Marine Geosciences in Bretagne
Brest, May 24 - June 7, 2005

International Training Course in Marine Geosciences, UBO - Purdue 2005
Stage international de Formation en Géosciences marines, UBO – Purdue 2005

UBO
-Purdue
Earth Science
Exchange
Programme
Crozon Field Report:
The Armorican Sandstone of the Pen-Hir Cliffs
Guillaume Blondiaux, Master 1 SML-GO, Andrew Cyr, PhD Purdue, 2nd Year, Emre Unal, PhD Purdue, 2nd Year

 

Field Observations
This field trip was to the Camaret area in the western part of the Crozon peninsula, Brittany, France. We examined the Early Ordovician age Armorican Sandstone formation, which forms the base of the Paleozoic section at this locality. The outcrops observed were approximately 450 meters thick and can be divided into three members (Picture 1).
The Lower member is made of fine- to coarse-grained, white to tan sandstones and medium- to coarse-grained tan to grey quartzites and very thick to massive beds that are tabular over 100’s of meters. Some beds are heavily bioturbated (picture 2). The Middle member is composed of the Gador Schist, which is made of alternating medium to very thin beds of slate and fine- to medium-grained sandstones or medium-grained quartzites similar to the Lower member. The Upper part is made of medium to thin beds sandstones and quartzites similar to the Lower part. Some of the beds of this member contain either mud-draped ripple cross-laminated sandstones (picture 2) or tangential cross-lamination overlain by symmetrical ripple cross-lamination (Picture 2).
The strike of the beds changes from 221o to 194o dipping towards the SE at angles from 59° to 86°. We also observed meso-scale folding/faulting in the Lower and Middle members (Picture 3).

 

Interpretations
According to the grain size and observed inorganic and organic primary sedimentary structures, the depositional environment was shallow water with episodic unidirectional to oscillatory flow alternating with periods of still water (picture 2). The combination of these types of flow conditions is indicative of either tidal flat or near shore depositional environments.
The structure we observed indicate overall compressional tectonics. This is consistent with what we observed in the higher parts of the section. These structures may be the result of Hercynian aged deformation.

 

Speculations
It is likely that, during the Early Ordovician, this depositional system was located on the northern edge of the Armorican micro-plate facing towards the Armorican Ocean.

 

 

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Picture 3. Example of meso-scale folding and faulting in the Lower (left) and Middle (right) members of the Armorican Formation at Crozon. Unaltered photo (A) and trace of fault (B) showing tectonic transport was towards the southeast. Bunker for scale.

A


B
Picture 1. Complete section of the Early Ordovician age Armorican Formation as exposed at Pen Hir Peninsula, Brittany, France. From left to right, the medium to thickly bedded sandstones and quartzites of the Upper Armorican overlying the thinly bedded Middle Armorican Gador Schist (A). From left to right, the Gador Schist conformably overlying the thick- to massively-bedded sandstones and quartzites of the Lower Armorican Fm (B).

A .

B

C

Picture 2. Stratigraphic up and paleoenvironmental indicators of the Early Ordovician age Armorican Fm. Casts of horizontal burrows in sandstone of the lower Armorican Fm. indicate that stratigraphic up is to the south, into the plane of the photo. Arrow is ~2 cm long (A). Mud-draped unidirectional current ripples (B) tangential cross laminations topped by symmetrical ripple cross laminations (C) in the upper Armorican Fm are another indication the stratigraphic up is to the south.