SEDIMENTOLOGICAL AND GEOCHEMICAL FEATURES OF CHAOTIC DEPOSITS IN THE VENTIMIGLIA FLYSCH (ROYA-ARGENTINA VALLEY- NW ITALY)

The Ventimiglia Flysch is a Upper Eocene-Lower Oligocene turbidite succession deposited in the SE part of the Alpine foreland basin, truncated at the top by the basal thrust of the Helminthoides Flysch, a Ligurian tectonic unit that presently covers part of the Dauphinois and Briançonnais successions of Western Ligurian Alps.

The Ventimiglia Flysch is made of alternations of sandstones and shales. The upper part is characterized by chaotic deposits showing the classic block-in-matrix fabric. These deposits are constituted by:

- hm-sized intraformational blocks (i.e. portions of Ventimiglia Flysch slid down);

- hm-sized extraformational blocks (i.e. portions of Cretaceous sediments of the Dauphinois Domain, and portions of sediments of the Alpine foreland basin such as Nummulite Limestone);

- km to hm-sized exotic blocks (portions of succession derived from other tectono-stratigraphic units, i.e Helminthoides Flysch ).

The matrix of the larger blocks is made up of conglomerates with block-in-matrix fabric interpreted as debris flow deposits.

Debris flow clasts show:

- different sizes, ranging from metre to centimetre;

- different shapes, from rounded to subangular;

- different lithologies referred to Ventimiglia Flysch, Nummulite Limestone, Globigerina Marl and Helminthoides Flysch alike the larger blocks of the chaotic complex.

The clasts of debris flow deposits are disposed randomly into a chaotic matrix that consists of a dark mudstone in which submillimetre- to millimetre-sized lithic grains, with the same compositions of larger clasts, are present. Locally the matrix consists of sandstones with quartz and feldspar grains and fragments of nummulitids that suggest reworking of unlithified Eocene sediments.

Cathodoluminescence observations allow the distinction of two kinds of clasts: clasts with the same orange luminescence as the matrix that may be interpreted as soft mud clasts that were cemented together with the matrix, and dull-luminescing clasts that underwent a cementation before the formation of conglomerates.

Debris flow deposits are cross-cut by a network of crumpled and broken veins, 10’s mm to cm-large, filled with orange luminescing calcite and locally with quartz. Their complex cross-cutting relationships with clasts and matrix show that several systems of veins are present, that may be referred to different fracturing events.

Some clasts are crossed or bordered by veins that end at the edge of the clasts. These veins show the same features as those that crosscut the whole rock. This indicates reworking of plastic sediments crossed by calcite-filled veins by mass gravity flows. Polyphase debris flow processes, proceeding along with fluid expulsion and veining, are thus documented.

Ellipsoidal, dm-large concretions also occur in chaotic deposits. Stable O and C isotope analyses, performed also on matrix, clasts and veins, show:

- δ¹³C close to normal marine values (-3 to 0 δ¹³C ‰ PDB)

- δ¹⁸O markedly negative (-9 to –7 δ¹⁸O ‰ PDB) that could be related to precipitation from relatively hot waters (60-70 ° C).

Concretions consist of cemented pelites without block-in-matrix fabric: this aspect indicates that they are not the result of early cementation of chaotic deposits. Similar lithological features and absence of biostratigraphically significant fossils make difficult to ascribe these concretions to Ventimiglia Flysch or Helminthoides Flysch. In both cases these concretions document a phase of concretionary growth within homogenous pelites caused by an upward flow of relatively hot waters and subsequent involvement in the mass gravity flow.

The block-in-matrix fabric and the variable composition and size of blocks show that these sediments are a sedimentary mélange related to mass wasting processes involving both extrabasinal and intrabasinal sediments. These gravitational movements took place along slopes of submarine tectonic ridges created by transpressional faults (Piana et al., 2009) that juxtaposed tectonic slices of different paleogeographic domains (Dauphinois, Briançonnais, Ligurian Units) in Late Eocene-Early Oligocene times, and involved both rock fall processes of huge blocks of lithified, older formations, and debris flows of unlithified intrabasinal sediment. Faults also acted as conduits for an upward flow of hot fluids supersaturated in calcium carbonate. These fluids reached shallow buried sediments where caused a localized concretionary cementation and formation of vein swarms within unlithified sediments prone to subsequent mass wasting.