Petrographic and mineralogical characteristics of diagenetic overprinting in Neoproterozoic diamictites from Murchisonfjorden, Nordaustlandet, Svalbard

The Caledonian Hecla Hoek Series (Harland & Wilson, 1956) of north-eastern Svalbard hosts an almost continuous Neoproterozoic (850 Ma to 600 Ma) succession of carbonates to siliciclastics. Significant disconformities appear only in the upper part of the Neoproterozoic succession and are related to Cryogenian glaciations (e.g. Hoffman et al., 2012). According to Harland and Wright (1979) Svalbard can be divided into three crustal blocks or terranes that were assembled during Late Caledonian time along major sinistral strike-slip faults from locations along Eastern and North Greenland. Harland (1997) later proposed that north-eastern Svalbard constituted the separate Eastern Svalbard Terrane (EST). Many workers, following Harland and Wright (1979), accept that before Caledonian time the EST was positioned along the east Greenland margin (e.g. Gee & Page, 1994; Harland, 1997; Gee & Tebenkov, 2004). However, direct paleomagnetic data has so far failed to quantify the paleogeographic position of the EST in relation to Laurentia (e.g. Denyszyn et al., 2009; Eyster et al., 2020, Macdonald & Swanson-Hysell, 2023). Indeed, there is an ongoing discussion concerning the primary character of the paleomagnetic record in the Tonian rocks of the EST (Maloof et al., 2006; Michalski et al., 2012, 2023). Simply, robust paleomagnetic data from the Cryogenian sequences of Svalbard are lacking.

The purpose of this research is to document the petrographic record of the diagenetic processes and the genesis of Fe-Ti oxides and sulphides, which are potential carriers of paleomagnetic signal in two stratigraphic units of the Cryogenian Polarisbreen Group diamictites sampled in the Murchisonfjorden area of Nordaustlandet, Svalbard (Fig. 1). This study is the first stage of ongoing paleomagnetic investigations of the EST Neoproterozoic succession.

Figure 1.

Geological map of southwestern Murchisonfjorden modified after Hoffman et al. (2012) and online interactive geological map available at the NorskPolarinstitutt website (https://www.npolar.no/). Circles/dots represent the sampling locations of the Petrovbreen Member (Mb.)/Wilsonbreen Formation (Fm.) diamictites, respectively. Paleomagnetic site of the Wilsonbreen Fm. diamictite (TILL6), sampled in 2013 and analysed by Michalski et al. (2023), is marked by a star. Stratigraphic column after Halverson et al. (2022) with Re-Os ages after Millikin et al. (2022). BSA – Bittel Spring Anomaly.

This paper is the first to integrate optical microscopy techniques, scanning electron microscopy (SEM) and spectroscopic analyses to investigate the petrography and mineralogy of the Cryogenian EST diamictites.

The Neoproterozoic succession of EST is subdivided into the Tonian, Veteranen and Akademikerbreen Groups, succeeded by the Cryogenian to Ediacaran Polarisbreen Group (Harland, 1997). The target diamictites cropping out in Murchisonfjorden belong to two stratigraphic units: the lower unit belongs to the Petrovbreen Member of the Elbobreen Formation, the upper one represents the Wilsonbreen Formation (Halverson et al., 2004; Hoffman et al., 2012).

The first records of Svalbard’s Late Precambrian diamictites were by Garwood & Gregory (1898). Kuling (1934) was the first to recognize the diamictites in Eastern Svalbard. They were described in detail by many authors (Krasil’shchikov, 1967; Chumakov, 1968; Flood et al., 1969; Edwards, 1976; Hambrey et al., 1981; Hambrey, 1982; Fairchild & Hambrey, 1984; Harland et al., 1993). Using transmitted light microscopy and cathodoluminescence, these authors focused on the sedimentology, composition and detailed clast descriptions, which allowed for lithostratigraphic correlation of Spitsbergen (Ny Friesland) and Nordaustlandet diamictite units (Fairchild & Hambrey, 1984; Harland et al., 1993).

As direct radiometric age constraints are lacking, the stratigraphic control of Svalbard’s Neoproterozoic sections has been primarily on lithostratigraphic and chemo-stratigraphic correlations (Halverson et al., 2004, 2018a,b; Hoffman et al., 2012). Only recently, Halverson et al. (2022) calibrated the age of the Svalbard Tonian succession using carbon isotope chemostratigraphy correlation with other well-dated Tonian sections from Canada, Ethiopia, and Namibia. The most current published age by Zhang et al. (2023) combines a subsidence model with new radiometric Re – Os ages from Akademikerbreen and Polarisbreen Groups and U-Pb maximum depositional age obtained from Veteranen Group of Northeastern Svalbard (Millikin et al., 2022; Zhang et al., 2023). As a result of the significant hiatus between the Russøya Member of the Elbobreen Formation and the overlying Petrovbreen Member the age of the latter lacks precision (Halverson et al., 2018a,b, 2022; Millikin et al., 2022). Based on the isotopically diagnostic Ediacaran Dracoisen Formation cap dolostone, deposited above upper diamictites, the Wilsonbreen Formation is interpreted as a representation of terminal Cryogenian (Marinoan) glaciation (Hoffman et al., 2012; Halverson et al., 2018a, b, 2022; Millikin et al., 2022).

Structurally, the Murchisonfjorden area exhibits many characteristics of thin-skinned fold and thrust belts. W-E field traverses Sparenestet to Roaldtoppen and Kinnvika to Floraberget and Celsiusberget, revealing that the prominent anticlinal folds display steep to overturned western forelimbs, often with evidence of thrust fault displacements. Cleavage development is prominent in the anticline cores. Cleavage development is lithology controlled in the associated more open synclinal folds. Subsidiary folds vary from upright (e.g. Kinnvika, south shore exposure of Celciusbergert anticline and NE Sore Russoya) to inclined structures, some associated with small-scale east-directed back thrusts. Conjugate shear faults related to brittle failure are noted on all scales. Major E-W trending transfer faults, common features of fold and thrust belts, were not evident. The investigated Polarisbreen diamictites and underlying Akademikerbreen Group rocks of Murchisonfjorden show no recognizable evidence of metamorphic alteration.

During our 2022 field investigations, 53 independently oriented paleomagnetic samples were collected from eight sites in Murchisonfjorden (Fig. 1). Specifically, six to eight samples were taken from each site. Two sites were collected from the Petrovbreen Member of the Elbobreen Formation and six from the Wilsonbreen Formation (Tab. 1). Petrographic and mineralogical observations were conducted on ten polished-thin sections representing all sites (three from the Petrovbreen diamictites and seven from the Wilsonbreen diamictites).

The petrography and mineralogy of the target diamictites were documented using integrated optical microscopy techniques, SEM, and Raman spectroscopic analyses. Following equipment was used: the Keyence VHX-7000 and Nikon E600 optical microscopes and JEOL JSM-6380LA scanning electron microscope at the Inter-Institute Microanalytical Complex for Minerals and Synthetic Substances at Warsaw University (Faculty of Geology) and Field Emission Electron Probe Microanalyzer (JXA-8530F JEOL HyperProbe) at the Department of Earth Sciences, Uppsala University. Minerals were identified using their optical features in transmitted and reflected polarized light, Energy Dispersive X-ray Spectroscopy (EDS) and Raman spectroscopy data. Spectroscopic investigations were performed using a Renishaw inVia Qontor Raman spectrometer with 532 nm laser excitation, 50 mW laser power, 1800 lines/mm grating and a Peltier-cooled CCD detector. The spectra were recorded in the 100 – 4000 cm⁻¹ range and 10% of the laser power during the measurement experiments. Analytical points were selected under a Leica optical microscope with a 50x objective.

Saturation isothermal remanent magnetization (SIRM) experiments were conducted at the Laboratory of Palaeomagnetism, Institute of Geophysics, Polish Academy of Sciences using pulse magnetizer MMPM10 for sample magnetization up to 3T, non-magnetic furnace MMTDSC for thermal demagnetization of the samples and superconducting quantum interface device (SQUID; 2G Enterprise, Model 755, USA) to measure natural remanent magnetization (NRM) and residual magnetization of the samples after each demagnetization step.