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

Fig. 6. Detail of a spongy-like dissolution horizon A) showing three different zones with an increasing upward size of the cavities and different generationsof fillings. B) Particular of a
cavity. C) Schematic line drawing of the cavity fills figured in B.

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isotopic data of δ C(rangingbetween −1.44 and 3.66) and δ O(be- there are records of dense networks of burrows in different depositional
tween −2.87 and −0.19) suggest marine-phreatic conditions environments forming Thalassinoides, Ophiomorpha and Gyrolithes,
(Table 1, Fig. 9). mostly produced by decapods and resembling modern bioturbated sed-
3) The upper subzone consists of large and smooth cavities, partially or iments (De et al., 1996; Knaust and Bromley, 2012). In addition, the
completely rimmed by RFC cements similar to those within zone 2, ichnogenus Spongeliomorpha was well established in shallow-marine en-
but lacking any internal sediment. In this zone a mosaic of blocky vironments by the Early Mesozoic (Hary et al., 1981; Mayer, 1981;
calcite occludes the residual cavities (zone 3 in Fig. 6A). The crystal Dahmer and Hilbrecht, 1986; Zonneveld et al., 1997). According to
size of the blocky calcite ranges from 200 to 500 μmwith Carmona et al. (2002), the increase in ichnodiversity is most evident
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δ C= −3.65‰ and δ O= −4.04‰ (Fig. 7F). from the Middle Triassic onwards. The presence of Thalassinoides and
Ophiomorpha in lagoonal facies has also been documented during the
earliest Jurassic in the Calcari Grigi from the Trento carbonate platform
Electron microprobe microanalysis of the RFC cements, confirms that (Monaco and Garassino, 2001).
the cements are calcite. Sr concentrations are very low (b100 ppm) sug- We have found crustacean microcoprolites in the bioturbated la-
gesting that these cements were originally calcite, not aragonite (Fig. 8B). goonal muddy sediments that form the host rock of the spongy dissolu-
The concentrations of Fe and Mn are also mostly b100 ppm, except close tion, thus confirming the presence of such organisms. Senowbari-
to thereddish sediment whereFeconcentrationsincreasedramaticallyto Daryan et al. (1979) described the occurrence of microcoprolites in
N1000 ppm because the presence of hematite and siderite. These samples Upper Triassic reef limestone in Sicily. Because of the canal arrange-
have been excluded from the database. Although there is the potential for ment, the authors defined a new species named Helicerina siciliana.In
this Fe to control cathodoluminescence by quenching, the low overall our case, the shape and arrangement of internal canals of the
concentrations of Fe and Mn (Fig. 8A) imply precipitation under oxidizing microcoprolites are closer to Favreina (Bronnimann, 1955), (Fig. 4E).
conditions (Kim and Lee, 2003). There is some subtle zonation of the This ichnogenus contains 25 known ichnospecies from Triassic to Mio-
radiaxial fibrous cements defined by fluctuations in Mg concentration, cene, but most of the known ichnospecies of Favreina are from Jurassic
but the overall concentration of Mg is largely b1000 ppm. These values and Cretaceous (Senowbari-Daryan et al., 2007).
do increase locally however, within discrete inclusions of microdolomite, Based on these assumptions, we stress the role of bioturbation in the
suggesting a high-Mg calcite precursor. Overall, the petrographical mor- formation of the spongy-type dissolution by creating inhomogeneities
phology and trace element composition of the cements, coupled with in permeability and porosity of sediments that drive differential diage-
their isotopic composition are consistent with precipitation in the marine netic processes as in modern environments (Walter and Burton,
phreatic realm. 1990). In particular, we believe that the presence of more porous sedi-
ments filling the burrows favored a faster dissolution with respect to
7. Discussion the surrounding muddy sediment (Fig. 6).

7.1. Bioturbation 7.2. The Spongy-type Dissolution
The morphology of the spongy-like cavities strongly resembles a The establishment of a mixing water lens has been interpreted as a
complex network of burrows and in our study area, the association of key diagenetic modifier within modern and ancient carbonate plat-
cm-sized burrows and dissolution cavities in specific subtidal–lagoonal forms (Back et al., 1986; Smart et al., 1988; Melim et al., 2002; Baceta
units of some peritidal cycles is clear (Fig. 10). Within Triassic sediments, et al., 2007). The resulting diagenetic features are related to changes in