Limestone Components
The undolomitized Vajont ooids typically display cortices that are composed of radially oriented, small
subequant to bladed calcite crystals (Zempolich, 1995).
Radial calcite fabrics, nonluminescence, enriched 13C and 18O isotopic compositions (average δ13C = +2.13‰
and δ18O = –3.12‰), low covariant Sr-Mg contents, low Mn-Fe contents, and an absence of neomorphic texture collectively suggest that Vajont ooids were originally composed of radial low-Mg calcite and underwent lit-tle diagenesis prior to dolomitization (Zempolich, 1995). These data suggest that Vajont ooids were rede-posited in the Belluno Basin as relatively pristine, min-eralogically stable low-Mg calcite.
Aragonitic and high-Mg calcite skeletal grains in the Vajont limestone that were deposited along with radial ooids in gravity flows are now replaced by low-Mg calcite (Zempolich, 1995). These grains exhibit a spectrum of fabric-retentive and fabric-destructive neomorphic fabrics, and possess enriched 13C and 18O isotopic compositions similar to radial ooids. This sug-gests that original metastable components were altered to low-Mg calcite early in the diagenetic his-tory of the limestone.
Intergranular pores in resedimented ooid grainstone were first cemented by pore-lining, nonluminescent, equant low-Mg calcite cement. Nonluminescent equant calcite cement occurs as thin isopachous linings in resedimented grainstone, as intraskeletal pore fill, and as isopachous linings in skeletal molds formed through the dissolution of original aragonite (Zempolich, 1995).
Isotopic compositions of nonluminescent equant calcite are enriched with respect to other calcite cements and fall within the field defined by radial calcitic ooids.
Analogous, equant low-Mg cement has been described in modern slope and basin settings by Schlager and James (1978). Based on these data, isopachous nonlumi-nescent equant cement is interpreted as an early marine precipitate in slope settings. Furthermore, its occur-rence in the ooid grainstones as a pore-lining phase in primary intergranular voids, intraskeletal pore space, and within skeletal molds indicates that precipitation began soon after deposition of carbonate in slope set-tings and continued during shallow burial diagenesis.
Late diagenetic calcite fabrics include banded lumi-nescent equant calcite that overlies nonlumilumi-nescent equant calcite and fills remaining intergranular pore space, coarse luminescent calcite that fills molds of skeletal grains, and fracture-filling luminescent calcite that crosscuts all previously described fabrics. Progres-sive depletion in oxygen values from banded lumines-cent calcite to mold-filling lumineslumines-cent calcite to fracture-filling luminescent calcite suggests progres-sive cementation in a burial environment (Zempolich, 1995). Importantly, late calcite cement occluded the majority of intergranular, intragranular, and moldic porosity that remained after early cementation and dis-solution in slope and shallow burial environments.
In summary, early and late diagenesis of Vajont sediments in basinal settings resulted in the formation of a relatively impermeable and mineralogically stable (low-Mg calcite) volume of rock (Zempolich, 1995).
Limestone–Dolomite Reaction Fronts
Dolomite bodies within the Vajont Limestone in both the southern and northern study areas exhibit a Figure 15. Small-scale dolomite–limestone reaction
front, Villanova locality. This mineralogic transition forms the left side of the dolomite wedge observed.
Dolomite fronts (brown; DOL) emanate from a fault (F) and propagate (open arrows) toward the left into unaltered limestone (light blue; LS) and toward the right into the core of the dolomite wedge. Arrows along the fault point toward the probable direction of fluid flow.
spectrum of replacement and recrystallized dolomite fabrics and variable Ca-Mg compositions that illustrate the initial nonmimetic replacement of limestone and progressive stabilization of intermediate dolomite phases (e.g., Kupecz et al., 1993; Kupecz and Land, 1994). These textures and compositions are distributed over centimeter- to meter-scale transitions from partially dolomitized limestone to completely dolomitized lime-stone and exhibit a concomitant increase in the degree of neomorphism and recrystallization with increasing proximity to fluid conduits (i.e., fractures, faults, and bedding planes). The inherent metastability and evolu-tion of these initial and intermediate dolomite fabrics, as defined by petrographic and geochemical study, has been explored by Zempolich (1995).
Transitions from limestone to dolomite occur over several tens of centimeters to tens and hundreds of meters in relationship to faults and fractures (Figure 15). Macroscale replacement fabrics include the gross retainment of lithoclastic grains and sedimentary struc-tures through variations in the size of replacement dolomite rhombohedra (Zempolich, 1995). Microscale dolomite textures, which record the initial step-by-step replacement of limestone by dolomite and the neomor-phism and recrystallization of initial replacement dolomite fabrics, are distributed across dolomite–
limestone transitions. These petrographic data define the mechanism by which limestone was progressively replaced by dolomite, and by which initial replacement fabrics were progressively recrystallized.
Replacement Dolomite
Initial replacement fabrics are found toward the periphery of dolomite reaction fronts within partially dolomitized limestone (Figure 16). Initial replacement dolomite is composed of calcian dolomite that con-tains inclusions of relic calcite (Zempolich, 1995). Two types of initial replacement styles are exhibited by the Vajont dolomite: intragranular replacement—initial dolomitization begins with the selective dolomitiza-tion of ooids and other grains within oolite where intergranular calcite cement has completely occluded pore space; and intergranular replacement—initial dolomitization begins with the dolomitization of ooids and intergranular matrix (i.e., carbonate mud) prefer-entially along grain peripheries (Zempolich, 1995).
Field and petrographic data suggest that these differ-ent replacemdiffer-ent styles are dependdiffer-ent on the degree of cementation within the precursor limestone fabric and original carbonate mud content. Recrystallized replacement fabrics are found in completely dolomi-tized limestone nearest to faults and fractures.
Cathodoluminescent petrography and microprobe analysis of initial replacement fabrics indicate that replacement and recrystallized dolomite found in both the northern and southern study areas luminesces a homogeneous dull red color, and that individual crys-tals are not compositionally zoned. Cathodolumines-cence also reveals that replacement dolomite crosscuts ooid grains, pore-lining nonluminescent equant calcite cement, and banded-luminescent equant calcite spar
cement (Zempolich, 1995). The widespread uniformity in luminescence and lack of compositional zoning sug-gests that initial replacement by calcite-inclusion–rich, nonstoichiometric dolomite, and neomorphism of these phases to more stoichiometric compositions, was the product of one progressive dolomitization event (Zempolich, 1995). Postdolomitization processes include the recrystallization, dedolomitization, and dissolution of initial calcite-inclusion–rich replacement fabrics.
These petrographic observations are important for several reasons. First, a replacement origin for dolomite in the Vajont limestone is inferred by the pervasive retainment of ooid ghosts in both dolomitized matrix and lithoclasts (Figure 16). These observations indicate that precursor limestone was not wholly dissolved, and later reprecipitated as dolomite in voids. Second, a late replacement origin for the dolomite is indicated by cathodoluminescent study that indicates replacement dolomitization occurred sometime after calcite cemen-tation in burial settings.
Baroque Dolomite Cement
Baroque dolomite cement is found in association with replacement dolomite along both large-scale and small-scale fractures within the Mt. Grappa–Visentin anticline, and as massive pore fill within brecciated cores of dolomite plumes located in the northern study area (Figures 11, 12). Baroque dolomite cement was not observed within limestone or along fractures within undolomitized limestone. This suggests that faults and fractures were the conduits by which dolomitizing fluid circulated, and that baroque dolomite cement was a final pore-filling phase that precipitated after replace-ment dolomitization.
Regional Stable Isotopic Geochemistry Compositions of replacement dolomite exhibit a wide range of δ18O and a relatively narrow range of δ13C values that overlap the Middle Jurassic marine carbon-ate compositions (Zempolich, 1995). Regional δ13C and δ18O compositions of replacement dolomite and baroque dolomite cement are summarized in Figure 17.
The 18O of replacement dolomite in northern dolomite localities is depleted relative to replacement dolomite located along the Mt. Grappa– Visentin anticline. Com-positions of baroque dolomite cement exhibit depleted
18O compositions relative to replacement dolomite and Middle Jurassic marine carbonate, and possess variable carbon compositions. The 18O of baroque dolomite cement appears to be uniformly depleted throughout the region. These data, in addition to fluid inclusion data (Th= 80–132°C, mean = 125°C, n = 12), suggest that replacement dolomitization and baroque dolomite cementation occurred at elevated temperature, and that dolomite replacement in the northern study area took place at higher temperatures than that of the southern study area (Zempolich and Hardie, 1991a, b;
Zempolich, 1995).
Trace Elements
A characteristic of Vajont replacement dolomite in both the southern and northern study areas is a low con-centration of Sr, Fe, and Mn (e.g., Col Visentin: Sr = 62.6 ppm, Fe = 93.2 ppm, and Mn = 27.5 ppm; Vajont Dam: Sr
= 32.1 ppm, Fe = 92.1 ppm, and Mn = 42.6 ppm) (Zem-polich, 1995). These values are similar to or are much lower than estimates of “marine” dolomite (Sr = 50–850 ppm, Fe = 10–2000 ppm, Mn = 5–275 ppm) (Al-Aasm and Veizer, 1982; Major, 1984; Saller, 1984; Aissoui, 1988;
Dawans and Swart, 1988; Vahrenkamp and Swart, 1990); “deep marine” dolomite (Fe = 2100 ppm, Mn = 590 ppm) (Lumsden, 1988); late-stage recrystallized and burial dolomites (Sr = 35–147 ppm, Fe = 287–5115 ppm, Mn = 0.1–1069 ppm) (Montañez and Read, 1992; Mon-tañez, 1994); dolomites of various depositional settings (average Fe = 2790 ppm, Mn = 245 ppm) (Weber, 1964);
and dolomites of hydrothermal brine origin (Gregg, 1985; Gregg and Shelton, 1989). This comparison sug-gests that Vajont trace element compositions are not compatible with dolomite replacement, neomorphism, or recrystallization involving fluids enriched in Sr, Fe, and Mn (i.e., burial fluids or hydrothermal brine). While recrystallization of replacement dolomite and loss of Sr, Fe, and Mn through time is a possibility (Kupecz et al., 1993), most Vajont replacement dolomite exhibits petro-graphic evidence of initial replacement crystal fronts and engulfment of dissolution-resistant precursor calcite (Zempolich, 1995). The retention of these microfabrics suggests that geochemical compositions of replacement dolomite were emplaced during initial dolomitization and not through recrystallization (Zempolich, 1995).
However, modeling of isotopic, trace element, and fluid inclusion data collected from both limestone and dolomite components indicate that Vajont stable iso-topic compositions and trace element concentrations are compatible with initial dolomite replacement and neo-morphism or recrystallization by seawater-derived fluid at elevated temperature (Zempolich, 1995). If correct, these data and models may suggest that circulation of seawater at temperatures ≤100°C may have caused dolomitization along faults and fractures within the Mt.
Grappa–Visentin anticline (southern study area), and that circulation of seawater and/or modified seawater at temperatures ≤200°C may have caused dolomitization along faults and fractures within synclines in the north-ern study area. Moreover,87Sr/86Sr values of replace-ment dolomite from southern (87Sr/86Sr = 0.707104–
0.707570; N = 9) and northern study localities (87Sr/86Sr
= 0.707040–0.708180; N = 2) overlap model ranges that utilize early Eocene and late Oligocene to early Miocene seawater values (Zempolich, 1995). Collectively, these results suggest that (1) dolomitization of the Mt.
Grappa–Visentin anticline occurred by the circulation of Early Tertiary seawater at temperatures of 35–100°C concomitant with initial early Eocene compression and (2) dolomitization of the northern dolomite localities occurred by the circulation of Early to Middle Tertiary seawater or modified seawater at temperatures ≤200°C concomitant with initial early Eocene or late Oligocene to early middle Miocene compression.
Figure 16. Dolomite replacement textures (plane light and cross-polarized light photomicrographs. (A, B) Partly replaced oolitic limestone. Replacement rhom-bohedra have preferentially nucleated within inter-granular matrix and along the periphery of ooids.
Precursor ooid structures are defined by a greater density of calcite inclusions within replacement rhombohedra (arrows). The dolomite–limestone con-tact between radial ooid cortices and replacement rhombohedra is sharp. (C) Replacement dolomite (partially recrystallized) with moldic pores “P”.
Complete replacement dolomitization of oolite results in formation of ≤10%–15% porosity.