ULTRA-HIGH RESOLUTION PARASEQUENCE AND SEQUENCE ARCHITECTURE IN A RECENT, POSTGLACIAL BASIN, VARANGER, ARCTIC NORWAY

Abstract:

The postglacial latest Pleistocene and Holocene Sandfjorden Basin (Varanger, Arctic Norway) is filled with an unconsolidated sedimentary record of nonmarine and nearshore marine sediments deposited during the last 15,000 years under conditions of overall relative sea-level fall due to postglacial isostatic uplift in northern Scandinavia. The basin fill is mapped by a combination of surface geomorphological and sedimentary data combined with high-resolution ground-penetrating radar lines.

The basin fill presents unique insight into the depositional genetics of parasequences, systems tracts, and a multicyclic, ultra-high resolution sequence stratigraphic architecture. The sediments were deposited as a series of clinothems in four orders of cyclicity driven by postglacial latest Pleistocene to Holocene climatic changes during the Younger Dryas (12,800–11,650 years before present [BP]) and Tapes (9000–6500 years BP) events. Periodic intense storms reworked the successive shorelines. Sediment was supplied from two distinctively different sources through local streams and along strike from a series of talus fans to a coarse-grained shoreface.

The sedimentary response to the external drivers and sediment supply resulted in a sequence stratigraphic architecture dominated by a hierarchy of downward-stepping forced regressive parasequences. The Sandfjorden Basin fill is interpreted to be a sixth-order, downward-stepping sequence with a duration of at least 15,000 years, from inception of the basin filling subsequent to deglaciation in the area until present. This sixth-order sequence is further divided into three seventh-order sequences by the Younger Dryas and the Tapes events, both generating initially transgressive surfaces followed by regression and seaward translation of the shoreline and formation of sequence boundaries. The seventh-order sequences are further divided into at least 13 eighth-order sequences by sequence boundaries that separate seaward-stepping parasequences. The eighth-order sequences are in turn divided into 44 ninth-order sequences bounded by surfaces that mark even higher-frequency downward stepping of the shoreline.

The ultra-high resolution Sandfjorden Basin sequence architecture is anomalous and nonuniformitarian because of the exceptional combination of allocyclic rapid drivers, postglacial isostatic uplift, climatic changes, and periodic storm activity, creating noncontinuous deposition. The basin fill is unlikely to be preserved in the geological record and forms an end member in sequence stratigraphic analysis. Yet it is applicable as an analogue to both recent and ancient deposits in which combinations of rapidly changing external forcing mechanisms control deposition.