Earthquake and flood-induced turbidites in the Saguenay Fjord (Québec): a Holocene paleoseismicity record

Abstract

A 38 m-long piston core from the deep basin of the Saguenay Fjord, Québec, recorded rapidly deposited layers (RDL) interpreted to represent major floods and earthquakes over the past ∼7200 years. High-resolution physical, magnetic and sedimentological analyses revealed at least 14 RDL, generally with a sandy base and a light gray color, interbedded with hemipelagic sediments. Digital X-radiography and grain size analyses at <1 cm spacing showed that six RDL have normal grading and likely resulted from earthquake-triggered slumps. Six other RDL have a similar normally graded basal bed, overlain by a coarsening-upward unit that underlies a fining-upward unit, interpreted as a deposit of flood-induced hyperpycnal flow. By analogy with the deposits that followed the AD 1663 earthquake, such beds are inferred to result from the breaching and rapid draining of a natural dam generated by an earthquake-triggered landslide. Based on this interpretation, the chronology derived from paleomagnetic secular variation, paleointensity and one AMS 14C date suggests that earthquake frequency was dramatically reduced at ∼4 ka, consistent with ice-load modeling during deglaciation in Eastern Canada. This study demonstrates that hyperpycnal and slump-generated turbidites can be readily distinguished in small basins such as the Saguenay Fjord and that hyperpycnal turbidites tend to be much thicker, because of greater flow duration and constriction.