The Ichnocoenosis of the Bottom Nepheloid Layer (BNL) Deposits: a Case Study from the Scaglia Toscana Formation (Paleogene, Central Italy)

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A Bollettino della Società Paleontologica Italiana, 56 (2), 2017, 243-251. Modena

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S. P. I.

The ichnocoenosis of the bottom nepheloid layer (BNL) deposits: a case study from the Scaglia Toscana Formation (Paleogene, central Italy)

Paolo Monaco, Francisco J. Rodríguez-Tovar & Alfred Uchman

P. Monaco, Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, via G. Pascoli snc, I-06123 Perugia, Italy; [email protected] F.J. Rodríguez-Tovar, Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, ES-18002 Granada, Spain; [email protected] A. Uchman, Institute of Geological Sciences, Jagiellonian University, Gronostajowa 3q, PL-30-387 Kraków, Poland; [email protected]

KEY WORDS - Ichnocoenosis, bottom nepheloid layers, deep-sea sediment, Paleogene, Italy.

ABSTRACT - Some fine-grained sediments from the Eocene in age Scaglia Toscana Formation in the Northern Apennines (Trasimeno area), previously interpreted as mud-silt turbidites, have been reinterpreted herein as bottom nepheloid layers (BNL). They contain a rich ichnocoenosis dominated by endichnial forms, that formed progressively in line with the slow accumulation rate of mud transported by the oceanic thermohaline bottom currents. In a BNL idealized sequence, a slight upward increasing density of trace fossils, suggests some differences with typical muddy turbidites. Together with sedimentary structures, trace fossils form an ichno-sedimentary sequence through the bed, which is explained by a step by step colonization that accompanies deposition of the bottom nepheloid layer by continuous currents. It is possible that these ichnological features are recurrent and helpful in recognition of similar deposits within other geological contexts.

RIASSUNTO - [Icnocenosi degli strati nefeloidi di fondo: un esempio dalla Scaglia Toscana (Paleogene, Italia centrale)] - Alcuni sedimenti a granulometria fine, presenti nella Formazione della Scaglia Toscana dell’Appenino settentrionale (area del lago Trasimeno), in precedenza reinterpretati come torbiditi fangose, vengono reinterpretate qui come depositi prodotti da correnti di fondo o strati nefeloidi di fondo (BNL, “bottom nepheloid layers”). Essi contengono una ricca icnocenosi dominata da forme endostratali (endichia), formatesi progressivamente seguendo il lento accumulo di sedimento fangoso trasportato dal movimento di correnti termoaline profonde. In una sequenza BNL ideale si osserva un lento ma progressivo incremento della densità delle forme, manifestando delle differenze con le torbiditi fangose che, diversamente, vengono bioturbate dall’alto verso il basso, essendosi deposte in un brevissimo lasso di tempo. Insieme con le strutture sedimentarie, le tracce fossili costituiscono un’icno-sequenza che è spiegata da una colonizzazione graduale dell’infauna, che segue la deposizione delle correnti nefeloidi continuamente attive sul fondo marino. È possibile che queste caratteristiche icnologiche siano assai più ricorrenti di quanto documentato e quindi utili da riconoscere rispetto alle torbiditi fangose, utilizzabili quindi in altri ambienti geologici.

INTRODUCTION Sea, in the North Atlantic and the South Atlantic, or offshore the Namibian coast (e.g., Inthorn, 2005), but Fine-grained gravity flow deposits are well studied in their sediments are poorly studied in the geological record recent years with several models and suite of structures with a few exceptions (McCave, 1986, 2001; Thomsen & (physical and biogenic) through the bed (e.g., Stow & van Weering, 1998; McCave & Hall, 2006). According to Piper, 1984a, b; Stow & Wetzel, 1990; Monaco, 2000; some authors, turbulent flow in BNL could be even more Wetzel et al., 2008; Monaco et al., 2010; Amendola et al., responsible for deposition of mud in flat basin plain settings 2016). In these deposits, also trace fossil suites forming than fine-grained turbidites (McCave, 1986; Weaver et ichnocoenoses, can greatly help to improve the knowledge al., 1992; Peine et al., 2009). Similar observations are about infaunal response and biogenic behavior under recently highlighted from the dedicated workshops in 2010 different silt-mud depositional processes and rates (Stow and 2011 (P. Pilgrim personal communication; Rutgers & Wetzel, 1990; Monaco et al., 2010, 2016a). However, van der Loeff & Kretschmer, 2011). The thickness of many deep-sea depositional processes of mud are poorly BNL depends on bottom current velocity and the balance known, including their ichnological features. This concerns between gravitational settling of particles, basin plain the so-called bottom nepheloid layers (BNL; Hunkins et morphology and turbulence of the current (McCave, al., 1969; McCave, 1986, 2001; McCave & Hall, 2006; 1986, 2001). Tracemakers intensively exploit organic Puig et al., 2013; Dutkiewicz et al., 2016). matter slowly deposited on the sea floor by the BNL; in The bottom nepheloid layers are deep dense waters, fact, in the Canadian Atlantic basin plain, some sediment where the sediment is suspended and transported by the surfaces are extremely bioturbated with hundreds of burrow density-driven oceanic thermohaline bottom currents. openings per m2 (Hunkins et al., 1969, figs 3-4) and benthic These currents transport suspended clay and silt with populations are significantly affected by organic matter and organic matter particles from margins to the central part phytodetritus within the BNL (Puig et al., 2001). of a basin, with usual suspended sediment concentrations The aim of this work is to present peculiar sediments of < 0.1 mg/l above background levels, reaching higher from the Scaglia Toscana Formation in the Trasimeno area concentrations close to the continental rise and slope with (central Italy, Figs 1-2), previously considered as muddy near-bottom peaks >1 mg/l and lesser concentrations ≤ 1 turbidites, although some doubts arise for the presence of mg/l progressively towards the basin plain (Puig et al., extremely abundant endichnia such as Avetoichnus luisae 2013). BNL are very common at recent basin margins, Uchman & Rattazzi (2011) and other endichnial-epichnial such as on the continental rise of the French Mediterranean trace fossils (Monaco et al., 2010). In this work they are

ISSN 0375-7633 doi:10.4435/BSPI.2017.13 244 Bollettino della Società Paleontologica Italiana, 56 (2), 2017 better reinterpreted as a typical ichnocoenosis of the BNL the Scaglia Toscana Formation have been introduced (Figs 2-4). to explain their very great lithological differences in the Northern Apennines (Fazzuoli et al., 1996, 2002). In the Trasimeno area, lithology of the Scaglia Toscana GEOLOGICAL SETTING Formation is highly variable; it includes limestones, marly limestones, variegated marls (yellow, reddish, The Scaglia Toscana Formation (Merla & Abbate, black to green, often rhythmically arranged), calcarenites 1967) in the Trasimeno area (Northern Apennines) (from coarse- to fine-grained), mud beds and mudstone consists of rhythmically bedded deposits showing shales (Damiani & Pannuzi, 1982; Damiani et al., 1987; different colours. They are dated to the Late Cretaceous- Piccioni & Monaco, 1999; Amendola et al., 2016). In Early Oligocene. This formation has been called also the the most complete sections (M. Solare, M. Buono and Scisti Policromi (Fazzuoli et al., 2002 and references Montanare, Figs 1-2), thick-bedded limestones dominate therein) or the Argilloscisti Varicolori in western Umbria in the lower part of the formation (early-middle Eocene) (Principi, 1924; Damiani & Pannuzi, 1982; Piccioni & and fine-grained calcarenites and calcilutites in the Monaco, 1999). Some members and subdivisions of middle-upper part (middle Eocene, P10-P12 zones), while

Fig. 1 - a) The study area with the geological map and five studied sections (modified from Monaco et al., 2012). b) Synthetic stratigraphic succession of the studied sections with indication of differentiated formations and members. P. Monaco et alii - Ichnocoenosis in BNL of Paleogene, Scaglia Toscana 245 contribution of shales increases gradually upwards in Fig. 2); the development can be found typically in the the middle-late Eocene part (Monaco & Uchman, 1999; Monte Solare section. Epichnia are poorly visible due to Piccioni & Monaco, 1999; Trecci & Monaco, 2011; intense mottling (Fig. 4a, white arrows). Some scattered Monaco et al., 2012). These lithological differences have Nereites missouriensis (Weller, 1899) (rare) are present. been found also in northernmost Tuscany, for instance in Vertically elongated spots are locally abundant (shafts of the Chianti region (Canuti et al., 1965; Bortolotti et al., Ophiomorpha rudis, Chondrites targionii and probably 1970; Fazzuoli et al., 1996), the Falterona-Cervarola- Cladichnus; Fig. 4i, arrows). Trasimeno Basin (Boccaletti et al., 1990) and towards the Marchean Apennines (Centamore et al., 1986, 2002). Very abundant coarse-grained calcareous turbidites, THE BNL ELEMENTARY SEQUENCE polymictic breccias and debris flow deposits with neritic bioclasts indicate delivery from nearby ridges and The studied hemipelagic or pelagic deposits of the intrabasinal highs implying an eastern provenance in the Scaglia Toscana Formation in the study area, show five, Tuscan Domain and a western provenance in the Umbrian usually strongly bioturbated intervals named BNL1, Domain (Amendola et al., 2016, with references therein). BNL2, BNL3, BNL4 and BNL5 (Fig. 2a, e). The studied sections crop out in inactive quarries in M. BNL1. The basal white interval consists of silt to mud Solare (N43°043´618´´; E12°148´536´´), M. Buono with thin bioclastic horizons, locally forming parallel (Fornaciari quarry, N43°070´850´´; E12°189´918´´), M. laminae and very thin rippled laminations; broad scoured Maggio (N43°283´641´´; E12°145´461´´), at Passignano base and alternations of erosive surfaces are present (N43°181´382´; E12°173´769´´) and at Montanare (Fig. 2a, e). Grains are represented by minute fragments (Scanizza, N43°24´165´´; E12°070´831´´) (Fig. 1). For a of undetermined shells, foraminifers and lithic grains detailed description of all ichnotaxa and their abundance (mainly carbonatic). The BNL1 is pervasively bioturbated in the central Apennines see Monaco & Checconi (2008). (exhibiting e.g., Thalassinoides, Palaeophycus, Chondrites intricatus, Halopoa, Fig. 2e), and continuous many hypichnia (Fig. 4b-c, e). Graphoglyptids are very few in ICHNOTAXA contrast to non-graphogliptid forms. BNL2. The second white interval (Fig. 2e) consists The trace fossil suite of the Scaglia Toscana Formation usually of mud, showing (in thin sections) silt laminae in western Umbria as preliminary stated by Monaco & often scoured having discrete erosive surfaces. Ripple Uchman (1999), Piccioni & Monaco (1999), and Monaco cross lamination is present. This interval is strongly et al. (2012, 2016a) comprises many ichnotaxa. The trace bioturbated too, also with horizontal, scattered, extent fossil assemblage includes hypichnial Bergaueria isp. cross-section of Zoophycos and Planolites (Fig. 2a, e). (rare), Cardioichnus isp. (rare, Fig. 4c), Halopoa isp. BNL3. This interval is composed of ungraded mud, (common), Helminthopsis isp. (rare), Lorenzinia isp. some centimeters thick, intensely burrowed exhibiting (rare, Fig. 4b), Megagrapton isp. (rare), Ophiomorpha endichnial Alcyonidiopsis longobardiae, Planolites, annulata (Książkiewicz, 1977) (common), Paleodictyon Chondrites targionii, C. intricatus, Cladichnus and strozzii Meneghini in Savi & Meneghini, 1850 (rare), Taenidium (Fig. 2a, e). Scolicia strozzii (Savi & Meneghini, 1850) (common), BNL4. This interval is built up by reddish intensely Spongeliomorpha isp. (rare), Thalassinoides isp. (very burrowed mud. Chondrites targionii, C. intricatus, common in some levels of the M. Solare section), Cladichnus, Avetoichnus luisae are the main trace fossils. Ophiomorpha isp. (common) and Urohelminthoida isp. Ichnodensity may be very high (50-70%, Fig. 2a). (rare). Crossichnia (sensu Monaco & Caracuel, 2007) BNL5. The mottled reddish interval at top of the that commonly cross cut beds completely or only parts sequence represents a transition to a hemipelagite (He in of them are limited to Ophiomorpha rudis (Książkiewicz, Fig. 2e) that consists of a laminated red clay. The mottled 1977), a system of oblique to vertical straight cylinders, interval is intensely burrowed (e.g., by Nereites producers) 15 to 30 mm in diameter, which differ in colour from the but locally some graphoglyptids are present. host rock (Monaco et al., 2016a, fig. 3,p l. 2) (Figs 2 and 4a). This trace fossil, usually found in the Ophiomorpha rudis ichnosubfacies in deep-sea fan deposits (Uchman, BOTTOM NEPHELOID LAYERS 2009), appears in a wide depth range (from outer shelf with storm deposits to the deep sea; Monaco, unpublished). Nepheloid layer or nepheloid zone is a layer of water in Endichnia are very abundant and characterize BNL; the deep ocean basins, above the ocean floor, that consists they include Alcyonidiopsis longobardiae (Massalongo, of a body of dense water with a high concentration of 1856) (common), Avetoichnus luisae (very common with suspended sediment close to the base of the continental about 100 specimens measured and studied, Monaco et slope. Nepheloid layers are usually several centimeters to al., 2012; Fig. 4f, h), Chondrites intricatus (Brongniart, meters thick and they carry sediment particles up to 12 μm 1823) (abundant), Chondrites targionii (Brongniart, in size in concentrations of 0.3-0.01 mg/l. The sediment 1828) (abundant), Cladichnus isp. (abundant), Halopoa is suspended and transported by the movement of the isp. (common, Fig. 4j), Palaeophycus tubularis Hall, oceanic thermohaline bottom currents (McCave, 2001). 1847 (rare), Planolites isp. (common), Taenidium isp. Some authors tend also to include the gravity transport (rare), and Thalassinoides isp. (common, Figs 2e and 4i). (Inthorn, 2005). The name “nepheloid” comes from the Endichnia occupy different levels with a characteristic Greek: nephos, meaning “cloud”. The thickness of cloud increase in density towards the top of a bed (ICD, depends on bottom current velocity and is a result of 246 Bollettino della Società Paleontologica Italiana, 56 (2), 2017 P. Monaco et alii - Ichnocoenosis in BNL of Paleogene, Scaglia Toscana 247 balance between gravitational settling of particles, strength and turbulence of the current. A vertical gradient in the concentration of particles of any given size is observed, with the gradient being steepest for the fastest settling of particles (Fig. 3). In a similar way, a surface nepheloid layer may be created, due to particle flotation, while intermediate and bottom nepheloid layers are different because they may be formed at the slopes and basin plain of the oceans due to the dynamics of internal waves, gravity and downward currents (Dickson & McCave, 1986; McCave, 2001). Differences among hemipelagites, muddy turbidites and muddy contourites and their ichnotaxa suites result from the mode of the sediment accumulation being very rapid in muddy turbidites and very slow and discontinuous in muddy contourites (Wetzel et al., 2008). In any case, bottom nepheloid layers (BNL) are marginally included by Wetzel et al. (2008) in muddy contourites. But BNL are induced also by gravity and not only by bottom currents and, therefore, they may be a mixing phenomenon from muddy gravity flows and muddy contourites (Weaver et al., 1992; McCave, 2001; McCave & Hall, 2006; Peine et al., 2009; Rutgers van der Loeff & Kretschmer, 2011). In fact, the accumulation rate of clay and coarse silt in a typical BNL may be very high and can reach 20 cm/ka with a prevalence of clay + fine silt (C+FS) and sortable silt (SS) (Fig. 3, modified from McCave & Hall, 2006). In any case, pure contourites that exhibit a discontinous rate of accumulation, show complete bioturbation with many syn- but a few post- depositional trace fossils (Wetzel et al., 2008). As indicated by McCave (2001) and McCave & Hall (2006), three divisions can be distinguished in BNL, from the base to the top: 1) the basal mud with silt laminae, from l up to 5 cm thick, which become finer, thinner and less frequent upwards; 2) the intermediate, graded Fig. 3 - Resuspension and sorting in bottom nepheloid layers (BNL). mud or fine silt; and 3) the top composed by ungraded The model shows distribution of sediment and their depositional mud or clay + fine silt, respectively. The upper mud was rates (modified from McCave & Hall, 2006). produced by turbulent flow of suspended mud that was transported by currents and settled very slowly to the sea floor. The consistency of mud was probably very similar Piper & Stow, 1991; Milighetti et al., 2009; Monaco et to the “cloudground” described by Monaco et al. (2016b) al., 2010; Amendola et al., 2016). These deposits are that persists for weeks as an independent layer that moves not discussed further herein. A large discussion about horizontally, as monitored for weeks in different lakes of history and definition of “turbidites”, “debris flow Umbria. Thus, the bioturbation rate of BNL cannot be deposits” and “contourites” has been largely reported distinguished easily from muddy turbidites because of the by Shanmugam (2002, 2017). In his revision of the similar accumulation rate. In any case, botton nepheloid “ten myths of turbidites”, examining the Grès d’Annot layers can be reworked successively by post-depositional Formation (SE France), Shanmugam evidenced the bottom currents, as in BNL1 and 2 in the investigated importance of basical models of Kuenen (1950) and deposits. Sanders (1963), which restrict turbidites only to a few types of deposits with simple structures, considering the major other part of deposits to medium- to coarse- FINE-GRAINED TURBIDITES grained debris-flow deposits. Mud layers (locally with AND HEMITURBIDITES thin intervals of silts laminae) are genetically attributed by this author to “bottom current reworking” and to A lot of literature concerns fine-grained turbidites pelagic/hemipelagic deposits (Shanmugam, 2002, fig. 17). (O’Brien et al., 1980; Stow & Shanmugam, 1980; Stow Nothing was mentioned about BNL deposits, although the & Piper, 1984a; Stow & Wetzel, 1990; Einsele, 1991; thickness and abundance of mud layers are conspicuous

Fig. 2 - a) The elementary sequence of thin-bedded, mud deposits, attributed to bottom nepheloid layers (BNL), with ichnotaxa and bioturbation rate. b) The Mt. Solare section in an abandoned quarry with many mud deposits of Scaglia Toscana Formation, Eocene, scale bar = 1 m. c) The M. Buono section in active quarry with different reddish beds, Eocene. d) The Montanare section in abandoned quarry, Eocene, scale bar = 1 m. e) The elementary sequence with 5 BNL intervals (1-5) and hemipelagic mud (He); arrows indicate endichnial trace fossils, scale bar = 12 cm. 248 Bollettino della Società Paleontologica Italiana, 56 (2), 2017 P. Monaco et alii - Ichnocoenosis in BNL of Paleogene, Scaglia Toscana 249 in sandstones of the Grès d’Annot Formation in France. to analyze accumulation rates and biogenic events in Fine-grained turbidites were discussed in the Miocene basinal environments. A detailed study of such events in Marnoso-arenacea Formation (Muzzi Magalhaes & a regional context requires further sedimentological and Tinterri, 2010; Tinterri et al., 2016) and in the Macigno ichnological analyses in order to differentiate between Formation (Amendola et al., 2016), both in the Northern bottom, intermediate and surficial nepheloid layers. In Apennines, but without any references to BNL. Similar any case, this study represents the first description of these attention has been devoted to transitional deposits between poorly known deposits in the deep sea environments, with “hemipelagite” sensu stricto and “muddy turbidite”; the respect to their ichnological features. “hemiturbidite” concept, which involves a muddy deposit formed by a vanishing turbulent flows on sea floor that became progressively hemipelagic, is burrowed from the ACKNOWLEDGEMENTS top with post-depositional trace fossils (Stow & Wetzel, 1990). Therefore, following these authors, a hemiturbidite This work is a contribution to the call for papers arising from the Fourth International Congress on Ichnology “Ichnia 2016 - Ichnology could represents a transition between a hemipelagite and for the 21st century: (palaeo)biological traces towards sustainable a turbidite (Stow & Wetzel, 1990). development”, held in Idanha-a-Nova (Portugal), 6-9 May 2016. Many thanks to Tiziana Trecci and Ugo Amendola for help in the paleoenvironmental reconstruction and depositional setting DISCUSSION of Paleogene deposits in the Trasimeno area. The work largely benefited by fundamental improvements by two reviewers, A. Although some authors have demonstrated the Wetzel and D. McIlroy. Funds by P. Monaco RicBas 2014-2016, prevalence and importance of modern BNL (McCave, Perugia University. 1986, 2001; Inthorn, 2005; McCave & Hall, 2006; Puig et al., 2013), detailed studies on the subject are absent in the REFERENCES geological record. 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Fig. 4 - Ichnotaxa and some mud deposits, Paleogene, Trasimeno area. a) Two elementary sequences of BNL with crossichnion Ophiomorpha rudis (Op) (M. Solare section). Note the change in colour, scale bar = 5 cm. b) Hypichnial Lorenzinia isp. (Lo), M. Buono section, scale bar = 1 cm. c) Cardioichnus isp. (Ca) and other unknown tubular trace fossil, M. Buono section, scale bar = 1 cm. d) White calcareous turbidite bed with sand at base (double arrow) and overlying mud (black arrow) (M. Solare section). e) Hypichnial Urohelminthoida dertonensis Sacco (1888) (Ur), M. Buono section, scale bar = 1 cm. f) Endichnial Avetoichnus luisae (Av), M. Solare section, scale bar = 2 cm. g) Rhythmical mud beds, M. Buono section, hammer for scale. h) Long and arcuate endichnial specimen of Avetoichnus luisae (Av), M. Solare section, scale bar = 2 cm. i) Endichnial level BNL4 with several trace fossils, Thalassinoides isp. (Th), Planolites isp. (Pl), Avetoichnus luisae (Av), Ophiomorpha rudis (Op), M. Solare section, scale bar = 3 cm. j) Endichnial/hypichnial Halopoa isp. (Ha), M. Buono section, bar = 5 cm. 250 Bollettino della Società Paleontologica Italiana, 56 (2), 2017

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