Algal crusts, autochthonous and clastic gypsum in a cannibalistic evaporite basin: a case history from the Messinian of Northern Apennines

Abstract

The Messinian Vena del Gesso Basin in the Northern Apennines is filled by very thick (up to 35 m) beds of coarse crystalline gypsum (selenite) associated with thinner carbonate and shaly (euxinic) intercalations. The conventional Usiglio model of salt fractionation does not apply to this evaporitic sequence for the following reasons: carbonate which underlies gypsum is not evaporitic but algal in origin; most gypsum did not precipitate from surface brines but at and below a sediment‐water interface occupied by algal mats; a significant portion (10–80%) of gypsum beds is composed of redeposited selenite which was removed from the margins and transported toward the centre of the basin by slope‐controlled currents and gravity flows (debris flows). We call this process cannibalistic because of its intraformational character (connected with evaporative fall of water level) and volumetric importance. A recurrent vertical pattern of six main facies (euxinic to gypsum fanglo‐merates) is interpreted as a bathymetric, regressive cycle controlled by both sedi‐mentological and tectonic‐eustatic factors. The inferred environmental setting is a residual turbidite trough (Marnoso‐arenacea) evolving abruptly toward lagoonal conditions and filled up to sea level by evaporitic and mechanical (mostly fluvial) processes. Repeated inundations of restricted‐marine water started the depositional cycle thirteen or fourteen times.

Evidence for gammacerane as an indicator of water column stratification

“…Desulphurisation of the polar fractions of the extracts of ten subsamples of the marl of the evaporitic cycle IV (Vai and Ricci Lucchi, 1977) of the Messinian Gessoso-solfifera Formation, Italy, released substantial amounts of gammacerane (O-200 pg/g TOC). Accordingly, large amounts of S-bound gammacerane must be bound in macromolecular aggregates present in the polar fraction.…”

The record of the Messinian salinity crisis in the Tertiary Piedmont Basin (NW Italy): The Alba section revisited

“…Very rare "spaghetti like" filaments (e.g. Vai and Ricci Lucchi, 1977;Panieri et al, 2010) have been observed.…”

“…10C). These features have been previously interpreted as early diagenetic products resulting from the displacive growth of coarse-grained selenite masses within the laminated matrix (Sturani, 1973;Clari et al, 2008), because in cross section they show an ellipsoidal shape that gives to the bed a characteristic flaser-nodular structure (embrechitic structure in Sturani, 1973;Vai and Ricci Lucchi, 1977) (Fig. 10D).…”

“…The real depth of formation of these facies is debated, since no modern analogues exist: however the occurrence of cyanobacteria filaments, often included in the core of the crystals in many Mediterranean examples (e.g. Vai and Ricci Lucchi, 1977;Panieri et al, 2010) has suggested relatively shallow depth, within the photic zone, although the depth of growth of such bacterial mats is not clear (Lugli et al, 2010). These features are rare in the studied outcrops, possibly suggesting slightly deeper depositional conditions or a turbid water column, due to abundant suspended clayey particles, not suitable for the penetration of light.…”

“…This change is evidenced by the disappearance of the massive and banded selenite and by the appearance of the branching selenite facies; moreover, it is accompanied by the homogenisation of the depositional conditions, with the ubiquitous deposition of the upper PLG cycles both in marginal and distal setting. The branching selenite facies was originally described in the Vena del Gesso basin as nodular and lenticular selenite (Vai and Ricci Lucchi, 1977) and was considered as rehydrated secondary gypsum, resulting from previous anhydrite nodules formed in a sabhka environment. Recently, Lugli et al (2010) have re-interpreted this gypsum facies as primary; according to this new hypothesis these deposits consist of bottom grown small-sized selenite crystals grouped in branches projecting outward from a common nucleation cone.…”