Shallow-water, eruption-fed, mafic pyroclastic deposits along a Paleoproterozoic coastline: Kangerluluk volcano-sedimentary sequence, southeast Greenland

Abstract

The 200–300 m thick, volcano-sedimentary sequence at Kangerluluk is part of the psammite zone, one of four major zones, which constitute the 1.8 Ga Ketilidian orogen in south Greenland. Three lithofacies are emphasized in the study: (1) the conglomerate-sandstone; (2) the volcanic; and (3) the pyroclastic lithofacies. The 2–40 m thick conglomerate-sandstone lithofacies represents a subaerial to subaqueous fan-delta deposit. Matrix- and clast-supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits. Erosive-based conglomerate channel fills attest to stream incision. Trough crossbedded sandstone, interpreted as lunate megaripples, planar-bedded sandstone indicative of upper flow regime bar-top sands, and small-scale trough crossbeds reflecting ripples follow up-section, form collectively with the conglomerate, 0.40–2.50 m thick fining-upward sequences. The sandstone-dominated unit, up-section from the conglomerates and composed of planar and low-angle crossbeds, minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base. The clastic sedimentary rocks are suggestive of a fan-delta setting. The 100–200 m thick volcanic lithofacies, composed of pillowed and pillow brecciated lava flows, is consistent with shallow-water deposition. Interstratification of lava flows with both conglomerate-sandstone and pyroclastic lithofacies, intrusion of dykes into volcaniclastic rocks, and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs. The 1–50 m thick, pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate-sandstone lithofacies, was emplaced in a subaqueous setting. The lithofacies is divided into a planar- to crossbedded tuff-lapilli tuff and a bedded lapilli tuff breccia, whereby both deposits are inferred to result from shallow-water surtseyan-type eruptions. The 5–15 m thick, bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes. The deposits are considered proximal to the vent. The 2–50 cm thick, planar- to crossbedded tuff-lapilli tuff featuring abundant euhedral and broken crystals of feldspar (≤2cm) and minor pyroxene (≤1 cm), are massive, graded, crossbedded and stratified. The planar but laterally discontinuous beds, characterized by abundant low-angle scours, are interpreted as low- to high-concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion. Local breccia-size pyroclasts disrupting beds are interpreted as bomb sags. The mafic, eruption-fed, Surtseyan-type deposits, postulated to be a subaqueous counterpart of cold, subaerial base surges, originate from subaqueous tuff cones formed along a rugged volcanic-dominated shoreline featuring high-energy fan-deltas.