Ichnology

The passive and active fill of burrows potentially stores information about sedimentary processes that are
otherwise not preserved in the rock record. In recent years, abandoned passively-filled vertical burrows were
introduced as “tubular tidalites” when their infilling displays rhythmic lamination reflecting a tidal signature. In
the shallow-marine Miocene sandstones exposed at Oura (southern Portugal), 36 tubular tidalites occur in a
1.5 m-thick interval. Their high abundance is likely a consequence of both an environment favourable for the
production of open burrows in a tidal setting, and post-depositional conditions facilitating the preservation of
the tubular tidalites. Besides vertical tubes, 13 horizontal burrows preserve a tidal signature indicating draughtfill
processes. All specimens belong to Thalassinoides and, for the first time, to Gyrolithes. The rhythmic infill of
two well-preserved specimens shows two significant features: (1) The thickness pattern allows for differentiation
into groups having 7 couplets (consisting of a dark and a light lamina) or multiples thereof, and (2) the thickness
patterns of both, consecutive couplets as well as dark and light laminae match sine curves. Both patterns indicate
a diurnal tidal cyclicity. The tidalites record up to four spring-tide and three neap-tide cycles. In addition to the
neap-spring cycles, a long-period lunar fortnightly tide regime can be envisaged. The tubular tidalites imply
diurnal tides during the Miocene in contrast to the Recent semidiurnal tides affecting southern Portugal.

The Ediacaran–Fortunian ichnofauna from Central Brittany (NW France) is revised for the first time since the pioneering work by Lebesconte at the end of the 19th century. The study is based on fossils from the type-localities of the historical Brioverian taxa Montfortia (traces from Montfort-sur-Meu) and Neantia (wrinkle surfaces from Néant-sur-Yvel), and on two new outcrops from Saint-Gonlay. The ichnofossil assemblage includes Helminthoidichnites tenuis, Helminthopsis tenuis, Gordia marina, Palaeophycus tubularis, ?Neonereites uniserialis, and ?Spirodesmos archimedeus. Locally, the grazing traces are associated with wrinkle surfaces considered as microbially induced sedimentary structures (MISS). The sedimentological characteristics of these deposits correspond to a shelf marine environment. Both U-Pb datings on detrital zircon and ichnostratigraphic criteria tend toward an Ediacaran age of the fossiliferous deposits, but we cannot exclude the possibility of a Fortunian age at the present state of knowledge.

Filled mammal tunnels (krotovinas) are the most common traces in modern grassland soils and trace fossils in loess paleosols. Krotovinas are still insufficiently used in paleogeographic reconstructions owing to the absence of a procedure for documenting their features. When their tracemaker can be identified, krotovinas complement body fossil evidence for past geographic ranges of fossorial mammals. Moreover, local palaeoenvironments can be reconstructed more precisely based on the preferred habitat of the identified species. However, the morphology of krotovinas has rarely been reported in detail that would allow their identification. In this article, we suggest five groups of diagnostic features that should be recorded and assessed for taxo-nomic identification of krotovinas. (1) Ornamentation on the burrow walls, which can be observed when there is sediment compaction contrast between infill and the host material, provides the most taxonomically specific features. (2) Infill type distinguishes between subterranean forms and other burrowers. (3) Tunnel repair and reburrowing patterns. (4) Motifs representing distinctive architectural elements: shafts, spirals, their connections to horizontal tunnels, the shape of chambers, and the predominant orientation of tunnels (horizontal, inclined, vertical). (5) Diameter, which has to be measured transverse to each tunnel or estimated from oblique sections. The measurements and observations can be compared against a reference collection of burrow plaster casts. Synoptic diagrams are suggested as a method for condensing images of diagnostic features from several sections. We suggest that the presence of krotovinas is one criterion for distinguishing grassland paleosols from humus-rich colluvial deposits.

A new ichnotaxon, Lapillitubus montjuichensis nov. igen. nov. isp., is described from the middle Miocene (Serravallian) of Montjuïc mountain (Barcelona, northeastern Spain). This ichnotaxon consists of a horizontal to vertical, cylindrical burrow with an agglutinated lining exclusively composed of lithoclasts. Lapillitubus montjuichensis is interpreted as the result of the burrowing activity of a deposit- or suspension-feeding annelid worm. This new ichnotaxon extends the record of the informal group known as clast-armored or agglutinated trace fossils. In addition, since part of its type material is located in the blocks that make up the façades of several modernist buildings in the city of Barcelona, this new ichnotaxon highlights the importance of fossils in urban settings for those cases in which natural outcrops are reduced, restricted or even missing.

The Pisco Formation consists of a succession of sandstone, siltstone and diatomaceous and tuffaceous mudstone with some interspersed layers of phosphate nodules, biogenic conglomerate, calcareous sandstone and mudstone, gypsum, and volcanic ash. The succession corresponds to the last basin-wide marine transgression that extended over the Pisco basin during the Middle Miocene to lower Pliocene. The base of the formation is an erosional unconformity directly overlain by a layer of phosphate pebble-size nodules within and matrix of fine sandstone. Igneous cobbles and boulders are locally abundant within this layer. In some areas the basal layer consists of fine sandstone characterized by Glossifungites ichnofacies (firmground), which locally grades into a Trypanites ichnofacies in hardground surfaces and calcareous lithoclasts (rockground). The Glossifungites ichnofacies is dominated by Thalassinoides and Gyrolithes burrows with sharp, unlined walls that penetrate a few tens of centimeters into the fine sandstone of the underlying Chilcatay Formation. Unlined, sub-vertical to vertical Gastrochaenolites and Trypanites borings characterize the hardground and rockground surfaces. These borings are approximately 18 mm long, lack ornamentation, and their openings are not truncated by erosion. Abundance of burrows and borings varies laterally and between the lower and upper surfaces of lithoclasts. Borings do not intersect but may be in contact with one another. Both burrows and borings are filled with fine-to medium-grained sandstone, though locally they are filled with matrix-supported phosphate pebbles. Commonly this horizon lacks invertebrate fauna, except for some articulated bivalves and oysters, and clusters of cirripeds preserved against some boulders. Marine mammals fossils, both articulated and disarticulated, and tree logs and fragments of wood are locally abundant. This hardground represents an omission surface in an upper foreshore, coastal environment that was cemented and perforated before the onset of the marine flooding of the last Pisco basin-wide transgression in which siliciclastic sediments were deposited. Moderately bioturbated fine-grained sandstones and siltstones overlie the basal transgressive surface. Burrows are relatively abundant in the lower sandstone layers with sub-vertical to vertical Skolithos, Ophiomorpha and Gyrolithes, and thick horizontal Thalassinoides. Burrow filling is similar to the embedded sediment. These burrows lack ornamentation and lining, except for Ophiomorpha, which shows walls clad in millimeter-size pellets. Scattered articulated bivalves, clusters of cirripeds, and claws of decapod crustaceans are locally abundant. The lower layers within this succession contain abundant marine vertebrates in some areas. This set of burrows corresponds to a Skolithos ichnofacies formed in the foreshore to upper shoreface, sublittoral, moderate-to-high energy environment. Overlying these lower fine sandstones are layers of siltstones with abundant hummocky cross-stratification and water current structures. These layers are mostly barren of burrows, with some levels showing a low to moderate degree of bioturbation, mostly Thalasssinoides, and Skolithos, representing the lower shoreface to inner shelf environment. Overall the degree of bioturbation decreases with the fining-upward succession, although there are several moderately bioturbated levels, some of them underlying fossil-rich, thin phosphate beds. Abstract Book of 31 st IAS Meeting of Sedimentology, 22