Depositional dynamics and holostratigraphy of the Lower Cretaceous Rurikfjellet Formation in Arctic Svalbard: From local storm sedimentation to global carbon cycling

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

The Early Cretaceous was characterized by important plate tectonic reconfigurations, volcanism, sea-level oscillations, and in particular oceanographic and climate fluctuations which resulted in major carbon-cycle perturbations. Long-lived humid greenhouse conditions prevailed with giant storms of non-actualistic size, magnitude and processes governing significant and relatively frequent event sedimentation of storm beds (tempestites) in nearshore and shallow-marine settings. In the high latitudes, the large circum-Arctic Boreal Realm encompassed vast epicontinental platform conditions with widespread, relatively shallow epeiric seas at the northern margin of Pangaea. However, the depositional dynamics, carbon-cycle mechanisms and stratigraphy of the Boreal basins is still to be fully understood, and precise correlation with lower-latitude basins, such as the Tethyan Realmand Atlantic and Pacific oceans, is thus commonly prohibited. This is particularly true for the lowermost Cretaceous and Jurassic–Cretaceous (J–K) boundary interval, being one of the most enigmatic stratigraphic intervals in the Phanerozoic. In addition, a unified model for the hydrodynamic mechanisms governing deposition of tempestites and the sedimentary structure of hummocky cross-stratification (HCS) most typically associated with tempestites, remains to be developed, despite their ubiquitous use in facies analysis of ancient shallow-marine systems. The High-Arctic archipelago of Svalbard represents the uplifted north-western corner of the Barents Sea Shelf and exposesa relatively continuous Upper Jurassic –Lower Cretaceous succession, allowing detailed examination of the depositional dynamics and stratigraphy of this important part of the Boreal Realm and its link to lower-latitude basins. Significantly, the lowermost Cretaceous Rurikfjellet Formationis situated immediately above the J–K boundary and forms an up to ca 400 m thick succession of well-exposed marine shale and a spectacular variety of sandstone tempestites. However, the sedimentology and integrated stratigraphy of the formation is still to be documented in detail. Consequently, this PhD study set out to conduct a detailed sedimentological and integrated stratigraphic analysis of the Rurikfjellet Formation, primarily in order to: (i) unravel the Early Cretaceous storm-depositional processes governing sand and mud sedimentation in the Boreal epeiric seas, with wider implication for general tempestite and HCS facies models; and (ii) establish a bulk-rock organic carbon-isotope (δ13Corg) stratigraphy within a biostratigraphically calibrated age framework, which can be used for correlation and inferences of carbon-cycle perturbations and dynamics at a regional and global scale.For this purpose,atotal of 33 sedimentological logs were retrieved from 19 field localities and onshore drill cores distributed across ca 9000 km2 on Spitsbergen, supplemented by 389 δ13Corg analyses and 99 source-rock screening (Rock-Eval) analyses, as well as collection of several hundreds of palynological samples and macrofossil (ammonites, belemnites and bivalves) specimens for biostratigraphic analyses. The study area covers the entire extent of a hitherto unrecognized northern relatively fine-grained clastic wedge of the Rurikfjellet Formation. The data set has led to the contribution of two authored and six co-authored published articles, one co-authored resubmitted manuscript, 21 authored and co-authored abstracts, and an authored published popular paper, which are included in the issuing thesis; as well as aco-authored unpublished atlas, which is not included in the thesis. The contributions presented in the thesis collectively provide a new depositional, palaeogeographic, holostratigraphic and agemodel for the Rurikfjellet Formation. In the study area, deposition took place across a storm-dominated and wave-dominated and fluvial-influenced prodelta to distal delta front within a high-fetch, shallow-marine to open-marine and low-gradient ramp setting. Sediment was shed from a SSW–NNE-trending shoreline towards the ESE, with sand-grade sediment delivery governed by wave reworking, downwelling storm flows and feeding from distributary channels, and with mud deposition during intermittent fair-weather conditions. The tempestite facies variability and various simple and complex configurations of HCS collectively indicate that penecontemporaneous storm deposition was controlled by relatively steady and highly unsteady waves, downwelling storm flows, as well as various storm-wave-modified hyperpycnal flows generated directly from plunging rivers during coupled storm-floods. Tempestite facies variability and resulting stratigraphic architecture of shallow-marine systems are inherently controlled by the shelf morphology, and proximal–distal facies trends of the tempestites highlight the need for reevaluation of general tempestite facies models, which may only be applicable tomoderate to low-gradient settings. In addition, a polygenetic model is presented to account for the various configurations of HCS that may commonly be produced during storms by relatively steady and highly unsteady wave oscillations, hyperpycnal flows and downwelling flows. Correlation and calibration of dinoflagellate cyst biostratigraphy, belemnite biostratigraphy, ammonite data, and Bathonian –lower Barremian δ13Corg and total organic carbonstratigraphy, permitdating of the Rurikfjellet Formation to the Valanginian – early Barremian. The base of the formation coincides with the Ryazanian–Valanginian boundary, which represents a depositional hiatus possibly combined with significant stratal condensation. The sequence stratigraphic stacking pattern of the formation tied to the calibrated age model reflects long-lived Valanginian – earliest late Hauterivian shoreline progradation, followed by shoreline retreat in late Hauterivian – early Barremian times, and a final renewed episode of shoreline progradation by forced regression in the early Barremian. The δ13Corg stratigraphyprovides a well-calibrated tool for Boreal and Boreal–Tethyancorrelations, and is predominantly characterized by: (i) a large negative excursion (≤6.4‰) in the upper Kimmeridgian – middle Volgian, consistent with the ‘Volgian Isotopic Carbon Excursion’ (VOICE); (ii) an upper Volgian – Ryazanian positive recovery, which may be useful for stratigraphic correlations across the J–K boundary; and (iii) a Valanginian prominent positive excursion (≤5.5‰), interpreted to represent the first record of the well-documented and heavily debated Weissert Event in Svalbard, followed by overall steady-state conditions in the upper Valanginian –lower Barremian. Time-calibrated correlation of the observed trends with existing δ13C records representative of the entire exchangeable carbon reservoir reflects major global carbon-cycle adjustments, including: (i) Boreal decoupling from the carbon reservoir of lower latitudes during Late Jurassic and J–K boundary times due to isolation of high-latitude basins in response to a global eustatic sea-level lowstand; and (ii) global carbon-cycle recoupling at the onset of the Weissert Event, coinciding with incipient oceanographic connectivity controlled by a global eustatic sea-level rise. Boreal variability in the decay of the Weissert Event was probably controlled by continued restricted connection between high-latitudeand lower-latitudebasins causing reduced ocean circulation in the Boreal Realm and associated partial deviation from prevailing global carbon-cycle dynamics, perhaps due toprolonged cooling.
ForlagDepartment of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen
Antal sider372
StatusUdgivet - 2020

ID: 250967163