Ana Santos

Trace fossil assemblages on Miocene rocky shores of southern Iberia

The use of rocky palaeoshore bioerosion analysis in the study of palaeontological and geological questions is beginning to bear fruit. Five southern Iberian Neogene rocky shores have been analysed and their bioerosion structures have been identified. The observed ichnodiversity is rather low; eleven ichnospecies were identified. These include bioerosion structures produced by polychaete annelids (Caulostrepsis, Maeandropolydora), clionaid sponges (Entobia), echinoids (Circolites), and endolithic bivalves (Gastrochaenolites). The different ichnoassemblages present in Miocene rocky shores in both Portuguese and Spanish sectors correspond to the Entobiaichnofacies. Comparison with the northeastern counterparts of these shores has also been carried out. The study of southern Iberian Miocene rocky shores made it possible to correlate them with the regional tectonic evolution and the main Neogene transgressive events affecting the region.

Extreme habitat adaptation by boring bivalves on volcanically active paleoshores from North Atlantic Macaronesia

A bstractExtensive bivalve borings are described in detail for the first time from basalt rockgrounds in the North Atlantic volcanic islands of Macaronesia. They occur on a Middle Miocene rocky shore of a small islet of Porto Santo (Madeira Archipelago of Portugal), as well as on Plio-Pleistocene rocky shores on Santiago Island (Cape Verde). A basalt substrate is widely penetrated by clavate-shaped borings belonging to the ichnogenus Gastrochaenolites interpreted as dwelling structures of suspension-feeding bivalves. Some of these borings still retain evidence of the alleged trace-makers preserved as body fossils, while others are filled with their casts. The ichnofossil assemblage present on these bioeroded surfaces belongs to the Entobia ichnofacies. Recognition of Gastrochaenolites borings in volcanic rocks provides useful paleoenvironmental information regarding an expanded strategy for hard-substrate colonization. Preliminary results from fieldwork in the Cape Verde Archipelago indicate that such borings are more widespread through Macaronesia than previously thought.

A History of Ideas in Ichnology

Abstract Although the concept of ichnology as a single coherent field arose in the nineteenth century, the endeavor of understanding traces is old as civilization and involved cultural areas worldwide. In fact, fossil and recent traces were recognized since prehistoric times and their study emerged from the European Renaissance. This progression, from empirical knowledge toward the modern concepts of ichnology, formed a major research field which developed on a global scale. This report outlines the history of ichnology by (1) exploring the individual cultural areas, (2) tracing a comprehensive bibliographic database, and (3) analyzing the evolution of ichnology semiquantitatively and in a graphical form (“tree of ichnology”). The results form a review and synthesis of the history of ichnology, establishing the individual and integrated importance of the different ichnological schools in the world.

Taphonomic Range and Sedimentary Dynamics of Modern and Fossil Rhodolith Beds: Macaronesian Realm (North Atlantic Ocean)

Distribution of living rhodoliths in the Macaronesian realm is limited by extensive rocky shores and narrow insular shelves that rapidly drop off beyond the 50-m isobath. Wind and wave erosion is most intense on north and northeast-facing shores due to the prevailing northeasterly trade winds over much of the region. Southern shores offer more sheltered, leeward settings. Rhodolith beds tend to thrive on eastern shores with strong long-shore currents and southeastern shores that benefit from wave refraction. Rhodoliths are not entirely absent off northern shores, but may fail to reach maximum size before being washed ashore to make berms and beaches. Islands considered in greater detail in this survey include Santiago, Maio, and Sal from the Cape Verde Islands, Fuerteventura and the related islet of Lobos in the Canary Islands, Selvagem Grande and Pequena from the Savage Islands, Porto Santo in the Madeira Islands, and Santa Maria in the Azores. This contribution expands on the concept that living rhodoliths enter the fossil record through a range of taphofacies defined by the degree of breakage and corrosion and further characterized by sedimentological criteria regarding the amount of matrix and packing among bioclasts. Rhodolith deposits in Macaronesia seldom reflect settings under natural growth conditions. Rather, rhodoliths are subject to transportation and post-mortem disintegration resulting in the accumulation of rhodolith materials captured by subtidal storm deposits, tidal pools and platform over-wash deposits, as well as beachrock, beach, berm, hurricane, tsunami, and coastal dune deposits. Some of this material is transferred farther offshore, but exposed island strata show a tendency for shoreward migration of taphofacies. Rhodolith beds provide a habitat for some species of marine invertebrates, including epifaunal and infaunal elements directly associated with whole rhodoliths and these features play a role in rhodolith biostratinomy.

Reworked marine sandstone concretions: a record of high-frequency shallow burial to exhumation cycles

Concretions, with abundant calcite-dolomite cement-replacement textures originally hosted in shallow-marine sandstones, were reworked into Lower Cretaceous fluvio-deltaic conglomerates and shoreface sandstones (External Zones, Betic Cordillera). A cycle of host sand deposition, early diagenetic concretion formation and concretion reworking is documented: (1) Well-sorted shoreface sandstone deposited. (2) Spherical to ovoid, non-ferroan calcite-cemented concretions formed below flooding surfaces at shallow-burial depths during early eodiagenesis. Non-ferroan calcite cements were precipitated from the bicarbonate derived from seawater and from dissolution of marine bioclasts, as shown by isotope analyses. (3) Concretions were reworked and exposed on the seafloor in a high-energy setting as indicated by the presence of numerous bivalve borings (Entobia ichnofacies), laminated micritic microbial crusts around the concretions, and epilithobionts (oysters, barnacles and corals) on the concretion surface. Concretions also appear as erosional remnants on the floor of channels which were incised into the shoreline sandstone when sea-level fell. (4) The fluvio–deltaic channels were filled with sediment during flooding in the late lowstand of sea-level. (5) The concretions are partly dolomitized, and the presence of siderite, pyrite and barite in the outer part of the concretions precipitated before the dolomite, suggests that the latter formed during shallow burial.

Basalt mounds and adjacent depressions attract contrasting biofacies on a volcanically active Middle Miocene coastline (Porto Santo, Madeira Archipelago, Portugal)

Small basalt mounds with encrusting corals and inter-mound carbonate sandy zones with abundant rhodoliths corresponding to an ancient intertidal to shallow-water sea floor are exhumed from overlying volcaniclastic deposits and basalt lava flows at Pedra de Água on Ilhéu de Cima off Porto Santo, one of the islands of the northeastern Atlantic Madeira Archipelago (Portugal). The mounds rise above the surrounding surface to attain a height of about half a meter. The mounds exhibit an in situ assemblage of hermatypic corals, dominated by Tarbellastrae and Solenastrea. They formed as massive (4.2xa0×xa01.9xa0m average length), isolated patches in a protected bay close to shore eroded from an uneven basalt substrate dated to the Middle Miocene (14–15xa0Ma). The slightly deeper zones between basalt mounds, which alternate with them over a distance of more than 20xa0m, are covered mainly by coarse carbonate sand on which rhodoliths up to 14.8xa0cm in diameter are preserved in situ. Many rhodoliths have grown around a basalt core, which indicates a local, near-shore source for development. Complete burial of the elevated coral settlements and intervening low zones populated by rhodoliths occurred when volcanic lapilli and other tephra catastrophically buried this part of the rocky shore. The rhodoliths and coral assemblages exposed in an area of 12xa0m2 were canvassed systematically using census quadrats to quantify community relationships.

Storm‐driven bottom sediment transport on a high‐energy narrow shelf (NW Iberia) and development of mud depocenters

Bottom sediment transport on the NW Iberian shelf was monitored during a downwelling storm in September 2014. Collected data was analyzed and fed into a 3D coastal ocean model to understand storm-driven sediment transport on the shelf and its impact on mid-shelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the pre-storm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short-lasting strong bottom current in combination with enhanced wave-current interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from downwelling. An across-shelf horizontal density gradient as high as 0.32 g/m4 occurred at the interface between the downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward-directed coastal jet. Simulation results suggest that during the storm, local near-bottom sediment suspensions with concentrations on the order of 10 kg/m3 would be triggered by wave-current interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine-grained sediment. This article is protected by copyright. All rights reserved.

Recent Rhodolith Deposits Stranded on the Windward Shores of Maio (Cape Verde Islands): Historical Resource for the Local Economy

ABSTRACT Johnson, M.E.; Baarli, B.G.; da Silva, C.M.; Cachão, M.; Ramalho, R.S.; Santos, A., and Mayoral, E.J., 2016. Recent rhodolith deposits stranded on the windward shores of Maio (Cape Verde Islands): Historical resource for the local economy. Maio is a volcanic island with an area of 269 km2 in the Cape Verde archipelago off the west coast of Africa. Although considered a leeward island, it absorbs NE trade winds that typically register 5 to 6 on the Beaufort Scale (moderate to fresh breeze). The trade winds produce ocean swells commonly 3.5 m in height that scour the islands north coast but also generate eastern longshore currents. Outcrops with Pleistocene rhodoliths occur on the SE and south shores and include lithified dunes mainly composed of crushed rhodolith debris. In contrast, the modern beaches and Pleistocene dunes on the more sheltered west coast are practically devoid of rhodoliths. Present-day rhodolith banks off the north coast would seem to be precluded by intense wave action. This study examines rhodoliths from overwash and beach-rock deposits around Ponta Cais in the far north. Lumpy rhodoliths (likely Lithothamnion sp.) are concentrated in a sheltered corner on the bay south of Ponta Branca. A more extensive overwash deposit covers an area of 27,000 m2 that is 1 m above mean sea level with a surface exposure of 450 rhodoliths/m2. A unique specimen nucleated around a ceramic fragment indicates that the deposit is historical in context. Rhodolith beach rock extends all along Praia Real east of Ponta Cais. A northern bank clearly exists, but it does so at a water depth normally adequate to protect larger rhodoliths from all but major storms. Abandoned limekilns behind Praia Real demonstrate that the local economy on a volcanic island used rhodoliths as a source of mortar and whitewash.

Rocky-shore unconformities marking the base of Badenian (Middle Miocene) transgressions on Mt. Medvednica basement (North Croatian Basin, Central Paratethys)

Badenian (Middle Miocene) transgressive deepening-upward successions located in the NE part of Mt. Medvednica (North Croatian Basin, Pannonian Basin System) unconformably overlie Mesozoic basement. Triassic and Upper Cretaceous limestone pebbles, cobbles, and boulders of the Badenian basal conglomerates display abundant in situ bivalve borings of Gastrochaenolites and sponge borings of Entobia. This Gastrochaenolites-Entobia ichnoassemblage is related to the Entobia subichnofacies of the Trypanites ichnofacies, characterizing littoral rocky-shore environments (wave-cut platforms and marine transgressive surfaces with a low or null rate of sedimentation). Gastrochaenolites torpedo, Gastrochaenolites lapidicus, and Entobia recorded in Badenian basal conglomerates (compared with modern Northern Adriatic rocky-shore environments), enabled more precise palaeoenvironmental interpretations. The occurrence of G. torpedo (produced by lithophaginid bivalves) on all sides of individual limestone lithoclasts in the Gornje Orešje basal conglomerate, coupled with truncation of the formation (possibly indicating multiphase colonization), reflect gravel transport, roll-over, overturning and erosion by wave action in high-energy rocky-shore settings. Gornje Psarjevo-2 basal conglomerate boulders were probably not subjected to significant movement and abrasion, as suggested by good preservation of both G. lapidicus (potentially produced by gastrochaenid bivalves), associated G. torpedo, as well as abundant shallow Entobia borings. The Badenian Gastrochaenolites-Entobia ichnoassemblage also could be the result of a composite development. However, direct cross-cutting relationships between G. torpedo and G. lapidicus and/or Entobia were rarely observed. In addition, Badenian boring tracemakers might have coexisted at the same water depth. Northeast Mt. Medvednica Badenian successions probably formed during different Central Paratethys transgressive pulses (NN5 and NN6 Zones). However, exact timing of Badenian transgressions, stratigraphic correlations and tectono-eustatic implications are unresolved, due to sparsely integrated biostratigraphic and high-precision geochronological studies of Early–Middle Miocene North Croatian Basin deposits and due to the absence of a uniform biostratigraphic zonation and regional chronostratigraphic division of Central Paratethys.

The bioeroded megasurface of Oura (Algarve, south Portugal): implications for Neogene stratigraphy and tectonic evolution of southwest Iberia: reply to Pais and Legoinha (DOI 10.1007/s10347-011-0268-y)

We start by acknowledging that comments are welcome and the ensuing discussion generally contributes to a better understanding of the scientific questions at stake, in the present case a paleoichnological study of a major Neogene bioeroded surface associated with an ancient rocky shoreline, which may be followed for about 1.5 km along the Oura sea cliffs on central Algarve, in southern Portugal. The aim of Cachão et al.’s (2009) paper was to show the geological significance of major bioeroded surfaces, their identification, and interpretation, by demonstrating the importance of the Oura hardground for the understanding of the stratigraphic record of the Neogene of southern Portugal and its correlation with Iberian tectonic development. For this purpose, a detailed paleoichnological study of the Oura hardground allowed to confirm the existence of an important intra-Miocene stratigraphic gap, represented by a razor-sharp erosional contact that separates the two main Neogene units in the Algarvian region (southern Portugal). The existence of these bioerosive structures allowed the reconstruction of the sequence of events related to this episode. At the same time, it was possible to state the relationship between this stratigraphic gap and the tectonic phase that took place in the Betic Chain and Alboran Sea domain and highlight the main differences between a foreland basin of the Betic orogen (the Oura bioeroded unconformity) and the Spanish unconformities in the inner sector of the Betic orogen (Santos et al. 2008). The Cachão et al. (2009) paper is a result of a line of research started by this international team more than 10 years ago (da Silva et al. 1999) dealing with the paleoichnology and the paleoecology of Neogene rocky shores in the Iberian and Eastern Atlantic Islands Neogene and their possible extra-paleontological implications (e.g., Domènech et al. 1999; Santos et al. 2008, 2010, 2011). We will continue by stressing that none of the comments produced by Pais and Legoinha (2010) directly dispute or even remotely refer to any of the major aspects focused on in our paper and as so stated in its title; namely, the occurrence of a bioeroded megasurface in the Oura region, the paleoichnological identification of the ichnofossils involved and their paleoenvironmental interpretation, as well as the existence of an intra-Miocene stratigraphical discontinuity (as shown by the bioeroded surface), and its regional tectonic implications. Thus, Pais and Legoinha (2010) statement ‘‘(...) to support very speculative and most likely, erroneous conclusions’’ without presenting any alternative interpretation or a paleogeographic and tectonic model that might contradict our conclusions is, at least astonishing. More specifically, Pais and Legoinha (2010) failed to interpret our Fig. 2 (Cachão et al. 2009) as a synthetic stratigraphical columnar section for the Oura Neogene sequence (as mentioned in its caption), i.e., of the Neogene sequence in the Oura region, and confused it with their own M. Cachão (&) C. M. da Silva Centro de Geologia e Dept. de Geologia, Faculdade de Ciências, Universidade de Lisboa, C6, Campo Grande, 1749-016 Lisbon, Portugal e-mail: mcachao@fc.ul.pt