Sediment routing evolution in the North Alpine Foreland Basin, Austria: interplay of transverse and longitudinal sediment dispersal

Abstract

Integration of detrital zircon geochronology and three-dimensional (3D) seismic-reflection data from the Molasse basin of Austria yields new insight into Oligocene-early Miocene palaeogeography and patterns of sediment routing within the Alpine foreland of central Europe. Three-dimensional seismic-reflection data show a network of deep-water tributaries and a long-lived (>8 Ma) foredeepaxial channel belt that transported Alpine detritus greater than 100 km from west to east. We present 793 new detrital zircon ages from 10 sandstone samples collected from subsurface cores located within the seismically mapped network of deep-water tributaries and the axial channel belt. Grain age populations correspond with major pre-Alpine orogenic cycles: the Cadomian (750-530 Ma), the Caledonian (490-380 Ma) and the Variscan (350-250 Ma). Additional age populations correspond with Eocene-Oligocene Periadriatic magmatism (40-30 Ma) and pre-Alpine, Precambrian sources (>750 Ma). Although many samples share the same age populations, the abundances of these populations vary significantly. Sediment that entered the deep-water axial channel belt from the west (Freshwater Molasse) and southwest (Inntal fault zone) is characterized by statistically indistinguishable age distributions that include populations of Variscan, Caledonian and Cadomian zircon at modest abundances (15-32% each). Sandstone from a shallow marine unit proximal to the northern basin margin consists of >75% Variscan (350-300 Ma) zircon, which originated from the adjacent Bohemian Massif. Mixing calculations based on the Kolmogorov-Smirnoff statistic suggest that the Alpine fold-thrust belt south of the foreland was also an important source of detritus to the deepwater Molasse basin. We interpret evolving detrital zircon age distributions within the axial foredeep to reflect a progressive increase in longitudinal sediment input from the west (Freshwater Molasse) and/or southwest (Inntal fault zone) relative to transverse sediment input from the fold-thrust belt to the south. We infer that these changes reflect a major reorganization of catchment boundaries and denudation rates in the Alpine Orogen that resulted in the Alpine foreland evolving to dominantly longitudinal sediment dispersal. This change was most notably marked by the development of a submarine canyon during deposition of the Upper Puchkirchen Formation that promoted sediment bypass eastward from Freshwater Molasse depozones to the Molasse basin deep-water axial channel belt. The integration of 3D seismic-reflection data with detrital zircon geochronology illustrates sediment dispersal patterns within a continental-scale orogen, with implications for the relative role of longitudinal vs. transverse sediment delivery in peripheral foreland basins.

Selective Recording of Tectonic Forcings in an Oligocene/Miocene Submarine Channel System: Insights From New Age Constraints and Sediment Volumes From the Austrian Northern Alpine Foreland Basin

“…The maximum SARs from 19.0 Ma onward are interpreted to result from unroofing and redeposition of the Augenstein Formation. This interpretation is furthermore supported by a shift in sediment-transport direction from east to north via northward prograding clinoforms and a major shift in sediment provenance (Sharman et al, 2018). During this time, the study area was transformed from a deep-marine transfer zone to a final sediment sink, efficiently preserving the tectonic signal in the SARs.…”

“…The migration direction of the clinoforms (from S to N, Figure 2 and Supplementary Figure S1) indicate a southern source. Detrital zircon U-Pb ages further suggest a change in provenance, when compared to the underlying UPF (Sharman et al, 2018). The provenance change and the predominantly northward directed sediment supply is consistent with redeposition of the Augenstein Formation on top of today's Northern Calcareous Alps (Frisch et al, 2001).…”

“…The so-called Puchkirchen Trough was fed by three different sources (Kuhlemann and Kempf, 2002). (1) A large, eastward prograding delta close to the present position of Munich received sediment from the Central Alps (Kuhlemann and Kempf, 2002;Sharman et al, 2018) (Figure 4). (2) The paleo-Inn Fan fed by intra-Alpine material served as a second source for detritus to the Puchkirchen Trough (Kuhlemann and Kempf, 2002).…”

“…During the deposition of the Oligocene to Miocene Zupfing Formation, Puchkirchen Group -consisting of the Lower (LPF) and Upper Puchkirchen (UPF) Formations -and the lower Hall Formation, sediment routing in the basin was largely controlled by a submarine channel system along the basin axis (Figures 2, 3) (De Ruig and Hubbard, 2006;Hubbard et al, 2009). This channel system received detritus (Figure 4) from the Central and Eastern Alps (Sharman et al, 2018) and separated the basin into a wide, gently sloping northern (De Ruig and Hubbard, 2006) and a steep tectonically active southern margin (Covault et al, 2009). The deep marine conditions of the Upper Austrian NAFB are in contrast to the fluvial, continental depositional environment in Oligocene/Miocene (Lower Freshwater Molasse, Figure 3) times in the central and western part of the NAFB (Kuhlemann and Kempf, 2002).…”

Down‐slope facies variability within deep‐water channel systems: Insights from the Upper Cretaceous Cerro Toro Formation, southern Patagonia

“…This uncertainty could be resolved by improved constraints on the relative depositional ages of each outcrop location. Contrasting detrital zircon proportions between tributaries and the main channel belt may also result from the unrecognized presence of other tributaries that are not represented by available data or by changes in the partitioning of sediment pathways in more source-proximal regions (Sickmann et al, 2016;Sharman et al, 2017).…”