Relationships between trace-fossil communities and substrate characteristics in some Jurassic pelagic deposits in the Umbria-Marche basin, Central Italy

RELATIONSHIPS BETWEEN TRACE-FOSSII, COMM-UN1TIES AND SUBSTRATE CHARACTERISTICS IN SOME JURASSIC PELAGIC D E P O S I T S IN THE UMRRIA-MARCHE BASIN, CENTRAl, ITALY PAOLO MONACO MONACO P. 1995. Relationships between trace-fossil communities and substrate characteristics in some Jurassic pelagic deposits in the Umbria-Marche Basin, Central Italy. [Relations entre traces fossiles et caract~ristiques du substrat remarqu~s darts certains d~pSts p~lagiques jurassiques du Bassin Marches-Ombries (italie Centrale)] GEOBIOS, M.S. 18 : 299-311.

ABSTRACT The study of trace fossil assemblages is a very useful tool for defining depositional palaeoenvironments, such as those of some early Jurassic pelagic sequences in the Umbria-Marche basin (late Carixian to early Aalenian). These sequences -Valdorbia and Fiuminata Colle Corno- are represented by thick bioturbated marls and limestones which alternate locally with poorly-bioturbated shales. Clay and silty-sandy deposits representing turbidites and storm beds (HCS = hummocky cross-stratified deposits) are very common, especially in the Toarcian interval. A comparison between trace fossil assemblages and sedimentologic/stratigraphic features permits an accurate paleoecologic and palaeoenvironmental analysis of sediments deposited in regressive conditions such as, for instance, the sequences studied. Trace fossil communities provide evidence of periods of non sedimentation or erosion during the Jurassic, when firm- or hardgrounds were commonly developed on muddy or sandy substrates. Penetration depth, burrow diameter, trace fossil density and species diversity emphasize the relationships with substrate characteristics and oxygen variations on the seafloor or in the sediment column. Consequently, analysis of different types of substrate and of substrate-controlled ichnofacies permits an evaluation of the paleoecologic strategies of the endobenthos in low to high-stress environmental conditions. KEY-WORDS : TRACE-FOSSILS, TURBIDITES, HUMMOCKY CROSS-STRATIFICATIONS(HCS), LOWER JURASSIC, UMBRIA-MARCHEBASIN, CENTRAL ITALY.

RESUMt~ L'~tude des traces fossiles repr~sente un excellent instrument pour la d~fmition du milieu de d~pSt du Jurassique (Carixien sup~rieur - Aal~nien inf~rieur) de certains s~diments p~lagiques presents dans le bassin Marches-Ombrie (Italie centrale). Ces s~diments -coupes de Valdorbia et de Fiuminata Colle Corno - sont repr~sent~s par des marnes peu bioturb~es riches en mati~re organique et par des calcaires argileux rouges et grumeleux tr~s bioturb~s, d~pos~s dans un milieu bien oxygenS. Ces sediments montrent des turbidites et des niveaux ~ stratifications entrecrois~es CHCS= hummocky cross-stratification"). Le passage des turbidites du Dom~rien ~ la pattie inf~rieure du Toarcien, puis aux d~pSts HCS du Toarcien moyen et sup~rieur, indique une tendance r~gressive darts les coupes ~tudi~es. Une comparaison entre les traces fossiles et les caract~ristiques s~dimentologiques du substrat permet de mettre en evidence diff~rents degr~s d'~nergie et des p~riodes de non-sedimentation ou d'~rosion dans le Toarcien moyen et sup~rieur Cfirmgrounds" et "hardgrounds"), sous un r~gime d'~coulement oscillatoire proche du fond (action des vagues de temp~te). A c e moment, les traces fossiles d~notent que les organismes d~veloppent des adaptations opportunistes ~ cause des conditions du milieu. De plus, ces traces augmentent en nombre, en diversit~ et en infiltration au sommet des couches ~ HCS et dans les s~diments typiques de bassin, en relation avec raccroissement de la disponibilit~ d'oxyg~ne et des conditions favorables du substrat. MOTS-CLI~S : TRACES FOSSILES, TURBIDITES, STRATIFICATIONSENTRECROIS]~ES, JURASSIQUE INF]~RIEUR, BASSIN MARCHES-OMBRIE,ITALIE CENTRALE.

300

INTRODUCTION In recent years stratigraphers have moved away from pure lithostratigraphic analysis and have approached the rock record in terms of Event Stratigraphy (Seilacher 1982), Genetic Stratigraphic Sequences (Galloway 1989), Sequence Stratigraphy (VanWagoner et al. 1990), and allostratigraphy (NACSN 1983). In particular, allostratigraphy permits subdivision of mappable stratiform sedimentary bodies on the basis of their bounding discontinuities. Such a stratigraphic paradigm provides a valuable framework for sedimentological and ichnological studies. Trace fossils and substrate analysis can be employed effectively both in aiding the recognition of various types of discontinuities and in assisting their genetic interpretation. In fact, trace fossils are proving to be one of the most important groups of fossils in demarcating stratigraphic boundaries (MacEachern et al. 1992). Specific studies of trace fossil communities in the Jurassic pelagic sediments of the Central Apennines are rare (see the literature summarized in Elmi 1981 ; Farinacci & Elmi 1981 ; Farinacci et al. 1981 ; Monaco 1992 ; Monaco in press). In Italy and Spain trace fossils are considered as being of marginal stratigraphic and sedimentologic importance, and are used mainly in order to recognize the origins of some nodular facies (Braga et al. 1981 ; Eller 1981 ; Elmi 1981 ; Massari 1981 ; Molina et al. 1987). Moreover, discontinuities in the nodular facies of "condensed" sequences (e. g. Bugarone Fm. and Rosso Ammonitico unit of reduced sections of Italy) are often unrecognizable, owing both to the intense burrowing which destroyed erosion surfaces and produced biogenic homogenisation, and to the diffuse nodularity related to postdepositional diagenesis. During the late Lias the clay content increased remarkably (Ortega-Huertas et al. 1993) and sedimentation rates in some Jurassic sequences of the Umbria-Marche basin were generally high ; these "intermediate" and "extended" sequences (Colacicchi et al. 1988) are compared since a great variety of lithotypes -including reworked sediments- and different muddy to sandy substrates are present. Dm-rythmites consisting of couplets formed by hthologic alternations of marly limestones and marls are common. In these sequences homogenisation due to bioturbation is not so evident and is only found in a few beds. Consequently, although discontinuities are less abundant t h a n in "condensed" sequences, they are clearly visible and substrate-trace fossil relationships are readily distinguishable. The aim of

this paper is to describe the trace fossil assemblages, to emphasize the substrate-ichnofacies relationships and to interpret some minor discontinuities m a r k e d by firm- hardground surfaces. In particular, those sequences where trace fossils are better preserved are considered. This study was made in order to evaluate their abundance, diversity and penetration depths in variablestress environments (Bromley & Ekdale 1984 ; Bromley 1990). The analysis of the type of substrate allows some burrowing strategies (Ekdale 1985) and other palaeoenvironmental phenomena from poorly to well oxygenated conditions to be evaluated.

G E N E R A L C O M M E N T S ON TRACE FOSSILS AND SUBSTRATES SUBSTRATE CHARACTERISTICS Depending upon their composition and consistency, the substrates occupied by infauna are usually indicated as various types of "grounds", such as soupground, softground, firmground, hard ground (see extensive literature reported in Bromley 1990 ; Ekdale 1985 ; Ekdale 1988). a) In the first case (soupground), the water content of the sediment is high and, therefore, has the consistency of a dense fluid. The organisms thus swim through the substrate, homogenizing it completely. Only if a large quantity of mucus is produced can tubes with well-defined walls be formed ; b) In the second case, the soft sediment (softground) still has a high water content but there is grain to grain contact..Thixotropic and dilation pheonomena are common in these sediments. The formation of mucus in these sediments also helps to aggregate particles of various origins, c) Further dehydration and compaction of the sediment produced a hardened substrate (firmground). This type is generally found below the two previous types and is often exhumed and exposed by energetic events which remove the soft sediments above. If such a substrate remains covered, it is refered to as a "concealed" firmground (Bromley 1990). When this type of substrate is exposed, it is penetrated by excavating endobenthic organisms and the compact nature of the sediment precludes further compression. Finally, d) increasing consolidation of the substrate, without reaching the cementation phase, results in a h a r d and compact sediment (hardground), characterised by generally vertical bioerosional processes. Composition, texture, stability, and hardness of the substrate are critical factors influencing the types of organisms t h a t inhabit the sea floor and

301 the types of behavior that are preserved in the sediment ; moreover, the diversity of the trace fossil assemblages reflects the nature of the substrate and is low in soupgrounds, high to very high in softgrounds, low to moderate in firmgrounds, and moderate to high in hardgrounds (Ekdale 1988). Locomotion trails (repichnia) and grazing traces (pascichnia) dominate in soupgrounds ; all types of trails and burrows are present in sol, grounds (see for example several ichnofacies described by Frey & Pemberton 1984 ; Seilacher 1967) ; dwelling traces (domichnia) dominate in firmgrounds and hardgrounds as special types of burrows excavated in stiff, compacted but uncemented sediment, or borings and bioerosion traces produced in hardgrounds (Bromley 1990 ; Ekdale 1985 ; Ekdale 1988 ; Pemberton & Frey 1985). From a physical point of view, the grain size of a substrate is very important : for example, more or less consolidated m u d d y bottoms are chosen by m a n y soil-bodied organisms (mostly worms) which possess specific locomotory characteristics adapted to this type of substrate ; or sandy and sometimes unconsolidated bottoms, colonised by certain types of invertebrates which sectrete mucus and thus consolidate the walls of their burrows in order to reduce the energy spent during locomotion (Bromley 1990). The physical consistency of the substrate can vary spatially, both vertically as well as horizontally. This fact m a y control the distribution of the endobenthic species and their behaviour within the substrate. Another important modification of the substrate by bioturbation is of a chemical nature : much material is transfered between zones of different chemical reactions during feeding and during the construction of galleries, both in a vertical and in an horizontal sense (Bromley 1990). A progressive increase in the concentration of oxygen in the interstitial waters of the sediments leads to specialized behaviour : there is a gradation from almost anoxic conditions in which the sediment is eaten (fodinichnia), to the grazing of food on the sediment surface (pascichnia) where there is more oxygen and, finally, to the formation of dwelling galleries within the sediment (domichnia) (Ekdale & Mason 1988). However, in these cases, it is not so much the depth which influences the behaviour of the infauna as the oxygen concentration of the interstitial waters or at the sea floor. Consequently, the critical environmental factor controlling the composition and distribution of some trace fossil assemblages is not variation in

water depth but rather a difference in sediment texture and stability which, in turn, are related to the energy of deposition of the lithofacies (see Ekdale 1988). Storm-generated sedimentary sequences m a y exhibit alternations between different ichnocoenoses that were produced under alternating high-energy (hummocky cross-stratified sands deposited by major storm events) and low-energy conditions (muddy deposits deposited during quiet non-storm intervals) (Pemberton et al. 1992b). HORIZONTAL AND VERTICAL TRACE FOSSILS Subdivision into vertical and horizontal trace fossils is essentially based on the energy level of the environment, the amount of nutrients in suspension (Bromley 1990 ; Ekdale 1985) and on the concentration of interstitial oxygen in the sediments (Ekdale & Mason 1988). In general, closer to the coast line where the energy level is related, for example, to the wave base (normal or storm), the most abundant forms are those which feed mainly on sediment in suspension while remaining firmly vertically anchored to the substrate, or which move vertically within the sediment in search of food (Bromley 1990). Further away from the coastline towards the lower energy zone (below the storm wave base and up to quite considerable water depths), vertical biological activity tends to decrease and, instead, horizontal activity increases. This is due to the fact that food needs to be increasingly looked for and also because there is less competition between the organisms. Below a particular depth and in certain conditions the amount of oxygen at the seafloor is lower due to the lower energy level (Ekdale & Mason 1988). Most of the organisms in this environment feed on deposited sediments because suspended sediments are rare. On the other hand, the complexity of the traces left by certain invertebrates within and on the sediment surface increases, as well as their areal extent (Gaillard & Olivero 1993 ; Romero-Wetzel 11989 ; Seilacher 1974). This behaviour is probably due to a change in the level of specialisation ~'m the search for food, which m a y be available under stable or only slightly variable environmental conditions for relatively long periods, b u t only to a few individuals. Nutrient intake is generally in the form of a systematic, meticulous and remarkably long search by each organism, even those of very small size (Ekdale & Lewis 1.991 ; Gaillard & Olivero 1993 ; Wetzel 1992). Some of these organisms have colonised the sediment in very low densities of about one animal per

302 100 m 2 (Wetzel & Werner 1981). In oxygen-poor environments, it is very important to distinguish between the oxygen content of the bottom water and t h a t of the interstitial water (Ekdale 1988). In such environments, the sediment often contains a b u n d a n t unoxidized organic m a t t e r for deposit-feeding organisms, and, consequently, horizontal deposit-feeding burrow systems, such as fodinichnia and pascichnia, prevail. Moreover, the lack of current activity on the sea-floor prevents the distribution of suspended food particles in the water colnmn for the consumation by filter feeders and, therefore, vertical filter-feeding burrow systems (domichnia) are generally rare under reducing conditions (Ekdale 1988).

re Jurassic sequences have been recently studied by means of a number of different methods (see the extensive literature on stratigraphy, micropaleontology, geochemistry and sedimentology summarized in Centamore et al. 1971 ; Colacicchi et al. 1988 ; Colacicchi et al. 1970 ; Cresta et al. 1988 ; Farinacci & Elmi 1981 ; Monaco et al. 1994 ; Ortega-Huertas et al. 1993). The largely bioturbated sequences considered here are represented by the F i u m i n a t a "Colle Corno" section (Pioraco) and the Valdorbia section (Fig. 1). The trace fossil study was undertaken on the interval spanning the late Carixian to the early Aalenian (Fig. 2). The lithostratigraphic bioturbated units considered here consist of : 1) well bioturbated gray to reddish mudstones/wackestones, 10-15 m thick, from the late Carixian to the late Domerian (upper part of COR = Corniola unit) ; 2) poorly-bioturbated shales of early-middle Toarclan age (MS = Marne del M. Serrone Formation), 20 to 55 m thick, very rich in clay. In the Valdorbia section the black color and geochemical anomalies emphasize a b u n d a n t organic m a t t e r

TRACE FOSSIL CHARACTERISTICS OF THE JURASSIC IN THE UMBRIAMARCHE AREA BIOTURBATION IN THE LITHOSTRATIGRAPHIC UNITS The area investigated belongs to the centralnorthern sector of the Umbria-Marche basin whe-

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Carte de localisation schdmatique indiquant les affleurernents principaux du Jurassique dans la rdgion Marches-Ornbrie (Italie centrale).

303 ~

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Distribution verticale des traces d'assemblages fossiles, leur abondance, dvdnements de sgdimentation (turbidites, ddpSts H C S et WB) et milieux dans la coupe de Valdorbia, MarchesOmbrie, Italie centrale, pendant le Jurassique infdrieur. Granulomdtrie du sable : v= tr~s fin, f=fin, m=moyen, c=gros, v=tr~s gros.

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content ("black shales", see Bartolini et al. 1992 ; Jenkyns1988 ; Monaco et al. 1994 ; Ortega-Huertas et al. 1993). Calcareous turbidites -mainly crinoidal and peloidal packstones- t h a t are slightly bioturbated and gravity flow deposits, such as slumps and pebbly mudstones are common in the COR and MS units. 3) Well-oxygenated, red and nodular marly limestones, 5 to 15 m thick, largely bioturbated of the middle-late Toarcian (RAUM = Rosso Ammonitico Umbro-Marchigiano unit). The RAUM unit is

characterized by sharp-based hllmmocky crossstratified (HCS) calcarenites, ranging in thickness from 20 to 60 cm (Monaco 1992). 4) moderately-oxygenated, gray marly limestones and marls of "Calcari a Posidonia" = CP u~fit, 10 to 40 m thick, of late Toarcian/early Aa]Lenian age. This unit in the F i u m i n a t a section is lJthologically sligh]y different (named " Calcari e IVIarne a Posidonia" = CMP ~;:in this section the CMP is represented by dm-rythmic alternations of marly limestones and clays very rich in bivalves., such

304 as Bositra buchii and Lentilla h u m i l i s (Conti & Monari 1992), whereas in the Valdorbia it is mainly represented by densely-packed bivalve limestones. Within the CP unit cm-thick bivalve gr~inatones - termed winnowed beds = WB (Monaco 1993) - are interbedded in the late Toarcian and early Aalenian sequences.

At Valdorbia (and to a smaller extent in the Fiuminata area) the vertical distribution from turbidites to sharp-based HCS deposits and WB beds probably reflects a generally regressive trend (Monaco et al. 1994). In fact, the HCS beds are considered to be diagnostic of fairly shallow-marine storm environments, below the daily wave influence and above the effective storm wave base and formed under the influence of a combined flow regime (see the literature summarized in Duke 1985 ; Monaco 1992): On the other hand, the WB beds were probably formed in a purely oscillatory regime, without nnidirectional currents. The causes of depth changes during the Toarcian are mainly related to sea-level variations and/or tectonic activity (Colacicchi et al. 1988 ; Farinacci et al. 1981 ; Hallam 1988 ; Monaco et al. 1994). STUDY METHODS The autochthonous marly levels deposited before and after high-energy events in the Fil~minata and Valdorbia sections are considered first, especially as regards their trace fossil content (preand post-depositional traces) (Seilacher 1982 ; Uchman 1991). These trace fossils represent a powerful tool for the recognition and interpretation of event beds, such as turbidites (Seilacher 1962) and storm deposits (Aigner 1982 ; Aigner 1985 ; Pemberton et al. 1992b). In the Fiuminata and Valdorbia sections the storm units represent, in the middle Toarcian, individual events that were deposited rapidly (during the period of a few hours to several days). They can superficially resemble turbidites but are generally distinguished by the presence of combined flow sedimentary structures (HCS) and shallow marine faunas in the intercalated shale deposits (Monaco 1993). Horizontal and vertical trace fossils can be differentiated. Horizontal trace fossils are observed on the bottoms and tops of calcarenitic beds, where these forms are more easily preserved (Seilacher 1962). The presence of vertical burrows was noted, together with percentual values of their density, the orientations of the burrows, and the maximum burrow diameter that was measured, and the results reported in several diagrams (Monaco et al. 1994).

Burrow abundance was evaluated according to the method of Baccelle & BoseUini (1965), although not based on thin sections as reported by these authors, b u t rather considering the trace fossil occurrence within a 40 cm diameter area of the outcrop. The percent values of trace fossil occurrence were calculated on the horizontal surfaces of the beds (bottom and top) and on the vertical sides of beds (internal biogenic structures within a bed, such as mottled structures). Burrow abundances were indicated as follows : absent = 0% ; sparse = 1-20% ; uncommon = 20-40% ; common = 40-60% ; abundant >60%. Calculations of bioturbation in repetitive thinner beds (0.2-0.5 cm in thickness) took into account the mean of the bioturbation values. Moreover, the substrate grainsize (lutite, siltite or arenite) and the sole marks (mainly scour and tool marks) are also considered. Vertical forms, types of traces, penetration depths, burrow diameters, inclinations and densities were measured, compared and reproduced in block diagrams; photographs together with drawings, were introduced into the data-base records for statistical comparisons (see methods reported in Monaco, et al. 1994, Fig. 17). RELATIONSHIPS B E T W E E N TRACE FOSSILS AND SUBSTRATES IN D I F F E R E N T ENVIRONMENTAL CONDITIONS The relationships between trace fossils and peculiar characteristics of the substrate are reported below, taking into account the different palaeoenvironmental conditions (turbidites or storm deposits) that were found in the Valdorbia sequence (Monaco et al. 1994) and in the early Jurassic interval of the Finminata sequence). Trace fossil distributions and substrate consistencies were reported considering the changing pattern of bioturbation in response to increasing energy in the depositional environment (Howard 1978). a ) Burrowing during the pre-turbidite interval (late Carixian-middle Domerian) In the Valdorbia section, the nodular reddish •marly limestones that were deposited during the pre-turbidite interval (late Carixian-middle Domerian stratigraphic interval) are largely bioturbated. Approximately 10 to 40% of the original bedding is homogenized by horizontal bioturbation. Nodular and highly mottled textures are present and trace fossils are largely compressed ; a few forms are taxonomically identffiable : Planolites, Zoophycos, Chondrites and Thalassinoides that have dimensions ranging from 5 to 70 mm

305 (5-15, 10-35, 0.2-10 and 25-70 mm~ respectively). The penetration depth in general is extremely variable and changes, respectively, from 0.5-10, 2040, 30-40 mm to 40-80 ram. M a ~ m u m burrow diameters are necessarily influenced by the effects of compression and vary within each layer (20-45 mm in most cases). The presence of feeding and grazing structures, constructed horizontally by deposit feeders and largely compressed in the post-depositional phase, indicates that the substrate consistency was originally a so,ground, without excluding that a firm substrate, not exposed at the seafloor ("concealed firmground" of Bromley,1990), was present (N. B. the presence of Chondrites). Current energy on the bottom during the late Carixian-middle Domerian interval was much reduced, as far as is indicated by the total lack of detritic impulses, such as turbidites and gravity flow deposits (only one thin debris flow deposit, 8 cm thick, was found). b) Burrowing during the turbidite interval (Domerian/Toarcian transition) Tectonic activity at the Domerian/Toarcian transition (Colacicchi et al. 1988) and a sea-level rise in the lower part of the Toarcian (Hal]~m 1988) promoted the deposition of calcareous turbidites and gravity flow deposits. These deposits reach 40 to 150 cm in thickness at Valdorbia (Monaco et al. 1994) and 15 to 50 cm at Fiuminata, and are intercalated with thick clayey deposits that in the lower part of the Toarcian (Tenuicostatum Zone) are gray to dark green in colour. In this interval sedimentation rate is very high and reaches about 40m/1Ma. At Valdorbia, geochemical anomalies in As, Sb, Zn, Co, Cu, Pb, V, Cr and Ba, ~mong other elements, testify that environmental conditions were poorly-oxygenated during the deposition of the MS Formation, and that reducing subenvironments must already have existed from the lower part of the Toarcian in the sequences studied as well as in other areas of the Umbria-Marche basin (Monaco et al. 1994 ; Ortega-Huertas et al. 1993). During this interval trace fossil associations became rare (Fig. 2) and burrow densities very low (K-selected endobenthos, Ekdale1985). Sparse ornate burrow-systems of 3-8 mm in diameter could be related to a geometrically patterned agrichnia (mainly Protopaleodictyon). Other seldom identifiable horizontal traces (? Gyrochorte) are locally present. These horizontal traces could be considered predepositional in the sense that they were preserved by slight erosion of a softground and sudden burial as "semirelief' casts (Seilacher,1964) on the sole of the overlying turbidite

(Bromley 1990). In the Domerian/Toarcian transition of the Valdorbia area, such horizontal trace fossils were generally formed during pre.- and postdepositional turbidite phases (Seilacher 1962), and were found in the hemipelagic mud that was deposited slowly between turbidite events. In such turbidite environments the calcarenitic material was deposited suddenly and rapidly, but very infrequently. On the contrary, the autochthonous substrate between each turbidity flow generally indicates quiet, stable conditions, and drastic changes are very limited (Pemberton et al. 1992a). Thalassinoides, Chondrites and Planolites show respective penetration depths of 8-15 ram, 0.5-2 mm and 3-10 rnm ; the burrow diameters range from 0.2 tol5 mm (10-15, 0.2-0.8 and 3-8 mm~ respectively) in the upper part of the Domerian stratigraphic interval. As from the lower Toarcian, burrow abundance, trace fossil density and dimensions were probably influenced by a progressive upward decrease in oxygen content at the seafloor and within the substrate (Fig. 2). Consequently, in the lower part of the early Toarcian, the burrow diameters of Thalassinoides (of about 3-10 ram) and Planolites (of about 0.5-5 turn) were considerably reduced. These forms disappear upwards in the middle/upper part of Tennicostatum Zone and only Chondrites persists in the autochthonous marly levels. The iclmological suite of the late Domerian/early Toarcian turbiditic interval may be regarded as a Zoophycos ichnofacies that indicates quiet-water, outer/inner shelf conditions, below the major storm wave base, in muds or muddy sands that are rich in organic matter and were associated with turbidity current deposits (Ekdale 1988).

c) Poorly-burrowed to unburrowed black shales (upper part of the early Toarcian) In the Valdorbia area laminated, pyrite-rich black sediments, related to the Early Toarcian anoxic event (Baudin et al. 1990 ; Jenkyns 1988), were deposited. Geochemistry, Total Organic Carbon content, micropaleontology (Bartolini et al. 1992) and trace fossils show that the anoxia was at a maximum in the upper part of the Tenuicostatum Zone, between 369 m and 360 m, with a peak around 364 m (Monaco et al. 11994). However, the positive geochemical anomalies indicate that the sea-floor was poorly oxygenated already in the older part of the Tenuicostatum Zone when medium to poorly oxygenated "Marne del M. Serrone" shales were deposited.

306

CHARACTERISTICS

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major storm wave

-

Figure 3 - Different substrate characteristics and the trace fossil response in a physically-controlled, storm-dominated environment. Rosso Ammonitico unit, middle-late Toarcian. T H = T h a l a s s i n o i d e s ; SK = S k o l i t h o s ; TR = T r y p a nites ; PAL = Paleophy. c u s ; PL = P l a n o l i t e s ; OP = Ophiomorpha ; C = Chondrites ; Z = Zoophyc o s . Diffgrentes caractdristiq u e s d u s u b s t r a t et rdponse d e s traces fossiles d a n s u n milieu d o m i n d p a r l'action p h y s i q u e des vagues de temp~te avec u n rdgime d'gcoul e m e n t oscillatoire proche d u fond. Unitd R o s s o A m monitico, Toarcien moyensupgrieur.

-

base

B1 a n d B2: regressive c o n d i t i o n s . Due to a regression in the middle/late Toarcian a fall in sea level causes a lowering of the major storm wave base level close to the seafloor. Large anomalous waves produce an oscillatory regime on the seafloor and winnow the soft mud (B1) or the soft sand (B2), causing exposure of firm to hard substrates (firm or hardgrounds). Non-sedimentation intervals due to an oscillatory regime at the seafloor occur and the old, formerly exposed substrates become progressively well bioturbated by opportunistic, vertical trace-makers. In the case of HCS deposits, burrowing activity is concentrated in a calcisiltite/calcarenite substrate that represents a fairweather deposit (B2).

nol--mnl storm wave base majors, ...... _~ base (storm ended)

_

I

Concealed

C1 a n d C2: post-regressive conditions. Sea-level raises the major storm wave base level and the depositional environment returns to quiet marine conditions; a normal sedimentation rate resumes on the seafloor and burrowing activity, within a muddy substrate (C1) or a sandy substrate (C2), occurs sporadically.

T r a c e f o s s i l s w e r e r e d u c e d i n d i m e n s i o n s a n d poo r l y d i v e r s i f i e d (Fig. 2). C h o n d r i t e s and rare Planolites were present in some cm-thick levels that

alternate with unbioturbated, laminated dark m a r l s . T h e p e n e t r a t i o n d e p t h i n g e n e r a l is v e r y l o w a n d r e a c h e s a m a x i m u m o f 3-5 r a m , M a x i -

307 m u m burrow diameters are 2-6 mm just below the black shales in the lower part of MS Formation. These features indicate that only a few feeding organisms could tolerate the oxygen-poor interstitial conditions of the substrate. In this oxygen-limited environment the nature of the bioturbation is episodic, superficial (few centimeters deep) and with burrow diameters in the range of one to a few millimeters. Traces are mainly dominated by the highly branched fodinichnial burrow Chondrites, which is characteristic of oxygen-poor interstitial conditions (Bromley & Ekdale 1984). Probably, as the concentration of dissolved oxygen both in bottom waters and within the sediment decreases, the size and density of organisms capable of inhabiting the underlying substrates also decreases (Rhoads & Morse 1971). d) Largely bioturbated storm deposits (middlelate Toarcian) Above the black shale interval (at the MS RAUM transition) trace fossils increase gradually in size and abundance, and horizontal forms are dominant at first. Planolites reach 10-20 rata in diameter, while Zoophycos is rare at Valdorbia and relatively more abundant at Fiuminata-Colle Corno before the deposition the RAUM. During the deposition of well-oxygenated and nodular sediments of the middle-late Toarcian (RAUM unit), both horizontal and vertical tracemakers become large, diversified and fairly common in the reddish nodular mudstones and wackestones. Thalassinoides, of 20 to 50 rata in diameter, Planolites of 5-15 rata in di,raeter and Chondrites represent the dominant burrow systems of the reddish marly deposits. The latter trace fossil is formed by darker vertical traces displaying an upside-down Y-shaped structure (Ekdale 1985). The maximum burrow diameter does not exceed 2 rata and penetration depths can reach 10 - 30 tara. Sub-elliptical concentrations of fragmented thin-shelled bivalves (Bositra) or echinoderm fragments produced by the activities of subvertical burrow organisms are very common in this interval. These bioclast concentrations could be due to the bioadvective activity of the infauna (Kidwell 1991). This infauna activity within the soft substrate exhumed and concentrated bioclasts -mainly thin-shelled bivalves- out of their life habitat ("biological agents", according to Kidwell 1991). In the middle Toarcian, calcisiltite beds showing hllmraocky cross-stratification (HCS) and locally oscillatory ripples are interbedded with marls (Monaco 1992). Trace fossils on the lower surface of HCS beds are represented by horizontal semi-

relief casts referable to Thalassinoides and Ophiomorpha having diameters of 30 - 60 rata and 30-80 tara, respectively, Planolites, and several other straight semirelief casts (?Gyrochorte). At the top of the HCS beds, oscillatory ripples become increasingly burrowed upwards by horizontal and vertical tracemakers. The maxim u m burrow diameter frequently exceeds 40 ram and penetration depths reach 70 - 100 ram (Monaco et al. 1994). Paleophycus, Planolites and Helminthopsis represent the dominant horizontal firmground-burrowers in the Valdorbia and Finminata sections, while vertical traces in semilithified or firm substrates are dorainated by Skolithos (Fig. 2). In fine-grained deposits (silt-fine sand) darker horizontal traces are referable to grazling/foraging structures in semiconsolidated sandy bottoms (post-HCS events, see MacEachern & Pemberton,1992 ; Pemberton et al. 1992b). The ichnological suite m a y be regarded as an "outer" Cruziana assemblage attributable to the variable degree of storm-wave action at the sediment-water interface, at depths corresponding to the maximum wave base (lower offshore/shelf transition, sensu MacEachern & Pemberton 1992). In very fine-grsined deposits (silt or mud) of middle-late Toarcian, hardground substrates are indicated by sharp changes in colours (from pink to dark brown). In the case of the sequences of the RAUM unit studied, the transitional layer that probably represents a discontinuity shows the vertical to subvertical dwelling structures of suspension / deposit feeding organisms (Fig. 3). Horizontal traces probably represent echinoid grooves, while vertical traces are mainly borings (Trypanites) that penetrated hard substrates to a depth of 2 to 10 cm (A. Uchman, personal commtmica tion). FIRM- AND HARDGROUNDS OF MIDDLE/LATE TOARCIAN AS EXPOSED SUBSTRATES Depositional trends in the Valdorbia section and in other Umbria-Marche sequences seem to indicate that a transgressive and a subsequent regressive phase are recognizable during the Toarcian (Monaco et al. 1994). A deepening trend occurred in the lower part of the Toarcian and a shallowing trend during the middle-late Toarcian. The latter fits better into a geological context clearly affected by a regressive phase (Hall~ra 1988) than into one affected by tectonic activity. During the middle Toarcian the sea-level fall shifted the major storm wave base close to the seafioor (at approximately 50-80 m depth). This

308 probably favoured the establishment of a highenergy regime at the seafloor that involved the sediment/water interface and the substrate (Fig. 3). In modern environments an oscillatory/unidirectional flow regime near the seafloor, due to strong storm conditions, produces hl~mmocky cross-stratified beds and oscillatory ripples (Walker 1984). Consequently, HCS beds may be diagnostic of repetitive, high-sedimentation events formed by storms in energetic, high-oxygenated environments (DAm 1990). Trace fossils in such environments are mainly monospecific and burrow density is often high (reselected ichnotaxa of Ekdale 1985). Sedimentary structures indicate that the burrows are produced over a fairly short period of time and that the environment is inhospitable due to very uneven sediment accumulation rates and oscillatory currents. Therefore, the typical ichnocoenosis of storm-derived sediments is made up of traces produced by opportunistic organisms in an unstable, high-stress, physicallycontrolled environment (Pemberton et al. 1992b). In these conditions food supplies are abundant in the water column but not available for a long time. The increasing penetration depth at the top of HCS beds, in the middle Toarcian, reflects low sedimentation rates during the waning phases of storms and testifies to the favourable conditions for several vertical burrowers. During the middle Toarcian, as a consequence of continuing and strong oscillatory movements on the sea floor, variable periods of non-sedimentation and/or erosion occurred, with exposure of firmground or hardground (Ekdale 1985). When erosion strips away a firm and/or soft surficial layer to expose hardgrounds (or firmgrounds), the sediment surface may be occupied by characteristic groups of hardground-burrowers, such as some echinoids, and exhibits Trypanites borings (or other firmground-burrowers in the case of firmground surfaces). Moreover, due to the oscillatory storm regime near the seafloor, the bottom is often winnowed (Fig. 3), and highly packed bivalve concentrations may be formed (Specht & Brenner 1979). As a consequence of these conditions the oxygen availability in bottom waters and within the sediment gradually increased through time (from the early to the late Toarcian). As a result, the progressive intensification of ichnofacies tiers from the MS to the RAUM Units indicates that dwelling organisms increased their penetration depth and occupied progressively deeper levels in the sediment column (see also the bioadvection activity of infauna). Consequently, the organisms inhabiting the reddish and autochthonous nodular facies of the inter-HCS beds of the RAUM are large, diverse and very abundant, and indicate well-oxygenated conditions on the sea-floor and

within the interstitial waters of the substrate (Savrda & Bottjer 1986 ; Savrda & Bottjer 1989).

CONCLUSIONS The study of trace fossils and substrate characteristics in some Jurassic sections of the UmbriaMarche area permits several conclusions : 1. The variations in oxygen concentration throughout the Domerian to Aalenian stratigraphic interval are sufficiently recorded in the UmbriaMarche sections studied. Trace fossil communities were negatively influenced during the early Toarcian by a reduction in the availability of oxygen in the bottom waters (lower part of Tenuicostatum Zone), and, subsequently, also in the interstitial waters within the substrate (during the upper part of Tenuicostatum and lower part of Serpentinus Zones). The reduced oxygen concentrations of bottom waters and interstitial waters, culminating during the deposition of the black shale, directly influenced the infaunal activity and trace fossil diversity. For this reason, trace fossil and substrate analyses, together with other studies (e. g. mineralogy, geochemistry, sedimentology and micropaleontology), can be employed in palaeoenvironmental reconstructions. 2. There is a relationship between the deposition rate and the burrowing intensity. High sedimentation rates in the Valdorbia and Fiuminata areas during the early Toarcian negatively affected the ability of the bioturbators to completely obliterate the event beds. High sedimentation rates preserved strata by not allowing the burrowers enough time to mix the sediment before the next clastic influx. On the contrary, reduced sedimentation rates during the middle/late Toarcian interval allowed some thin event beds to be totally destroyed by bioturbators. Moreover, a regressive phase in the middle Toarcian favoured a shift of the major wave base level close to the seafloor, thus exposing firm- to hardgrounds. These two types of "grounds" and related trace fossils indicate that several discontinuities in the sedimentation rate were present at that time. 3. Horizontal and vertical trace fossils were directly related to changes in palaeoenvironmental conditions and probably small-scale fluctuations in water depth. Turbidite or storm-influenced sedimentary sequences attest to shifts between lowenergy and high-energy depositional regimes. These changes were recorded by ichnocoenoses and burrowing patterns that were affected by the change from low-energy to high-energy conditions

309 related to the regressive phase that occurred in the middle/late Toarcian. - I a m v e r y grateful to Prof. M. Nocchi a n d Dr. F. W e s t a l l for critically r e a d i n g t h e m a n u s c r i p t a n d i m p r o v i n g t h e English. This s t u d y w a s u n d e r t a k e n w i t h t h e financial s u p p o r t of M.U.R.S.T. (40%, 60% R. Colacicchi).

Acknowledgments

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