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78 S. Todaro et al. / Sedimentary Geology 333 (2016) 70–83

Fig. 8. Scattered diagram of element composition for radiaxial fibrous cements: A) Mn vs. Fe; B) Sr vs. Mg. Note the low (b100 ppm) concentrations of Mn, Sr, and, in most cases, Fe and Mg.
Where Mg concentrations are elevated to N1000 ppm, analysis is of microdolomite inclusions. Higher concentrations of Fe (N100 ppm) are associated with the presence of hematite within
the sediment laminae.

spongy stratabound dissolution with present day analogues observed in 7.3. Source of Meteoric Water and Position of the Mixing Zone
interior zones of the Bahama Bank (Beach, 1995; Whitaker and Smart,
1997) or in the Yucatan platform (Back et al., 1986) supports the influ- The presence of small sinkhole/funnels in pedogenic caps of some
ence of dissolution by mixing waters (Fig. 11). Smart et al. (1988) de- cycles provides evidence of a humid climate, and a possible infiltration
scribe a highly porous ‘swiss-cheese’ texture in the upper mixing zone of groundwater (Fig. 10). The end-Triassic paleo-climatic conditions in
that is very comparable to that of our study, but with a greatest thick- the western Tethyan realm indicate a switch from a seasonal monsoonal
ness. In addition, the comparison of the studied morphologies with regime during Carnian and Norian times (Preto et al., 2010) to more
the fossil analogous from the Danian Platform of northern Spain de- humid conditions during latest Norian and Rhaetian (McElwain et al.,
scribed by Baceta et al. (2001) shows striking analogies. These latter au- 1999; Ahlberg et al., 2003; Korte et al., 2009). On the basis of the charac-
thors highlight the distinctive spongy-like dissolution porosity as a terization of paleosols at the top of the Dolomia Principale (Brenta Dolo-
criterion for recognizing fossil marine-meteoric mixing zones. Key diag- mites, Italy), Berra et al. (2010) suggest that close to the Norian–
nostic criteria in our study are: Rhaetian boundary the stratigraphic record on the northern coast of
the Tethys experienced a rapid increase of precipitation. This implies
• The shape of the cavities with evidence of enlargement of burrows, in the onset of a sub-tropical climatic setting with alternating dry and
zones 2 and 3 (Fig. 6), is a distinctive proof of corrosive dissolution. wet seasons (Retallack, 2001). The presence of thick paleosols with
• The sub-horizontal arrangement of the spongy horizons, the lateral Terra Rossa, rich in Fe-oxides and clay minerals, which are not present
continuity and discrete thickness within the outcrop (Fig. 10), are in- in the lower part of the Triassic succession, indicates that also the car-
dicative of the establishment of a sub-horizontal lens as observed in bonate platform experienced a more humid climate during Rhaetian
the Bahamas zone (Smart et al., 1988). This is supported by the dis- time (Fig. 3C).
tinctive layering of the sediment fill (Fig. 6); Nevertheless, the lack of evident penecontemporaneous stratabound
• The distinctive sequence of burrow infill-reddish silt-RFC. A similar se- dissolution morphologies as a result of karstification or mixing zone dis-
quence was described by Baceta et al. (2001), who proposed that the solution in other sectors of the platform with similar peritidal facies de-
presence of Fe-oxide crusts resembles those found in modern sedi- velopment, suggests that, in our case, either rainfall supply was not
ments (Whitaker and Smart, 1998), through the oxidation of sulfides sufficient to singularly result in the development of a mixing zone or ex-
produced by BSR in the lower mixing zone. In our case, there is distinct posure periods were too short. Therefore, we speculate on the contribu-
petrographic evidence that indicates an alternation of dissolution and tion of a fresh water recharge from an adjacent exposed landmass as
precipitation. In particular, the irregular shape of the cavities (zones 2 shown in the formation of the mixing zone in our study (Fig. 12). The
and 3) is indicative of dissolution while the presence of the reddish speculative profile of the flat-topped platform, given the compressional
sediments and the RFC cements provide evidence of sedimentation deformation related to the accretion of the Maghrebian fold and thrust
under marine phreatic conditions. belt, shows an uplifted inner zone formed as a result of the ongoing
rifting of the Alpine Tethys. In support of this, indicators of fresh to brack-
ish waters, such as characean gyrogonites are present in lagoonal sedi-
The spongy horizons overlie a zone that consists of irregular bur- ments deposited in coastal ponds or lakes close to the depositional
rows filled by sediment that is coarser if compared to the host rock basin of Monte Sparagio (Fig. 4D).
(zone 1, Fig. 6A). This type of filling is strictly related to a bio-
retexturing process due to bioturbation (Flügel, 2004)soweinter-
pret this lower zone to have not been affected by dissolution. In 7.4. Conceptual Model
zones 2 and 3, the dissolution was focused upon the burrow infills
since they had better porosity and permeability than the host rock, Based on the above considerations, we put forward a conceptual
enlarging the burrow cavities. Therefore, the boundary between model to explain the different steps leading to the stratabound porosity
zones 1 and 2 (see. Fig. 6A) can be interpreted as the base of the during the development of a shallowing upward peritidal cycle.
zone of dissolution, i.e. the boundary between the marine-phreatic Fig. 13 shows the interpreted stepwise formation of the spongy po-
zone and the overlying mixing zone. Based on this assumption, we rosity within the study area. Step 1 indicates intense bioturbation of a
evaluate the thickness of the mixing lens to be in the range of 50– lagoonal muddy subtidal unit. The progressive relative sea-level lower-
100 cm (Fig. 10). The position of the spongy-like dissolution within ing leads, during step 2, to the formation of the stromatolitic unit. The
subtidal units of specific peritidal cycles capped by thick paleosols, establishment of the mixing water lens occurred during the relative
would suggest that dissolution could have formed by the establish- sea-level lowstand responsible for platform exposure (Step 3) and the
ment of a mixing zone floating on the underlying marine phreatic permanence of the lens within the bioturbated horizons creates the
zone during the cyclic exposure of the platform. spongy-like dissolution. The subsequent filling of the cavities, after a