A possible tsunami deposit around the Jurassic–Cretaceous boundary in the Boulonnais area (northern France)

Abstract

An unusual succession of facies locally deposited around the Jurassic–Cretaceous boundary in the Boulonnais (northern France) is attributed to a tsunami event by comparison with recent tsunami deposits. This sedimentary succession includes basal erosion with reworked lithified blocks, soft-sediment deformations, an erosional conglomerate overlain by wood fragments and clays containing continental and marine fossils in one setting and conglomerate with mixed fauna in an other setting. The tsunami probably affected the coast of the Boulonnais area of the London–Brabant Massif. The origin of the event is unknown. It was most probably triggered by an earthquake, but other origins such as volcanic eruptions, a giant landslide, or even the impact of an extraterrestrial bolide into the ocean may also be considered.

Introduction

Instantaneous and/or exceptional events (e.g. meteorite impacts, earthquakes, tsunamis) greatly disturb sedimentary environments. Among such instantaneous events, tsunamis are a constant feature of the present world oceans. In the 1990s, for example, 83 tsunamis events occurred, mostly triggered by earthquakes. An average rate of 57 events per decade is globally recorded in historical times (Bryant, 2001). Many of these events had major effects on the coastlines and some caused great damage. Other mechanisms involved in generating tsunamis are submarine landslides, volcanic eruptions and impacts of bolides into the oceans.

Anomalous deposits attributed to tsunamis (tsunamites) have recently been described from a variety of ages and environmental settings, e.g. upper Archean (Hassler et al., 2000), lower Proterozoic (Bhattacharya and Bandyopadhyay, 1998), Cambrian (Pratt, 2002), mid-Cretaceous (Rossetti et al., 2000), the K/T boundary interval (e.g. Bourgeois et al., 1988, Takayama et al., 2000), Miocene (Shiki and Yamazaki, 1996), middle Pliocene (Massari and D'Allessandro, 2000), upper Pleistocene (Takashimizu and Masuda, 2000), and Holocene (e.g. Hemphill-Haley, 1996, Goff et al., 1998, Cita and Aloisi, 2000, Dawson and Shi, 2000, Dominey-Howes et al., 2000, Nanayama et al., 2000, Bondevik et al., 2003, Tuttle et al., 2004). Tsunamites range from deep-sea and abyssal plain settings (Cita and Aloisi, 2000) to marine coastal (Bhattacharya and Bandyopadhyay, 1998) and coastal lake settings (Dawson and Shi, 2000).

Although there is no reason to believe that tsunamis had a very different frequency in the past, whatever the age considered, and despite the above-mentioned recent studies, only few examples of ancient tsunamites are described in the literature when considering all the coastal sequences studied. Such events are in most cases not recognized, and their occurrence is therefore largely underevaluated in the geological literature, probably especially for pre-Quaternary times. This under-appreciation of the role of tsunamis in ancient coastal successions boils down two reasons. The first reason is that non-deltaic coastal environments are characterized by erosion as much as deposition, and therefore are not abundantly represented in the stratigraphic record. The second reason is that it is not clear how to separate the deposits of major storms and hurricanes from tsunamis.

In this paper, we describe the record of a probable tsunami occurring close to the Jurassic–Cretaceous boundary in a coastal marine sedimentary succession in the Boulonnais area (northern France). It is a contribution to the few ancient tsunamites described in the literature, and one of the very rare events of that type described to our knowledge in the European Mesozoic.