Storm Tracks Predict Land-To-Sea Sediment Transfer: Erosional Patterns from the Upper Ordovician (Hirnantian) in the Oslo Region, Norway

Abstract

Recent refinements in nomenclature with an emphasis on member-level organization of Hirnantian strata from Norway’s Oslo region provide a useful way to grasp depositional processes recorded in the last stage of the Ordovician system. Found in outcrops throughout the many small islands of the Oslofjord and the adjacent Bunnefjord, the multifaceted Langøyene Formation is differentiated into older, mostly stratiform deposits of sandstone and oolitic limestone in the Skauern, Høyerholmen, and Pilodden Members and younger cut-and-fill breccias and conglomerates in the Kalvøya Member. Combinations of sandy and calcareous conglomerates derived from erosion of the foregoing Høyerholmen, Pilodden, or older lithologies make up the preponderance of pebble- to boulder-size materials in the basal parts of the Kalvøya Member. As many as three generations of overlapping incisions into fully hardened strata resulted from cyclic changes in sea level during late Hirnantian time. Conditions under which channels were eroded and filled were variable, dependent on position along a preexisting gradient from land to sea. From 50 to 100 m in cross section, proximal streambeds accumulated fill characterized by chaotic debris-flow deposits as well as more normal riverine deposits with imbricated clasts. In a middle-slope position, channels exhibit a more mature organization, with clast-supported deposits that formed as flood jets off river mouths. Most distal in position, true tidal channels are eroded into strata with trace fossils diagnostic of normal marine conditions. Evidence from additional parts of the paleocontinent of Baltica in Norway, Sweden, and Estonia supports the argument that land-to-sea sediment transfer was induced by hurricane-related tropical storms bringing catastrophic rainfall to the Fennoscandian Shield. Similar evidence from other Late Ordovician continents is reviewed in the context of hurricane landfall patterns based on predictable storm tracks.