Photos, commentary and interpretation (or any misinterpretation) by Jim Locke, College of Marin, Kentfield, CAWhile many American tourists would probably have headed for Nice or Monte Carlo, I headed into the hills around Peira Cava.
From the village, I could see what I had come here to see,
the tilted
layers of sedimentary rocks in the distant ridge.
A closer view. The prominent layers are sandstones
and conglomerates
that are interbedded with shales.
The Peira Cava region is the type area where Arnold Bouma described the characteristics of "flysch" deposits. Flysch is used extensively in Europe for rocks that have been deposited on the deep-ocean floor. (Bouma, A. H., 1962, Sedimentology of some flysch deposits: Amsterdam, Elsevier, 168 p.)
The idealized Bouma sequence consists of
E (top) -- period of slow accumulation of mud
D unit -- deposition of parallel-laminated silts & muds
C unit -- deposition of rippled sands
B unit -- deposition of parallel laminated sands
A (bottom) -- deposition of massive sands or conglomerates (commonly graded in grain size, coarse at bottom to fine at top) with a sharp base.
Above you can see the sharp bottoms which sometimes undulate as shown in the third sandstone from the bottom.
Below is a contact at the bottom of a massive, graded
(larger particles
near the base) A unit overlying a E unit shale.
Below, with a very coarse base, the sequence grades
abruptly upward
into a parallel laminated sand (B) and eventually into a rippled sand
(C)
Below, this sequence appears to start with a rippled sand
(C)
Some of the more spectacular features in these sequences
are the
sole markings that underlie the bases of many of the sequences and
provide
information on the direction of flow. The linear bi-directional sole
mark
that runs across the center of the image from top left to bottom right
contrasts with the somewhat triangular, unidirectional markings (flute
casts).
More sole markings
Update: The features shown
below were graciously described in an email from Sébastien Migeon, a
marine geologist
from the University of Nice.
"I am a marine
geologist (sedimentologist) from the University of Nice and I just
found your web page about Peira Cava turbidite deposits. I mainly
work on modern turbidites but I am also interested in these flysch
deposits, mainly for teaching purposes. I have a comment about
the 2 last photographs of your page. What you show here are not flute
casts but soft sediment deformations like slump ball. Similar
features can be observed on the opposite flank of the Peira Cava
syncline. I don't know if the 2 layers exhibiting such features
can be correlated between the 2 flanks of the syncline but as you saw
on the field, it is possible to follow the layer over hundreds of
metres. The deformation is not local but affects sediments probably
over a large area. By comparison with similar features classically
observed in Morocco, such deformations probably result from
earthquake-induced horizontal and vertical acceleration of the seafloor
(Peira Cava basin was a small tectonic basin so frequency of
earthquakes was probably high)"
More soft sediment deformations (earth-quake induced
features)
Other turbidite sequences visited during the year.
Carbonate rich turbidites on the island of Rhodes (Fall 1999)
a folded and faulted sequence
less disrupted sequence
a massive, parallel laminae to rippled sequence
