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

A 38 m-long piston core from the deep basin of the Saguenay Fjord, Quebec, recorded rapidly deposited layers (RDL) interpreted to represent major floods and earthquakes over the past similar to7200 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 stumps. 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 C-14 date suggests that earthquake frequency was dramatically reduced at similar to4 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. (C) 2003 Elsevier Ltd. All rights reserved.