Bosiljka Glumac

Carbonate Deposition and Sequence Stratigraphy of the Terminal Cambrian Grand Cycle in the Southern Appalachians, U.S.A.

ABSTRACT The cessation of shale and carbonate deposition of the Conasauga Group grand cycles (Middle to Upper Cambrian) and the establishment of the widespread peritidal carbonate deposition of the Knox Group (Upper Cambrian to Lower Ordovician) represent a prominent change in sedimentation along the early Paleozoic passive continental margin in the southern Appalachians. To evaluate the causes for this change, this study focuses on the Maynardville Formation, which is the uppermost carbonate unit of the Conasauga Group. The Maynardville consists of: (1) a lower subtidal facies succession, which is underlain by the Nolichucky Shale and resembles the rest of the Conasauga Group carbonate deposits; and (2) an upper peritidal facies succession, which is conformably overlain by similar peritidal deposits of the Copper Ridge Dolomite (Knox Group). Deposition of shale and subtidal carbonate took place in deep-ramp (Nolichucky) to shallow-ramp, subtidal-shoal, and lagoonal settings (lower Maynardville). The carbonate ramp was westward sloping toward the Conasauga intrashelf shale basin. To the east, the ramp was linked to a broad, semiarid carbonate tidal flat encompassing a variety of peritidal environments (upper Maynardville and Copper Ridge). The Maynardville is a shallowing-upward succession that formed by carbonate platform aggradation and basinward progradation. The transition between the Maynardville and the Copper Ridge Dolomite is within a conformable peritidal carbonate succession that contains common siliciclastic sand-size detritus. This interval is interpreted as a sequence boundary correlative with the craton-wide late Steptoean (Dresbachian-Franconian or Sauk II-Sauk III) unconformity. The change from a Conasauga to a Knox depositional style may be related to distinct stages in passive-margin evolution. The end of grand-cycle deposition in the early Late Cambrian is coincident with the cessation of tectonic activity along extensional features (an intracratonic graben and other fault systems), and marks the transition into a mature-passive-margin setting. The mature margin was characterized by decreased rates of thermal subsidence, which, coupled with the infilling of the Conasauga intrashelf basin, favored shallow-water carbonate deposition. The final stabilization of the margin is reflected in the deposition of the thick peritidal carbonate strata of the Knox Group.

Selective dolomitization of Cambrian microbial carbonate deposits; a key to mechanisms and environments of origin

The Upper Cambrian Maynardville Limestone (Conasauga Group) and the lower Copper Ridge Dolomite (Knox Group) of the southern Appalachians contain a variety of microbial carbonate deposits, such as stratiform (cryptalgal laminates), laterally linked hemispheroidal (LLH), vertically stacked hemispheroidal (SH), columnar, and digitate stromatolites, as well as thrombolites. Digitate stromatolites and thrombolites, in most cases, do not exhibit evidence of significant dolomitization, even though they are commonly embedded within completely dolomitized deposits. These microbial deposits formed primarily by calcification of cyanobacteria in lower intertidal and upper subtidal environments, which were not primary sites for dolomitization. Early diagenetic calcification of cyanobacteria additionally reduced the susceptibility of these deposits to dolomitization. Extensively dolomitized microbial deposits (LLH, SH, and columnar stromatolites, and most of stratiform stromatolite laminae) formed primarily by the trapping of sediment in supratidal and intertidal environments on arid to semi-arid tidal flats. Pervasive syndepositional calcification of cyanobacteria may have been precluded by conditions of periodic emergence, hypersalinity, and the presence of finegrained sediment serving as competing sites for carbonate mineral nucleation. Extensive dolomitization altered these peritidal carbonate sediments early in their diagenetic history (penecontemporaneous or syngenetic dolomitization). The presence of both calcitic and dolomitic laminae is indicative of a combination of trapping and calcification for the formation of stratiform stromatolites. The formation of Upper Cambrian microbial deposits was primarily controlled by the conditions within the environments of deposition, but was also biotically influenced to some degree.

Stable isotopes of carbon as an invaluable stratigraphic tool: An example from the Cambrian of the northern Appalachians, USA

This study illustrates the usefulness of carbon isotopes in stratigraphic interpretations of poorly fossiliferous strata beyond the resolution possible by biostratigraphy. The Upper Cambrian strata of the lower Gorge Formation in northwestern Vermont have been interpreted as highly condensed continental-slope deposits with a hiatus bracketed by the trilobite fauna of the Dunderbergia zone (Steptoean) and the Saukia zone (upper Sunwaptan). This interpretation was based on information from two thin fossiliferous intervals, the occurrence of one of them being unconfirmed in a recent study. Carbon isotopes provide a means for testing this interpretation because marine carbonate rocks deposited during the Steptoean Age of the Late Cambrian record a large, global, positive carbon isotope excursion (δ 13 C values of up to +5‰ relative to the Vienna Peedee belemnite standard). If the proposed age for the lowermost Gorge Formation is correct, these strata should record the start of the excursion, and the hiatus should produce an abrupt termination in the record of the excursion near maximum values. The determined δ 13 C values (−0.57‰ to +0.39‰) indicate that the excursion is not recorded at this locality. The results question the age for these strata and suggest that the hiatus is greater than previously recognized, encompassing most of the Steptoean. This major sedimentary hiatus lends supporting evidence for a significant eustatic or pan-Laurentian sea- level event during the Steptoean perturbation in the global cycling of carbon.

Discontinuity surfaces in Upper Cretaceous to Paleogene carbonates of central Dalmatia (Croatia): Glossifungites ichnofacies, biogenic calcretes, and stratigraphic implications

Substrate-controlled ichnofacies and biogenic calcretes represent key features for identification and interpretation of discontinuities in the carbonate rock record, which are of great significance for stratigraphic interpretations and correlations. Intraformational firmground and composite surfaces, as well as a regional Cretaceous to Paleogene (K–Pg) subaerial unconformity, developed in Upper Cretaceous to Paleogene intra-platform peritidal successions in central Dalmatia, Croatia (Adriatic-Dinaridic Carbonate Platform, ADCP), were analyzed for their trace fossil and subaerial exposure features. Thalassinoides (probably T. paradoxicus) box-work burrow systems of the substrate-controlled Glossifungites ichnofacies characterize the two documented firmgrounds and one composite (polygenic) surface. Rhizogenic laminar calcretes developed subsequently inside burrows of the composite surface through diagenetic overprint of marine sediment that passively infilled the burrows. While the formation of the two firmgrounds was probably caused by cessation of precipitation and/or deposition of calcium carbonate due to relative sea-level fall, the recorded trace fossils associated with the composite surface indicate that this surface developed through both submarine firmground and subaerial exposure stages probably caused by several episodes of regression and transgression, and exemplifies the general complexity of hiatal surfaces in shallow-marine carbonate successions. The regional K–Pg subaerial unconformity is characterized by biogenic (beta microfabric) calcretes with rhizoliths including Microcodium aggregates, root tubules, as well as alveolar-septal structures. Laminar calcretes and pisoids, together with in situ and resedimented speleothems, and bauxitic deposits, were also recorded. The unconformity developed due to formation of a forebulge in front of the approaching Dinaridic orogen. Ichnological and subaerial exposure features, together with stratigraphic implications derived from the analyzed discontinuities, serve as examples that can be applied to discontinuities present in carbonate successions elsewhere.

Late Cambrian (Steptoean) sedimentation and responses to sea-level change along the northeastern Laurentian margin: Insights from carbon isotope stratigraphy

Carbon isotopes are applied as tools for stratigraphic correlation of poorly fossiliferous Upper Cambrian carbonate strata in the northern U.S. Appalachians. Upper Cambrian (Steptoean) marine carbonate rocks record a significant global positive carbon isotope excursion (δ 13C = +4‰ – 5‰ Vienna Peedee belemnite [VPDB]), the timing of which is well documented in fossiliferous sections elsewhere. The Steptoean excursion peaks at a sea-level lowstand that produced the Sauk II– Sauk III sequence boundary on the North American craton. In this study, this excursion is documented for the first time in the northern U.S. Appalachians in poorly exposed limestone debris flow and olistolith deposits interbedded within 20 m of continental slope shales of the Schodack Formation. These deposits contain the only reported pre– Elvinia zone Steptoean fauna in New York and record δ 13C values of up to +3‰ . The slope carbonate sediment was mainly derived from the shelf margin and is mixed with common coarse-grained silici-clastic material. These deposits reflect a seaward migration of the siliciclastic source area (exposed craton), suppressed carbonate platform sedimentation, and shelf bypassing during the Sauk II– Sauk III sea-level fall. Nonfossiliferous dolostones and dolomitic marbles of the proposed carbonate platform correlatives (the Pine Plains Formation in southeastern New York and the Stockbridge Formation from western Massachusetts) also contain common coarse siliciclastics ; however, sampled sections do not record elevated δ 13C values, indicating that these strata are probably not of Steptoean age. This suggests that Steptoean time is represented in the carbonate platform to slope succession of the northeastern (present-day) Laurentian margin by an extremely condensed stratigraphic interval or even a hiatus.

Influence of Early Lithification on Late Diagenesis of Microbialites: Insights from δ18O Compositions of Upper Cambrian Carbonate Deposits from the Southern Appalachians

Abstract This paper documents a difference in isotopic compositions between Upper Cambrian microbial and non-microbial micritic deposits and proposes implications for diagenesis of calcimicrobial deposits. The δ18O values (−10.98 to −8.71; average −9.88 ‰ VPDB) of calcimicrite comprising shallow subtidal microbialites from the southern Appalachians are more negative than: (1) the calcimicrite from associated subtidal non-microbial deposits (−8.98 to −7.16; average −7.82), suggesting a different diagenetic history; and (2) estimates of Late Cambrian marine calcite values (−5 to −3 ‰), indicating postdepositional modifications. Early diagenetic calcification of microbial deposits promoted the formation of growth cavities and borings rimmed with marine fibrous and prismatic calcite cement. Some of the voids remained open and provided pathways for fluids during later diagenesis. The microbial deposits, therefore, experienced more pronounced diagenetic alteration than the less porous non-microbial micritic deposits. The δ18O compositions provided invaluable insights into the influence of early lithification on the later diagenesis of microbialites and into the processes that can result in poor preservation of syndepositional marine isotope signatures in these deposits.

Effects of Grand-Cycle Cessation on the Diagenesis of Upper Cambrian Carbonate Deposits in the Southern Appalachians, U.S.A.

ABSTRACT The vertical transition from the mainly subtidal alternating shale and carbonate units (or grand cycles) of the Conasauga Group (Middle to Upper Cambrian) to the peritidal dolostone of the Knox Group (Upper Cambrian to Lower Ordovician) marks a major change in the early Paleozoic passive-margin sedimentation of the southern Appalachians. The grand cycles represent an interplay between intrashelf shale basin and carbonate-platform deposition. The end of grand-cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost Conasauga Group strata--the Maynardville Formation. The Maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone. This change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the Maynardville and, in conjunction with the changes in the regional facies distribution at the end of grand-cycle deposition, extended its influence upon the burial diagenesis of these deposits. The subtidal deposits of the Maynardville contain a variety of calcite cements. Dolomite is not abundant in these deposits, where it occurs mainly as a fine-crystalline ferroan phase associated with argillaceous layers and pressure-dissolution features. The local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits, diagenesis of clay minerals, and pressure dissolution of carbonates. Ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite. The subtidal deposits also contain saddle dolomite cement associated with Mississippi Valley-type (MVT) minerals, which indicate the involvement of externally derived fluids during late burial diagenesis of these strata. The peritidal deposits, on the other hand, are extensively dolomitized. Fine-crystalline dolomite most likely represents early diagenetic replacement of tidal-flat carbonate sediment. Coarse-crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines. Fenestrae, desiccation cracks, and evaporite-dissolution voids are occluded with dolomite cement, which is commonly complexly zoned. Zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement. Saddle dolomite in pore centers, dissolutional voids, and tectonic fractures formed during late burial from warm fluids associated with the migration of MVT brines. The effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal Maynardville and the underlying Conasauga Group carbonate deposits, and between the peritidal deposits of the upper Maynardville and the rest of the overlying Knox Group. The depositional environments and early diagenesis also affected the burial diagenesis. The changes in regional facies distribution, caused by carbonate platform progradation at the end of grand-cycle deposition, resulted in a decrease in the proportion of siliciclastic components. This decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits. The Maynardville is conformably overlain by carbonate deposits of the Knox Group. This allowed the Maynardville-Knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the Ordovician foreland (Sevier) basin to the southeast during the conversion from a passive into a convergent continental margin. The Nolichucky Shale separates the Maynardville-Knox succession from the carbonate deposits of the underlying Conasauga Group, which were, in contrast, affected by the migration of burial fluids from the Cambrian intrashelf shale basin to the west.

Polygonal sandcracks: Unique sedimentary desiccation structures in Bahamian ooid grainstone

Sandcracks, which are ubiquitous in Holocene eolian and beach backshore carbonate grainstone on Alligator Point, Cat Island, Bahamas, resemble polygonal mudcracks, but formed in ooid sand without muddy matrix. In experiments on Cat Island beach sand, sediment surfaces cracked polygonally in the absence of mud or biofilms while drying at room temperature due to contraction generated by capillary effects related to surface tension attraction of interstitial water. Gravitational collapse of irregular open pores and repacking of sand grains due to loss of cohesion between particles caused by evaporation of water enhance the cracking process and appearance of polygons by providing space for cracks to expand. The polygons are held together by any remaining capillary moisture and associated meniscus cement, which precipitates as the sand dries. Polygonal sandcracks can be preserved by rapid lithification of carbonate sand, but have been documented only rarely from other localities because their formation requires well-sorted, well-rounded spherical grains rather than those making up the more common, heterogeneous skeletal and peloidal sediment in carbonate settings. Interpretation of this primary sedimentary desiccation structure provides new insights into sedimentation and diagenesis of ooid-rich deposits and can aid in recognizing ancient subaerial exposure horizons.

Near-Surface Imaging (GPR) of Biogenic Structures in Siliciclastic, Carbonate, and Gypsum Dunes

High-resolution geophysical methods, such as ground-penetrating radar (GPR) imaging, are increasingly applied to ichnological research. Large vertebrate and invertebrate burrows and tracks can be detected and resolved using center frequencies of > 400 MHz. Geophysical images of bioturbation structures in siliciclastic, carbonate, and evaporite (gypsum) dunes exhibit characteristic electromagnetic signal returns, which are associated with active burrow openings (ground–wave gap), filled burrows (hyperbolic diffraction and “pull up”), and large tracks (concave up patterns). The noninvasive imaging can be used for pseudo-3D visualization (closely spaced survey lines) and monitoring of biogenic activity (repeated surveys). Because biogenic structures induce distinct anomalies in geophysical records collected at frequencies typical of many geological investigations, caution must be taken to avoid misinterpreting them as primary sedimentary structures.

Stratigraphic framework, discontinuity surfaces, and regional significance of Campanian slope to ramp carbonates from central Dalmatia, Croatia

The sedimentology, microfacies, and stratigraphic age (from planktonic and benthic foraminifera and strontium-isotope stratigraphy) of a 300-m-thick Upper Cretaceous carbonate succession from the Island of Čiovo (central Dalmatia, Croatia) were analyzed in order to determine the lithostratigraphic, depositional, and chronostratigraphic framework. The Cretaceous strata were deposited in the southern part of the long-lasting (Late Triassic to Paleogene) Adriatic-Dinaridic Carbonate Platform (ADCP), one of a few late Mesozoic, intra-Tethyan, peri-Adriatic (sub)tropical archipelagos. The succession is separated by a firmground formational boundary into two lithostratigraphic units: the underlying Middle to Upper Campanian Dol Formation consisting of slope pelagic limestone with intercalated turbidites and debrites, and the overlying Upper Campanian Čiovo Formation composed of outer-ramp bioclastic-lithoclastic and echinoderm-dominated packstone. Age, lithology, and depositional settings of the Čiovo Formation are different from other penecontemporaneous, regionally important inner-platform carbonate successions within the ADCP domain. Therefore, the Čiovo Formation is proposed here as a new lithostratigraphic unit. Regionally important condensed intervals in the form of at least two firmground surfaces, characterized by Thalassinoides burrows (with phosphatic mineralization) that belong to the Glossifungites ichnofacies, occur in the lowermost part of the Čiovo Formation. Abrupt shallowing of depositional environments at the boundary between the Dol and the Čiovo Formation, and the generation of the formational boundary firmground, likely correlate with the regionally recorded Upper Campanian Event that represents a global eustatic sea-level fall. A regionally important subaerial exposure surface with nodular calcrete, rhizoliths, and Microcodium aggregates in the upper part of the Čiovo Formation represents a regional subaerial unconformity that was recorded across the ADCP domain and was interpreted as a consequence of diachronous and differential uplift of various parts of the platform in response to the formation of a forebulge in front of the approaching Dinaridic orogen.