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

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 Ke tilidian orogen in south Greenland. Three lithofacies are emphasized in the study: (1) the conglomerate-sandstone; (2) the volcanic; and (3) the pyroc lastic lithofacies. The 2-40 m thick conglomerate-sandstone lithofacies rep resents a subaerial to subaqueous fan-delta deposit. Matrix- and clast-supp orted conglomerates are interpreted as debris flow and longitudinal gravel bar deposits. Erosive-based conglomerate channel fills attest to stream inc ision. Trough crossbedded sandstone, interpreted as lunate megaripples, pla nar-bedded sandstone indicative of upper flow regime bar-top sands, and sma ll-scale trough crossbeds reflecting ripples follow up-section, form collec tively with the conglomerate, 0.40-2.50 m thick fining-upward sequences. Th e 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 elastic sedimentary rocks are suggestive of a fan-delta setting. The 100-200 m thick volcanic lithofacies, composed of pillowed and pillow brecc iated lava flows, is consistent with shallow-water deposition. Interstratif ication of lava flows with both conglomerate-sandstone and pyroclastic lith ofacies, intrusion of dykes into volcaniclastic rocks, and peperite formati on accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs. The 1-50 m thick, pyroclastic lithofacies wi th sharp depositional contacts to the overlying volcanic and underlying con glomerate-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 lapi lli tuff breccia with abundant bomb sag structures and graded beds is consi dered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly col lapsed allowing for transport via mass flow processes. The deposits are con sidered proximal to the vent. The 2-50 cm thick, planar- to crossbedded tuf f-lapilli tuff featuring abundant euhedral and broken crystals of feldspar (less than or equal to 2 cm) and minor pyroxene ( less than or equal to 1 c m), are massive, graded, crossbedded and stratified. The planar but lateral ly discontinuous beds, characterized by abundant low-angle scours, are inte rpreted as low- to high-concentration sediment gravity flows produced direc tly from subaqueous tephra jets that collapsed due to massive water ingesti on. Local breccia-size pyroclasts disrupting beds are interpreted as bomb s ags. The mafic, eruption-fed, Surtseyan-type deposits, postulated to be a s ubaqueous counterpart of cold, subaerial base surges, originate from subaqu eous tuff cones formed along a rugged volcanic-dominated shoreline featurin g high-energy fan-deltas. (C) 2000 Elsevier Science B.V. All rights reserve d.