 1). Somewhat surprisingly, similar low abun- transform to calcite via dissolution-precipitation pro- dances also correspond to times of maximum continental cesses early in the diagenetic history of most sequences. PHANEROZOIC 34AR1NE OOLITES 173

[~] PA ~] NSD

I 1 NSN MS

'l N= 87 o~ ]5- tD

,o- i ~ 0

5 -

£ o IslDI l' J I K I T FIG. 2.--N¢omorphic sparry ooids; these grains which once consisted 50O 300 loo ofacicular crystals of aragonite have been replaced by ¢quant crystals of low-magnesium calcite. Pennsylvanian Plattsburg Limestone, Kan- FIG. 3.--Distribution of fabrics of aragonite cortices. PA = primary sas. aragonite; NS = neomorphic spar; NSD = neomorphic spar as deformed ooids; NSN = neomorphic spar as ooids with displaced nuclei. Tabulaled from the 179 Phanerozoic ooid occurrences for which fabric data were available. Note that neomorphic spar is the dominant fabric recording the former presence of aragonite. In general, these transformations result in a characteristic fabric of interlocking anhedral calcite crystals, either as neomorphic spar, which may preserve gross features of the aragonite precursor as solid and/or liquid inclusions, referred to as sparry ooids (Fig. 2). Component crystallites or as pore-filling spar, which precipitates following com- range in size from about 20 to 100 microns; original plete aragonite dissolution (e.g., Folk and Assereto 1976; concentric banding is commonly preserved through the Bathurst 1975). alignment of solid and/or liquid inclusions, and by virtue Originally calcitic components retain virtually all fabric of the fact that spar crystal boundaries commonly coin- elements during diagenesis. Because low-magnesium cal- cide with primary laminations. Such fabrics have been cite is a relatively stable phase in most diagenetic envi- reported in former aragonite marine cement (e.g., Maz- ronments, low-magnesium calcite components retain zullo 1980), in former aragonite fossils (e.g., James 1974), original fabrics as well as trace element and isotopic sig- in calcitized ooids (e.g., Sandberg 1975), and are now natures (e.g., AI-Assam and Veizer 1982). High-magne- generally accepted as the typical fabric developed during sium calcite, on the other hand, is a metastable phase aragonite to calcite transformation. which rapidly transforms to low-magnesium calcite via Neomorphic Sparry Deformed Ooids (NSD).--Two dissolution-precipitation reactions. Unlike aragonite cal- variants on sparry ooids occur commonly in Phanerozoic citizafion, however, this transition commonly occurs while carbonates. The first have been referred to as elephantine retaining the finest details of original fabric; extensive (connected trunk-to-tail) or spastolithic (deformed) grains compositional changes are accompanied by minimal which exhibit variable degrees of postdepositional defor- changes in the shape and orientation of constituent crys- mation. These forms arise when ooids are lithified by tallites (Towe and Hemleben 1976; Schlager and James calcite (commonly marine) cement before aragonite to 1978). calcite transformation. During aragonite dissolution, rig- Since originally calcitic components, either as low- or id calcite cement matricies collapse, giving rise to de- as high-magnesium calcite, retain fine details of primary formed and interconnected ooid shapes (e.g., Conley fabric whereas originally aragonite components are re- 1977). Such shapes record the former presence of ara- placed by sparry calcite, conical fabrics provide powerful gonite cortical laminae as well as early lithification by criteria by which to evaluate original mineralogies. With- calcitic cements. in this broad framework, however, Phanerozoic marine Neomorphic Sparry Ooids with Dropped Nuclei ooids exhibit a variety of fabrics and forms. Many of (NSN). -- Lowered, displaced, or dropped nuclei are a sec- these have been described elsewhere (e.g., Richter 1983) ond variant of sparry ooids that occurs commonly in the and therefore are only briefly summarized here. rock record. Like deformed grains, dropped nuclei record the physical settling of less soluble (in this case nuclear) ARAGONITE FABRICS phases during aragonite dissolution. In such cases, nuclei consist either of terrigenous or calcitic components. Like Neomorphic Sparry Ooids (NS).--Coatings which now deformed ooids, dropped nuclei record the former pres- consist ofequant interlocking crystals of calcite are herein ence of cortical aragonite. 174 BRUCE H. WILKINSON, ROBERT M. OWEN, AND ALAN R. CARROLL

CALCITE FABRICS Primary Radial Ooids (PR).--Cortices of acicular calcite crystallites normal to grain exteriors are the dominant coating in Phanerozoic ooids. In thin section, these grains exhibit radial variations in color reflecting variable densities of included noncarbonate material and variations in the size and density of constituent crystallites (Fig. 4A); individual crystallites range from 2 to 5 #m in width and 5 to 25 #m in length. Frequently, radial laminae may occur with micritic laminae of equant, anhedral nanno- gains. These are particularly common in those portions of radial ooids where banding densities are highest (Med- wedeff and Wilkinson 1983). As mentioned earlier, modem marine radial calcite ooids, although rare, exhibit fabrics nearly identical to those seen in ancient counterparts. Modem high-magnesium calcite radial grains are reported from Baffin Bay, Texas (Land et al. 1979), from the Amazon shelf (Mil- liman and Barretto 1975), and from deeper waters off the Great Barrier Reef (Marshall and Davies 1975). On the basis of this fabric similarity, and because other originally ealcitic components in ancient limestones retain fine fabric detail, there is little reason to doubt the interpretations of Sandberg (1975) and Heller et al. (1980) that radial fabrics in ancient ooids record originally calcitic cortices. Primary Radial Spalled Ooids (PS).--A variant of primary radial ooids occurs when outer layers are separated from inner cortical carbonate. Such grains, reported from a wide variety of oolitic units, have commonly been referred to as spalled because younger laminae have been physically detached from ooid interiors. Most workers have concluded that spalling occurs during burial and compaction when ooids are subjected to lithostatic stress at point contacts between individual grains. Wilkinson et al. (1984), on the other hand, suggest that spalled radial FIo. 4.--Fabrics exhibited by originally ealeitic cortices. A) Primary ooids may record the formation ofbimineralic aragonite- radial ooid from the Jurassic Twin Creek Formation, Wyoming. Such fabrics are the most common of all ooid fabrics. B) Sparry ooids from calcite cortices in which aragonite laminae underwent the Cambrian Bonneterre Formation of Missouri. Such grains are in- selective dissolution and in which (the then separated) termediate between sparry ooids and radial ooids in that constituent calcite laminae underwent collapse. In most cases, the crystals are coarse but oriented normal to grain exteriors. former presence of aragonite layers is impossible to demonstrate conclusively. Owing to the fact that bimineralic compositions can be documented only in a few occurrences, and that preserved laminae are exclusively radial, we have tabulated all occurrences of spalled radial ooids SECULAR VARIATION IN as examples of originally calcitic cortices. ARAGONITE FABRICS Primary Sparry Radial Ooids (PSR).--Thirteen early The abundance of ooids exhibiting aragonite fabrics Paleozoic oolites exhibit a fabric which is intermediate has varied greatly throughout the Phanerozoic (Fig. 3), between radial and sparry, consisting of coarse, radiating with abundances corresponding closely to variations in crystals which widen from their origin at the nucleus to the abundance of all calcareous oolites (Fig. 1). The rel- their termination at grain exteriors (Fig. 4B); wedge-shaped ative proportion of sparry ooids, deformed sparry ooids, crystals commonly attain widths of 100--200 ~m. Such and sparry ooids with displaced nuclei has remained rel- grains are commonly "cerebroid" (e.g., Carozzi 1962) in atively constant with sparry grains the most abundant, that radial crystals, extending a few microns to a few tens deformed grains the next most abundant, and grains with ofmiccrons beyond the grain exterior, give rise to scalloped dropped nuclei the least abundant (Fig. 3). On the av- margins in section. Poorly defined pseudouniaxial crosses erage, sparry, deformed, and displaced nuclear grains are common (Fig. 4B). When planes of sections pass either comprise 58%, 28%, and 14%, respectively, of grains ex- above or below grain centers, the very coarse size of com- hibiting fabrics recording the former presence of cortical ponent crystals gives rise to fabrics similar to those in aragonite. sparry ooids. In such transects, coarse, radial crystals ap- PHANEROZOIC MARINE OOLITES 175 pear as interlocking mosaics of equant, anhedral spar. • PSR Thus, it is often possible to observe a spectrum of fabrics ranging from what appears to be radial (nuclei transected) [] Ps to sparry (nuclei above or below planes of section) in one [] PR N= 145 thin section. The original mineralogy of sparry radial ooids is not known with any degree of certaintly. Three possibilities ED % are reasonable. The first is that they exhibit primary fabrics similar to those in nonmarine low-magnesium calcite grains from a variety of spelean and artificial (boilers, S~ ~o # water-treatment plants), low-salinity settings. Although the reasons for crystal-size variations are unknown, it is at least plausible that sparry radial fabrics are primary attributes of originally calcitic grains. A second possibility is that sparry radial ooids formed as normal radial ooids with fine crystallites, and that these radial crystallites "co- alesced" through the diagenetic infilling ofintercrystalline spaces by later calcite. Coalesence has been documented K [ r in radial calcite marine cements by Lohmann (in press), 500 300 ]00 a process that gives rise to coarse radial fabrics with char- F1o. 5.--Distribution of fabrics which record primary cortical calcite. acteristic undulatory extinctions. If crystals in sparry ra- PR = primary radial; PS = primary spallcd radial; FSR = primary spar- dial ooids were originally finer grained, coalesence could ry radial. Tabulated from the 179 occurrences for which fabric data was well have produced coarser crystal sizes. A third possi- available. Note that radial fabrics prcdorainate in calcitic ooids and that bility is that sparry radial ooids were precipitated as coarse sparry radial ooids only occur in middle Cambrian, upper Cambrian, and lower Ordovician units. Also note that the fabrics in this figure and aragonite crystals that transformed to calcite while re- in Figure 3 total 232. The mason for this is that of the 179 occurrences taining gross characteristics of primary fabric. Coarse ra- from which these data were tabulated, some have two or more fabric dial aragonite ooids, although unknown in modem ma- types. rine settings, have been reported from several hypersaline lacustrine systems, most notably the Great Salt Lake in Utah (e.g., Halley 1977). Crystals in these grains are truly these as radial, spalled, and sparry radial grains have spectacular, attaining dimensions of several hundreds of varied as well; sparry radial ooids are found only in mid- microns. Because aragonite to calcite transformations may die and upper Cambrian and lower Ordovician sequences. preserve original fabric elements when aragonite crystals While it might be argued that the reason for the lack of are large, it is at least plausible that, like Great Salt Lake correspondence between sparry radial ooid abundances grains, sparry radial ooids were precipitated as aragonite. and other calcitic ooid abundances is that they were in Other attempts to evaluate the primary mineralogy of fact aragonite, comparison of sparry radial ooid abun- sparry radial grains include those by Freeman (1980), who dances with aragonite ooid abundances (Fig. 3) yields an suggested that sparry radial grains from the Cambrian equally poor correlation. Radial sparry ooids are enig- Bonneterre Formation of Missouri were originally ara- matic, both with respect to their original mineralogy and gonite, and by Sandberg (1983), who suggested that sparry their clumped distribution in early Paleozoic rocks. radial fabrics in the Cambrian Mural Limestone of British Columbia were calcitic. Due to these uncertainties, choos- AMBIGUOUS FABRICS ing among the three possibilities outlined above is the least secure of the fabric evaluations of original ooid com- Two additional fabrics which occur commonly in Pha- position. However, given that sparry radial coatings do nerozoic ooids were not tabulated as recording either ara- retain at least some manifestation of original fabric, an gonite or calcite. attribute common to other originally calcitic components, Micrite Ooids.--Micritic ooid cortices are composed of and that preliminary trace elemental analyses do not in- equant nannograins, up to 10 #m in diameter, and lack dicate significantly elevated strontium concentrations, we either radial or sparry fabrics in thin sections (Fig. 6A) have tabulated occurrences of sparry radial ooids, albeit or SEM images. Such fabrics can arise in one of two ways. tenatively, as originally calcitic cortices. In radial ooids, the length of calcite crystallites (oriented normal to grain exteriors) is commonly the same as the thickness of concentric layers that define the banding typical of all such coated grains. Heller et al. (1980) have SECULAR VARIATION IN CALCITIC FABRICS pointed out that in most ooids, banding density increases (lamina thickness decreases) as grains increase in size. The abundance of ooids exhibiting calcitic fabrics has Such increases record the greater importance of grain been highly variable throughout Phanerozoic time (Fig. abrasion over grain growth as grain mass becomes larger. 5), corresponding closely to variations in the abundance Not infrequently, lamina thicknesses soon approach val- of all calcareous ooid grains (Fig. l). The proportions of ues of the widths of radial calcite crystals and, as a result, 176 BRUCE H. WILKINSON, ROBERT M. OWEN, AND ALAN R. CARROLL

tization of grain exteriors results (e.g., Kobluk and Risk 1977). Because micritic fabrics can arise either through the addition of equant nannograins in calcitic ooids or through the activity of microendoliths in calcite or aragonite ooids, and because both processes give rise to nearly identical fabrics, it is not possible to infer the original mineralogy of micritized grains. Moldic Ooids.--The second fabric not equated with an original mineralogy is molds developed during the selective dissolution ofooids (Fig. 6B). The origin ofoomoldic porosity is indeed problematic. Since aragonite grains transform to calcite through aragonite dissolution and calcite precipitation, moldic porosity might simply record the former presence of aragonite grains. Sandberg (1983) has pointed out, however, that such interpretations may be incorrect. It is well established that calcite with ap- proximately 13 mole% MgCO3 has a solubility equivalent to that of aragonite and, therefore, high-magnesium cal- cites should dissolve as readily as aragonite. Magnesium loss from grains that have undergone high- to low-magnesium calcite transformations also demonstrates that both aragonite and high-magnesium calcite stabilize by precursor-phase-dissolution-calcite-precipitation reactions. Selective dissolution of high-magnesium calcite components, resulting in moldic porosity, however, is a process virtually undocumented in natural carbonate sequences. Although individuals such as Schroeder (1979) and Budd (1983) report limited porosity developed during high-magnesium calcite dissolution in subaerially exposed Cenozoic sequences, these are rare instances. Vir- tually all magnesian calcite components in carbonate sequences, as stated earlier, transform to low-magnesium calcite while retaining the finest details of original fabric, processes far removed from the wholesale dissolution re- quired for the development of moldic porosity. FIG. 6.--Ambiguous fabrics in Phanerozoic calcereous ooids. A) Mi- The difficulty in evaluating the original mineralogy of critic ooid from the Mississippian Ste. Genevieve Limestone, Illinois. moldic ooids is that the basis for correlation between ooid Based on gross fabric, it is not possible to infer the original mineralogy fabric and original mineralogy is empirical, being based of such coatings. B) Oomoldic porosity in the Jurassic Great Oolite of on numerous observations of components of known com- England. Pores (dark) were developed during the dissolution of ooids. Negative print of a thin section. Small white ovoids are bubbles in the position in Cenozoic carbonate sequences, and lacks any thin section. theoretical chemical underpinning in support of these interpretations. While the bulk of natural observations strongly suggests that oomoldic porosity records the for- growth proceeds through the accretion of equant nan- mer presence of aragonite, equivalence of magnesian eat- nograins. Medwedeff and Wilkinson (1983) have further cite and aragonite solubilities in conjunction with rare shown that lamina thickness (and the accretion of radial reports of molds developed during high-magnesium cal- versus equant crystallites) depends both on rates of car- cite dissolution precludes a secure correlation between bonate precipitation (which is controlled by ambient water oomolds and aragonite grains. As a result, moldic porosity chemistry) and rates of cortex abrasion (which is deter- in oolite units has not been tabulated as recording the mined by grain mass and levels of physical energy in former presence of either aragonite or calcite cortices. formational environments). These factors may be nearly equal during the growth of an entire cortex and, as a consequence, resultant fabrics are micritic. SECULAR VARIATION IN MICRITIC AND MOLDIC FABRICS Micritic fabrics, however, also can arise through the activity of microendolithic organisms, primarily cyano- The abundances ofmicritic and moldic fabrics in Pha- phytic algae (e.g., Margolis and Rex 1971). These organ- nerozoic sequences are dissimilar. Micritic ooids, al- isms excavate tubular borings into nearly all carbonate though uncommon, occur with roughly equal frequency substrates, including ooid cortices during growth; borings in Paleozoic, Mesozoic, and Cenozoic sequences (Fig. 7). are subsubsequently filled with micron-size calcite ce- Moldic porosity, on the other hand, shows a clumped ment crystals. When borings are dense, complete micri- distribution; it is particularly common in Permo-Triassic PHANEROZOIC MARINE OOLITES 177

~® ' LS

] Micritic ~® LM w i I Moldic _J

~__~UM @ I MC "@-- d T ~U~

M,~ LP TI ~' ® 'L~

500 300 lO0

FtG. 7.--Secular variation in the abundance of micritic and moldic i [ I I I I I I i I 20 40 60 80 oolites. Note that micfitic stains are fairly evenly distributed whereas % /IRAGONITE FABRICS moldic grains occur most commonly in Permo-Triassic sequences. FIG. 9.-- Relationship between mean aragonite content of ooid cor- rices and positions of relative global sea level from Vail et al. (1977). sequences (Fig. 7), a distribution that is similar to that Bars encompass95% confidencelimits around mean compositions.Note a general trend relating increasing amounts of cortical aragonite to in- seen in the abundance of aragonite fabrics (Fig. 3). This creasing continental emergence. similarity suggests, but only suggests, that most oomolds were originally aragonite grains. mineralogy, and 4) the great variation in oolite abundances throughout Phanerozoic time. SECULAR TRENDS IN OOID MINERALOGY Calculation of 95% confidence limits on mean cortical The relative abundance of cortical aragonite and calcite compositions demonstrates that of the 26 time slices for in Phanerozoic marine carbonates has not remained con- which ooid fabric data is available in Figure 8 (no fabric stant, but, as suggested by MacKenzie and Pigott (1981) data for lower Devonian and upper Ordovician units), 11 and Sandberg (1983), bears a crude relationship to gross intervals are represented by a sufficiently small number positions of global sea level (Fig. 8). Quantification of of occurrences that mean compositions are not known such a relationship, however, is a less than straightfor- with any degree of certainty (95% confidence limits exceed ward exercise owing to l) a lack of definitive data on the _+ 50%). Only 15 of the 28 time intervals are represented absolute positions of global sea level over the past 570 by sufficient samples so that average cortical composi- million years, 2) a paucity of quality data on oolite fabrics tions are meaningful. Comparisons of mean compositions in Phanerozoic marine sequences, 3) the necessity of sam- for these intervals with positions of global sea levels from pling fairly small time intervals in order to establish sig- Vail and others (1977) indicate a general relationship be- nificant correlations between global eustasy and cortex tween conical mineralogy and global eustasy. Increased continental flooding corresponds to increased amounts of

CALCITE ,g, LO UC ARAGONITE t 20 LS

® LM .B Jd 15 J (D v I ® ~ MC .s E, w, w i• ® iUT > 0 12 D 10 g O ® , JP UIP M& uP ® ® z ]4 L~ Z .2 , ® ,Q

d~ i 21 I i i I i I I i 0 40 GO 80 500 300 100 % ARAGONITE FABRICS Fro. 8.-- Distribution of occurrences of originallycalcite and aragonite FIG. 10.--Relationship between mean aragordte in cortical carbonate ooid cortices. Data tabulated from the 179 Phanerozoic occurrencesfor and calculated areas of exposed (or flooded) continental crust. Bars which diagnostic fabric data were available. Note a general correspon- encompass 95% confidence limits as in Figure 9. This correlation is dvnce between abundant aragonite cortices and periods of continental significantat the 10% level, sliBhtlybetter than that for Figure 9. Con- emergence. tinental areas calculated from sea-level positions of Vail et al. (I 977). 178 BRUCE H. WILKINSON, ROBERT 3,'1. 0 WEN, AND ALAN R. CARROLL

during submergent modes is more problematic in that LM large volumes of relatively shallow marine carbonate oc- 60 U¥ LD cur in the rock record. Two scenarios for a paucity of LS @ UC M£ highstand oolites seem plausible. The first is that during t q!k q : @ i --K• i ~ q times of maximum transgression, epicratonic carbonate- o LIP producing environments, although extensive, may have UIP ~_ 4o been too deeply flooded to allow for development of the wave-agitated settings necessary for ooid formation. Al- ternatively, highstand marine environments may have L~ @ q LP ® been ideal for ooid generation from a physical standpoint, but some aspect of atmospheric-hydrospheric chemistry 20 UT may have precluded the widespread abiotic precipitation ® t of calcium carbonate. Ideally, quantitative data on the volume and lithology Q of various Phanerozoic shallow-marine carbonate facies i 20r I 4tO 6JO I t 80'• q, t would allow for the proper choice between these two % ARAGONITE FABRIC5 models. Unfortunately, to our knowledge, such a data set FIG. l l.--Mean cortical mineralogies versus percent of continent does not exist and any interpretation must rely on more flooded. Freeboard data from Hallam (1977). This represents the best indirect lines of evidence. Several of these, however, sug- correlation (significant at lhe 5% level) obtained between various data sets on global eustasy and ooid mineralogy. gest that highstand chemical parameters rather than physical parameters precluded the formation ofooids in highstand seas. First, rates of carbonate production determined cortical calcite; increased emergence corresponds to in- for a variety of shallow Holocene settings are on the order creased amounts of cortical aragonite (Fig. 9). A slightly of 1 m/1,000 yrs. Even if average Phanerozoic carbonate better correlation is obtained between mean cortical mi- production rates were several orders of magnitude lower, neralogies and calculated areas of continent exposed or as is suggested by data from Wilson (1975) and Sadler flooded (Fig. 10), with a statistical significance better than (1981), based on estimates of absolute sea-level positions the 10% level (i.e., the probability that this degree of by Vail and others (1977), they should have been high correlation is random is less than 10%). enough to bring carbonate depositional surfaces well above In addition to eustatic data from Vail et al. (1977), wave base rather rapidly dunng periods of maximum mean cortical mineralogies were also compared to epoch- flooding. In other words, it is difficult, given estimated interval freeboard data (as % of continent flooded) from rates of marine transgression and regression such as those Strakhov (1948), Termier (1952), Schuchert (1955), Vin- in Figure l, and reasonable estimates of rates of carbonate ogradov (1967-69), and Cook and Bally (1977), as sum- deposition, to envisage situations where epicratonic car- marized by Wise (1974) and Hallam (1977). Among these bonate environments would remain deeply flooded for data sets, variations in the degree of inundation of the any significant length of time, particularly during the long U.S.S.R. in Hallam (1977) yields the best correlation with middle Paleozoic and upper Mesozoic-lower Tertiary mean cortical mineralogies. The series ofpaleogeographic highstand time intervals during which oolites are rare and facies maps of the Soviet Union (Vinogradov 1967- (Fig. 1). 69) on which these freeboard data are based (Hallam Second, although carbonate oolites are uncommon in 1977) are as yet unmatched for any other continental highstand sequences, data from Van Houten and Bhat- region and allow for the most detailed analysis possible tacharyya (1982) demonstrate that just the opposite is of changes in the aerial extent of seas through the Pha- the case for chamosite-hematite ironstone oolites (Fig. nerozoic. The significance of the correlation between ooid 12). With the exception of lower and middle Jurassic mineralogies and these data is better than the 5% level examples, Phanerozoic ferruginous coated grains are (Fig. 11). largely restricted to highstand marine sequences. Their abundance in such units requires that highstand marine settings were sufficiently shallow that grains were agitated SIGNIFICANCE OF O01D ABUNDANCES on the sea floor. This dominance of ironstone oolites and Data on Phanerozoic marine ooids demonstrate that the rarity of calcareous oolites in highstand units strongly their abundance and composition have varied greatly over suggests that ambient water chemistry rather than am- the past 570 million years, with differences corresponding bient wave energy precluded calcite or aragonite ooid to variations in the position of global sea level in that growth in highstand seas. Such chemical variations also there is a general relationship between high oolite abun- relate to the mineralogy of cortical carbonates. dances during periods of transgression and regression, and low abundances during periods of continental emergence SIGNIFICANCE OF OOID MINERALOGIES and submergence (Fig. 1). While a paucity of oolite during emergent modes is readily related to restricted areas of Three aspects of secular variations in oolite mineralogy shallow marine deposition and lesser volumes of shallow are worthy of comment: 1) How significant is the corre- manne facies, as mentioned earlier, a similar lack of oolite lation between average cortical composition and global PHANEROZ01C MARI3t~E OOLITES 179

.xx eustasy? 2) What factors can be identified that might relate PHANEROZOIC IRO5 positions of global sea level to the variables which po- tentially influence cortical mineralogy? 3) Which of these 20 best accounts for variations in ooid abundance and composition through Phanerozoic time?

CORTICAL MINERALOGY AND GLOBAL EUSTASY 13B

Just how good is the relationship between cortical min- Z eralogy and global eustasy? We have determined that the best statistical correlation is that between mean ooid mineralogy and continental area flooded or exposed (Figs. 10 and 11). Do these correlations in fact manifest an un- derlying genetic relationship between these parameters? Each reader ultimately will have to decide both on the basis of the data herein as well as his own cynicism con- cerning such statistical relationships. It should be noted, FiG. 12.-Abundance of oolitic ironstones in Phanerozoic sedimentary sequences from Van Houten and Bhattacharyya (1982), who con- however, that these correlations are remarkably high in elude that these ferric oxide and chamosite ooids developed in detritus- light of two aspects of the data. The first deals with the starved settings of mild climate. Note that with the exception of Jurassic accuracy of estimates of global sea levels. Freeboard curves ironstones, these coated grains were most abundant during periods of for a variety of continents, particularly North America continental submergence. Calcareous ooids are rare in coeval limestone and Asia, and attempts to quantify such estimates (e.g., sequences. Schuchert 1955; Wise 1974; Vail et al. 1977; Hallam 1977) yield highly disparate results. Imprecision in such quantifications may be a significant source of"noise" in carbonate equilibria must be centrally involved. The two such correlations. The second of these aspects is probably of these that have received the widest attention involve even more important and is less easy to evaluate, let alone changes in either ambient Mg/Ca ratios of seawater quantify. This is the degree to which local variations in (Sandberg 1975) or atmospheric-hydrospheric CO2s (Pi- chemical and physical processes, rather than global sec- gott and MacKenzie 1979). Two aspects of each of these uiar variations, have determined the original mineralogy variables merit note: 1) What is their influence on the of abiotic carbonate precipitates. Considering the Holo- mineralogy of abiotically precipitated carbonate phases? cene as our best-studied time period, marine calcite and 2) How do potential changes in these variables relate to aragonite are known to coexist as both cement and cor- processes which control positions of global sea level? tical phases. With respect to cement, aragonite is domi- Mg/Ca, pC02, and Carbonate Polyrnorphs.--It has long nant in some localities; (e.g., James and Ginsburg 1979); been recognized that ambient Mg/Ca activities are di- calcite is dominant in others (e.g., Land and Moore 1980). rectly (although possibly not causally) related to the pre- Even in marine calcite cement, magnesium contents range cipitated calcium carbonate polymorph. Owing to the fact from less than 4 mole% MgCO3 (e.g., Schlater and James that low-magnesium calcite, then high-magnesium calcite 197g) to over 18 mole% MgCO3 (e.g., Longrnan 1980). and aragonite, then aragonite, is the sequence of phases In ooids, dominant mineralogies have been shown to vary commonly precipitated from fluids with increasing Mg/ from dominantly aragonite to increasingly calcitic along Ca ratios, Mtiller and others (1972), Folk (1974), and a mere 10 km of shore zone in Baflin Bay (Land et al. many others have suggested that the relative concentra- 1979). Thus, we observe that ooid mineralogies are highly tion of Mg +÷ ions versus Ca ++ ions may determine the variable and dependent on local conditions within a nar- mineralogy of precipitated phases. Because Mg/Ca ratios row spatial and temporal framework, yet are also signif- of modern seas are about 5, and because data from MOiler icantly correlated with certain eustatic indexes on a global and others (1972) suggest that low-magnesium calcite to scale throughout the Phanerozoic. Such a relationship high-magnesium calcite plus aragonite Mg/Ca threshold clearly supports the argument that local conditions which ratios are about 5, Sandberg (1975) and Milliken and determine the relative proportions of cortical calcite and Pigott (1977) have invoked variations in marine Mg/Ca aragonite are themselves ultimately influenced by secular ratios to explain inferred differences in cortical miner- chemical variations on a global scale. alogies in Phanerozoic seas. Pigott and MacKenzie (1979), on the other hand, recognized that the composition of carbonates precipitated GLOBAL EUSTASY AND ATMOSPHERIC- at saturation from seawater can be determined by at- HYDROSPHERIC CHEMISTRY mospheric pCO2s at an essentially constant Mg/Ca ratio. If it is accepted that some process or processes have They concluded that changes in cortical mineralogy were linked dominant cortical mineralogies to positions of probably due to changes m atmosphefic-hydrospheric COz global sea level, what then are these processes? Obviously, levels and not to major perturbations in the Mg/Ca ratio some parameter (or parameters) which influence marine of seawater. Low CO2 levels favor aragonite precipitation; 180 BRUCE H. WILKINSON, ROBERT M. OWE~. AND ALAN R. CARROLL

PRODUCE CARBONATE & PYRITE EMERGE TERRESTRIAL MATERIALS INPUT DIAGENETIC 8~ METAMORPHIC C02 OXIDIZE SULFIDES t ERODE SULFATE & ORGANIC CARBON PCO z EROOE CARBONATES PC02 RESTRICT TERRESTRIAL 8IOTAS ERODE METAL SILICATES INCREASE RESPIRATION 8~ OXIDATION INCREASE PHOTOSYNTHESIS PRECIPITATE EVAPORITES

SUBMARINE WEATHERING IMPORTANT

Mg/Co 1. PYROXENE + PLAGIOCLASE + Mg*~.-.-~,,- Q SUBMARINE WEATHERING LESS IMPORTANT CHLORITE + EPlDOTE + H* DIOPSIDE + ENSTATITE + PLAGIOCL.&SE + Mg --~ Mg/Co CHLORITE + EPIDOTE + QUARTZ + Co*~ EMERGENT MODE 2. H* ÷ HC03 ~ H20 + CO2+ Co*o+ 2HCO~, --]*'- CoCO3 + H20 '+ C02{ ARAGONITE SEAS SUBMERGENT MODE FIo. 14. -- Diagrammatic model of lowstand geochemical processes. Top = Subacrial oxidatafion ofsufides, erosion of carbonates and metal CALCITE SEAS silicates, and increased photosynthesis serve to depress atmospheric CO2 levels (from MacKenzie and Pigott 1981). Bottom = Decreased mag- FIG. 13.--Diagrammatic model of highstand geochemical processes. nesium removal at cooler ridges and addition of magnesium ions from Top = Diagenetic and metamorphic reactions at subduction zones, sul- the weathering of silicates serve to elevate ambient Mg/Ca ratios. fate reduction and carbonate deposition reactions in epicontinental seas, and decreased photosynthesis serve to elevate atmospheric CO2 levels (from MacKenzie and Pigott 1981). Bottom = 1) Idealized greenschist With respect to Mg/Ca ratios, it was originally proposed metamorphic reactions at hydrothermal ridges convert plagioclase and marie minerals to chlorite and epidote while removingmagnesium (and by Sandberg (1975) that the explosive diversification of adding calcium and hydrogen)ions from convectingsea water. 2) Both planktonic protists in the Mesozoic and the subsequent H+ and Ca+ ions liberated during basalt alteration drive additional CO2 deposition of large volumes of calcium carbonate in deep into the atmosphere. Mg/Ca ratios of sea water are depressed. ocean basins served to elevate Mg/Ca ratios (primarily through calcium removal) and cause aragonite production high CO2 levels favor calcite (and possibly dolomite) for- in post-Jurassic seas. As subsequently pointed out by marion. Sandberg (1983), however, such a mechanism for chang- Mg/Ca, pCO2, and Global Eustasy.--What processes ing Mg/Ca ratios is not in accord with the abundance of link positions of global sea level to Mg/Ca ratios and to lower Paleozoic or upper Paleozoic-lower Mesozoic cor- COz levels in marine systems? With respect to COz, tical aragonite, nor the dominance of cortical calcite in MacKenzie and Pigott (198 l) suggested that submergence upper Mesozoic-lower Tertiary seas (Fig. 8). gives rise to l) low rates of erosion and deposition of It is now recognized that the principal control on ocean- terrigenous elastic sediments, 2) widespread deposition ic Mg/Ca ratios relates not to the abundance of planktonic of carbonates, and 3) increased rates ofproducrion of CO2 protists, but to the hydrothermal alteration of basalts at from diagenetic and metamorphic reactions at subduction midoceanic ridges. Numerous studies (e.g., Bischoff and zones, all of which result in relatively high atmospheric Dickson 1975; Hajash 1975; Wolery and Sleep 1976; Bloch CO2 levels, and the dominance of calcite in Phanerozoic and Hofmann 1978; Edmond et al. 1979; Bender 1983) seas (Fig. 13). They further suggested that emergence pro- have demonstrated that rates of greenschist metamor- motes 1) weathering in tcrrcstfial environments, 2) ero- phism at spreading hydrothermal ridge systems may have sion of carbonatcs, 3) erosion of metal silicates, and 4) an overwhelming influence on Mg ++ and Ca ++ concen- increased photosynthesis, all of which result in low at- trations in ocean water. During such submarine weath- mospheric CO2 levels and the increased dominance of ering, the Mg/Ca ratio of circulating seawater is drastically aragonite in Phanerozoic seas (Fig. 14). lowered, primarily through magnesium removal (e.g., PHANEROZOIC A,tARINE OOLITES 181

Bloch and Hofmann 1978), but to some degree because threshold values that control cortical mineralogy. Un- of calcium addition (e.g., Bender 1983). Because changes fortunately, inadequate data now exist to make these cal- in the gross position of global sea level directly relate to culations. However, two qualitative aspects of the data global rates ofseafloor spreading (e.g., Pittman 1978) and on ooid abundances and cortical mineralogies suggest that because rates of ridge spreading directly relate to rates of MacKenzie and Pigott (1981) and Sandberg (1983) are convective heat loss during seawater-basalt exchange re- correct in their suggestion that ooid composition is pri- actions (e.g., Bender 1983), it is apparent that Mg/Ca marily (although indirectly) controlled by CO2 levels. The ratios should decrease during periods of submergence and first of these is that carbonate oolites are strikingly rare rapid spreading (Fig. 13), achieving minimum values dur- in highstand sequences, while both ferric oxide and cham- ing highstands, and should increase during periods of sea- osite-coated grains are common. If Mg/Ca ratios alone level lowstands and decreased rates of spreading (Fig. 14), were suppressed during these time periods, there is little achieving maximum values during lowstands. Hence, low reason to expect that such changes would inhibit abiotic Mg/Ca ratios (highstands) should facilitate calcite precip- carbonate formation. On the contrary, abundant exper- itation; high Mg/Ca ratios (lowstands) should facilitate imental data suggest that elevated Mg/Ca ratios suppress aragonite precipitation. rates of carbonate precipitation while lower Mg/Ca ratios In fact, changes in pCO 2 and ambient Mg/Ca as related enhance precipitation. Atmospheric CO2 levels, on the to rates of spreading and global eustasy (Figs. 13 and 14) other hand, may have reached sufficiently high levels dur- both should change in a simultaneous and complemen- ing periods of maximum submergence that physicochem- tary fashion to influence cortical mineralogies. During ical carbonate precipitation was inhibited. This is not to submergent modes, pCO2 elevating reactions as outlined imply that carbonate sediments were not forming; they by MacKenzie and Pigott (1981) and Mg/Ca ratio sur- surely were and in vast volumes. It does imply, however, pressing reactions at submarine ridges as outlined by that the degree of carbonate saturation may have been DeVore (1983) should both facilitate calcite precipitation lower, that spontaneous nucleation and growth ofabiotic (Fig. 13). In addition, Ca *+ and H + ions, liberated during carbonate crystals on ooids was inhibited, and that wide- magnesium-exchange reactions between seawater and hot spread and diverse communities of calcareous metazoans basalt (Fig. 13), should shift marine carbonate equilibria were, even more than today, responsible for carbonate such that additional CO2 is liberated to the atmosphere. production in shallow, highstand seas. Conversely, during emergent modes, CO= depressing re- The second aspect of the ooid data that implies a pre- actions as outlined by MacKenzie and Pigott (1981) de- dominant role for variations in atmospheric COz levels creased rates of magnesium removal at hydrothermal is that there is a better correlation between mean cortical ridges and increased delivery of magnesium (from sub- mineralogy and areas of continents flooded or exposed aerial weathering reactions) to oceans by rivers, all fa- than there is between mineralogy and positions of global voting cortical aragonite precipitation (Fig. 14). sea level. While less compelling than ooid abundance Recently, Berner and others (1983) have developed a data, this relationship suggests that ooid mineralogies were mathematical model which was used to hindcast atmo- more closely tied to areas of continents exposed than to spheric CO2 levels from the late Cretaceous (100 m.y.) to simple rates of spreading as reflected in vertical changes the present based on various seafloor-spreading curves. in sea level. MacKenzie and Pigott (1981) suggest that A significant conclusion of this modeling effort was that, rates of weathering, oxidation, and reduction of various regardless of which specific spreading-rate curve was con- earth-surface carbonates, sulfates, silicates, and oxides, sidered, all evidence suggests a marked decline in CO2 may have had a significant influence on atmospheric CO2 levels during this period. Based on the arguments made levels (Figs. 13 and 14). Because the importance of these above, this trend should be manifested by a transition processes relates to areas of continent exposed or flooded, from predominantly calcite (late Cretaceous) to predom- by implication it was CO2 levels as determined by the inantly aragonite (present-day) cortical mineralogies. This rates of these processes that controlled the mineralogy of is indeed what we have observed (Fig. 8). In general, both abiotic carbonate phases throughout the Phanerozoic. subaerial and submarine chemical processes favor emergent-mode calcite and submergent-mode aragonite pre- ACKNOWLEDGMENTS cipitation. Many students, both undergraduate and graduate, made numerous trips to libraries while assembling the data on SUMMARY which this paper is based. To them we offer our sincerest Given that expected variations in both Mg/Ca ratios thanks. Considerable tutoring on greenschist metamor- and pCO2s with changes in global sea level are in accord phic reactions by Eric J. Essene and help with binomial with observed variations in the cortical mineralogy of statistics by Karen Rose Cercone are gratefully acknowl- Phanerozoic marine oolites, can an estimation of the rel- edged. Robert L. Folk graciously loaned us the Texas- ative importance of these two variables be made? Since Austin oolite collection. SEM images were made at the the two variables are themselves related, a quantitative Microbeam Laboratory at the University of Michigan. answer to this question would require a comparison of Figures were drafted by Derwin Bell, Department of Geo- the partial time derivation of each parameter during pe- logical Sciences. Reviews of the original manuscript by riods of emergence and submergence with the critical Daniel Bernoulli, Hans Ffichtbauer, and Judith Mc- 182 BRUCE H. WILKINSON, ROBERT M. OWEN, AND ALAN R. CARROLL

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