Mario Coniglio

STABLE CARBON AND OXYGEN ISOTOPE EVIDENCE OF CRETACEOUS SEA-LEVEL FLUCTUATIONS RECORDED IN SEPTARIAN CONCRETIONS FROM PUEBLO, COLORADO, U.S.A.

ABSTRACT Limestone concretions in the Blue Hill Member of the Carlile Formation (middle Middle Turonian) exposed in the Lake Pueblo State Park, Colorado, U.S.A., range in size from 20-cm-diameter, approximately spherical forms to flattened spheroids >2 m in width and >1 m in height. The earliest matrix and septarian calcite cement, with 13C and 18O values ranging mostly from -18o/oo to -8o/oo (PDB) and -3o/oo to -8o/oo (PDB), respectively, precipitated from diluted seawater during shallow burial where sulfate reduction largely controlled pore-water bicarbonate composition. In contrast, the latest septarian cements exhibit very depleted 18O values (-16o/oo) with 13C values approaching values of -1o/oo to -3o/oo, reflecting a strong meteoric-water component to pore fluids. Overall, 13C and 18O values are inversely covariant. Concretion matrix calcite and septarian cements precipitated from mixtures of modified marine and meteoric fluids. Pore-water reversals resulted from sea-level fluctuations leading up to and associated with the formation of a sequence boundary, or possibly the development of parasequences near the top of the Blue Hill Member. Relative sea-level falls caused pore-water freshening as the coastal mixing zone migrated seaward. Similarly, modified marine-derived pore waters replaced fresher pore fluids accompanying subsequent marine flooding. Carbonate cements precipitated during this active hydrologic history recorded the isotopic characteristics of their parent fluids. Negative 18O values of the meteoric end-member calcite compositions suggest derivation from high-altitude precipitation in the Sevier highlands to the west. Septarian cements in concretions, despite their enclosure in relatively impermeable host sediment, can act as sensitive indicators of relative sea-level fluctuations.

Origin and diagenesis of cryptobiontic Frutexites in the Chapel Island Formation (Vendian to Early Cambrian) of Southeast Newfoundland, Canada

A stromatolitic limestone bed in the Late Proterozoic (Vendian) to Lower Cambrian Chapel Island Formation contains sheet cracks in which the microfossil Frutexites is entombed by marine calcite cement. The sheet cracks, which formed from expansion of the microbial mat and associated sediment, stayed in communication with sea-water until they became filled with marine cement. Sheet cracks within stromatolites are a previously undescribed habitat for cryptobionts

Neomorphism and cementation in ancient deep-water limestones, Cow Head Group (Cambro-Ordovician), western Newfoundland, Canada

Abstract Lithification of thinly bedded lime mudstones and grainstones in the Cow Head Group resulted mainly from precipitation of calcite cement and neomorphic spar at shallow burial depths below the sea floor. These crystals are predominantly non-ferroan although the iron content and the size of neomorphic spar commonly increase near the margins of nodular and continuous beds of lime mudstone. Displacive fringes of ferroan fibrous calcite represent the last stage of carbonate precipitation in some limestones. Most calcite δ13C values range between 0 and −3% (PDB) but δ13C values as negative as −15% indicate that bacterial degradation of orgnaic matter variably influenced pore-water composition. Serial analyses from the centres to the margins of nodular and continuously bedded lime mudstones suggest that many of the limestones in this sequence were lithified prior to the precipitation of calcite having the prominent negative δ13C values typical of concretionary carbonate. Calcite δ18O values range from −5 to −8ℵ (PDB), indicating precipitation from 18O-depleted pore-waters of uncertian origin. Cathodoluminescence shows that aggrading neomorphism in the lime mudstones did not take place by porphyroid or coalescive consumption of a more finely crystalline mosaic to yield a more coarsely crystalline mosaic. Instead, scattered crystals enlarged by precipitation of progressively younger and eventually ferroan calcite, especially at the margins of nodular and continuous limestone beds. The cathodoluminescence zoning in calcite crystals is similar to that observed in meteorically altered limestones where pore-waters were initially oxidizing but became progressively more reducing with time. The preservation of trace element zoning, crystal growth microfabrics, and δ13C values in authigenic calcite suggest collectively that recrystallization did not occur during subsequent burial.

Formation of complex fibrous calcite veins in Upper Triassic strata of Wrangellia Terrain, British Columbia, Canada

Abstract Fibrous calcite veins are ubiquitous throughout the thinly bedded, organic-rich Upper Triassic marine mdrocks of the Queen Charlotte Islands and their lateral equivalents on Vancouver Island. These veins show variable and complex morphologies and can be grouped into several types: (a) simple; (b) anastomosing or composite; (c) boxwork; and (4) polygonal network oriented normal to bedding. Field, petrographic, and geochemical evidence suggest that vein opening, resulting from hydraulic fracturing due to elevated pore-fluid pressures, was an early phenomenon and occurred prior to significant compaction of the host sediments. Calcite fibers in the veins are up to 30 mm long and commonly oriented perpendicular to the wall but locally display conical structures. Fibrous calcites, with the exception of those in boxwork veins, are generally non-ferroan and dull to very weakly orange luminescent. The boxwork calcites are ferroan, zoned and show dull luminescence with some bright rims. δ 18 O values range from −8.2 to −21.6‰ (PDB) and δ 13 C values range from 2.0 to −4.4‰ (PDB). Although some variations are present among the different morphological types of calcite veins, oxygen and carbon isotopic values display important variations when compared geographically. The most depleted oxygen and carbon isotopic values are those of boxwork calcite and they are associated with areas where the effects of early Mesozoic plutonism were most severe. Precipitation of boxwork fibrous calcites is interpreted to have been related to hydrothermal discharge into unconsolidated host sediment, rather than to later burial. Although the hydrothermal influence on the formation of vein calcite is related to geological events specific to the Wrangellia Terrain, this study provides an alternative mechanism for the generation of fibrous calcite veins and demonstrates the local importance of hydrothermal input in the evolution of pore-water chemistry.

Subfossil bioerosion of mollusc shells from a freshwater lake, Ontario, Canada

Numerous circular, 30 to 270 μm‐diameter bore holes in several species of gastropod and bivalve shells from Julian Lake, southern Ontario, Canada are similar in appearance to the ichnotaxa Oichnus simplex, Oichnus paraboloides, and Tremichnus isp. These ichnotaxa are well described from marine systems but are rarely reported from freshwater environments. The numerous bore holes and their random placement in individual shells from Julian Lake indicate that the borers were non‐predatory. The purpose of boring may have been to scavenge either the organic material contained within the shell microstructure or the shell carbonate to be used as a source of carbonate. The Julian Lake borings may have been created by either freshwater gastropods or bacteria. The identity of the trace maker (s), however, is uncertain. Similar holes have been previously described in lacustrine settings and interpreted as dissolution pits. SEM examination of the microstructure reveals no definitive evidence for dissolution. As with m...

Hydrothermal dolomite in the Timiskaming outlier, central Canadian Shield: Proxy for Late Ordovician tectonic activity

The Timiskaming Paleozoic outlier on the central Canadian Shield preserves a thin distal portion of the once extensive Taconic deep-water shale succession that extended across eastern Laurentia. Within this succession, a thin (5 cm) lithic and phosphatic sandstone occurs a few meters above the top of the buried shallow-water carbonate platform, and contains abundant (40-60%) euhedral dolomite, rare fluorite, pyrite-cemented shale microbreccia, and crystal mosaics of subhedral dolomite with pyritic veinlets. The dolomite formed local cement, and precipitated very near the sediment-water interface as illustrated by a relatively uncompacted sandstone framework and a 3-dimensional fabric of clay particles trapped by dolomite growth. The majority of dolomite has planar crystal faces, with ferroan (4.1 ± 0.3 mol% FeCO3) crystal cores and non-ferroan (0.9 ± 0.2 mol% FeCO3) rims. Fluid inclusions, too small (<2 μm) for reliable microthermic analysis, are mostly liquid, which, in keeping with interpreted near-surface diagenesis, as well as temperature-controlled dolomite-crystal roughening models, may indicate that formation temperatures were no more than ∼60 to 80°C. δ13CPDB values (∼1.2 ‰) are similar to a Late Ordovician seawater composition, but δ18OPDB values (−4.8 to −5.2 ‰) are too negative compared to the expected values for contemporary deep-marine dolomite. Combining the regional paleoceanographic framework with diagenetic constraints and revised models for Late Ordovician seawater temperature and δ18O compositions, the dolomite is interpreted to be a proxy for a low-temperature (at least ∼50°C) hydrothermal anomaly near the sediment-water interface. Fragments of shale microbreccia, pyrite veinlets, and subhedral dolomite, along with the regions structural history, allow speculation that this event coincided with local reactivation of a Precambrian fault, part of an ancestral fault system now manifest regionally by an extension of the Ottawa-Bonnechère Graben. Late Ordovician craton-interior tectonism, in a region previously considered tectonically stable at this time, is defined by local response of inherited Precambrian structure driven by, but distal to, Taconic orogenesis.

Petrology, Elemental and Isotope Geochemistry and Geomicrobiology of Carbonate Infillings and Biofilms Lining Cracks Below the Neoproterozoic (Sturtian) Cap Carbonate in the Mirbat Inlier, Southernmost Oman

Cryptic biofilms line cracks in granite basement below a Neoproterozoic cap carbonate in southern Oman. Their depth in the narrow cracks indicates that they grew in a dark non-photic environment, and signs of upward flow within the cracks suggest that they may have grown in waters expelled from depth, which may not have been in isotopic equilibrium with contemporary ocean waters. This study tested this hypothesis and disproved it. The biofilms and associated detrital carbonate were precipitated in isotopic equilibrium with contemporary seawater. The study also confirms the 13C secular trends previously observed and attributed to seawater changes during post-glacial transgression and accumulation of the cap carbonate.