Sten-Andreas Grundvåg

Sedimentology of the Lower Cretaceous at Kikutodden and Keilhaufjellet, southern Spitsbergen: implications for an onshore–offshore link

ABSTRACT Detailed sedimentological investigations of the Lower Cretaceous succession of southernmost Spitsbergen indicate deposition during a long-term fall and rise in relative sea level. The Rurikfjellet Formation shows an overall regressive development and consists of offshore deposits grading upwards into progradationally stacked shoreface parasequences. The overlying Helvetiafjellet Formation shows a two-fold division reflecting an overall transgressive development. The lower Festningen Member represents a lateral extensive sandstone sheet that was deposited in a braid plain setting with sediment dispersal to the south-east. The unit also includes a lower Barremian subaerial unconformity at its base, demonstrating that uplift and shelf erosion also took place in southern Spitsbergen. Clinoforms observed in seismic data from, amongst others, the Lower Cretaceous in the western Bjarmeland Platform suggest a potential link between the onshore unconformity and the offshore clinoforms. The Festningen Member is capped by a coaly shale unit that represents an expansion surface which marks a change into a high-accommodation distributary fluvial system of the overlying and heterolithic Glitrefjellet Member. The overall transgressive development recorded in the Helvetiafjellet Formation eventually resulted in a marine flooding that eroded and drowned the delta plain depositing an offshore mudstone unit, 5–10 m in thickness, that marks the re-establishment of open marine shelf conditions in the basal part of the Carolinefjellet Formation. The succeeding sand-rich part of the Carolinefjellet Formation contain abundant hummocky cross-stratified sandstones deposited in an inner shelf setting, and therefore represents renewed shoreline progradation onto the shallow subaqueous shelf.

Depositional environment, ichnological features and oxygenation of Permian to earliest Triassic marine sediments in central Spitsbergen, Svalbard

Late Early Permian–lowermost Triassic carbonate, siliceous (spiculites) and clastic marine sediments in the Marmierfjellet area (Isfjorden, central Spitsbergen) contain a relatively diverse and abundant trace fossil assemblage providing important information about the depositional processes. The Vøringen Member (Late Artinskian–Kungurian) of the Kapp Starostin Formation (Late Artinskian–? Changhsingian) contains trace fossils (Nereites, Phycosiphon, Zoophycos and Arenicolites—common in tempestites) typical of the proximal–archetypal Cruziana ichnofacies, which indicates lower shoreface. Nereites, Phycosiphon and Zoophycos, accompanied by other rare trace fossils, characterize the Svenskegga and Hovtinden members of the Kapp Starostin Formation. They are interpreted as the distal Cruziana ichnofacies, possibly transitional to the Zoophycos ichnofacies typical of the lower offshore zone. However, the sporadic occurrences of Arenicolites and Macaronichnus can point to episodic shallowing to upper offshore–lower shoreface. The lowest part of the Triassic Vikinghøgda Formation (Induan–Olenekian) contains a very low-diverse ichnoassemblage composed of a few simple and branched forms ascribed to the impoverished Cruziana ichnofacies (lower to upper offshore environment), which is attributed to the early recovery stage after the Permian–Triassic extinction. The trace fossils and loss of primary sedimentary structures caused by intense bioturbation throughout most of the section point to generally oxygenated pore waters on the sea floor. However, some horizons, especially laminated black shales, display reduced or no bioturbational activity. These horizons also show high V/(V+Ni) ratios, which indicate oxygen-depleted sediments with periods of anoxic conditions. A remarkable black shale unit deposited under anoxic and sulphidic conditions occurs at the Permian–Triassic transition.

Structural Inheritance and Rapid Rift‐Length Establishment in a Multiphase Rift: The East Greenland Rift System and its Caledonian Orogenic Ancestry

We investigate (i) margin-scale structural inheritance in rifts and (ii) the time scales of rift propagation and rift length establishment, using the East Greenland rift system (EGR) as an example. To investigate the controls of the underlying Caledonian structural grain on the development of the EGR, we juxtapose new age constraints on rift faulting with existing geochronological and structural evidence. Results from K-Ar illite fault dating and syn-rift growth strata in hangingwall basins suggest initial faulting in Mississippian times and episodes of fault activity in Middle-Late Pennsylvanian, Middle Permian, and Middle Jurassic to Early Cretaceous times. Several lines of evidence indicate a close relationship between low-angle late-to-post-Caledonian extensional shear zones (CESZs) and younger rift structure: (i) reorientation of rift fault strike to conform with CESZs, (ii) spatial coincidence of rift-scale transfer zones with CESZs, and (iii) close temporal coincidence between the latest activity (late Devonian) on the preexisting network of CESZs and the earliest rift faulting (latest Devonian to earliest Carboniferous). Lateto post-Caledonian extensional detachments therefore likely acted as a template for the establishment of the EGR. We also conclude that the EGR established its near-full length rapidly, i.e., within 4–20% of rift life. The “constant-length model” for normal fault growth may therefore be applicable at rift scale, but tip propagation, relay breaching, and linkage may dominate border fault systems during rapid lengthening.