4. STRATIFICAZIONE HUMMOCKY E CALCARENITI NEL GIURASSICO DEL BACINO UMBRO-MARCHIGIANO

Hummocky cross-stratification (HCS) and calcarenites in the Umbria-Marche basin, Middle Toarcian.

Coordinator: Paolo Monaco; e-mail: pmonaco@unipg.it
Collaboration: J.A.Vera & J.M.Molina (Granada, Jaén, Spain)

Introduction to HCS in the Jurassic

During the middle/late Lias, the Rosso Ammonitico of the Umbria-Marche Apennines received clay deposits and carbonate detritic beds. These sharp-based detrital beds, 0.5 cm to 100 cm thick, are not always induced by tectonic activity (gravity flow deposits as turbidites, slumps, breccias and pebbly mudstones, are present in the Lower Toarcian). In fact, new studies of Stratigraphy Lab of Perugia and Granada Universities seem to contrast with previous literature that refers all calcarenite beds with cross stratifications to turbidity flow deposits in a deep-water floor for tectonic pulses. In these interpretations metheorologic causes (storms) are totally rejected. The detailed sedimentological study of Rosso Ammonitico by means detailed measures of laminas and their relationships has revealed, instead, contrary to previous interpretations (Elmi, 1981), large and well preserved hummocky cross-stratified (HCS) beds. This low-angle stratification which form symmetric hummocks, 50-70cm thick, generally erosive, is well known in the literature (see the extensive bibliography in Monaco, 1992; 1994; et al. 1994). Abundant calcareous tempestites showing HCS are present in the Upper Jurassic of Spain (see Vera & Molina, 1998 with references therein). HCS beds in the middle Toarcian of Apennines, supply some proofs of vertical trends from turbidites to HCS deposits, and provide a tool to reconstruct the changes in the sea-floor depth. A result is the regressive trend of the Toarcian (see Monaco al.1994) or several shallowing-upward cycles in the Upper Jurassic of Spain, (Vera & Molina, 1998). In Italy the trace fossil behaviour is indicative for a paleoenvironmental analysis and shows variations in the oxygen content during high-energy, waning-energy and fair-weather phases of HCS events.

The HCS sequence

A complete sequence as shown in Fig.1 (upper and lower figures) consists of three divisions (see extended bibliography in Monaco 1992; 1994; et al.1994; for Spain see Molina et al.1997):
I). BIVALVE AND ECHINODERM LAG. Coarse-grained (grain-size up to 5 cm), densely packed rudstones, floatstones and/or grainstones (0.5 to 40 cm thick) constitute the lower part of a sequence (basal lag, Fig.1b). The basal contact is sharp and erosive surfaces seems to affect underlying shales. In general the lag is discontinous and massive, but direct graded bedding is present (Kreisa, 1981). The coarse fraction is composed of bivalves and/or echinoderms, micritic lithoclasts and other invertebrate remains. Crinoids ossicles locally make up to 90% of the coarse fraction (encrinites).
II). HUMMOCKY CROSS-STRATIFIED DIVISION. This division represents the middle part of a sequence (Fig.1a) and occurs generally in thick beds (up to 60 cm thick). The elliptical domes can reach 100 cm in diameter and are generally symmetrical (Fig.2), and, consequently, no consistent directional current trend can be detected. The HCS displays wavelengths mostly from 70 to 140 cm. The grain fraction consists of well sorted, fine-grained calcareous sand and silt (average grain sizes in the thin section being between 0.05 and 0.30 mm). Texturally, they are generally grainstones and matrix-poor packstones. Most grains (up to 70%) are echinoderm fragments. Micritic peloids are present (locally up to 40%), together with pebbles (up to 15%), fragments of benthic foraminifera, and/or radiolarians. Small, flat or concave, thin-shelled, disarticulated bivalves are fairly common. At least 50% of the concave valves lie with the concavity facing up.
III). CALCISILTITE-LUTITE DIVISION. This part (from 0.5 to about 20 cm thick) constitutes the upper part of the sequence (Fig. 2a). Here radiated echinid carcasses smothered by the sudden influx of mud wich blocked their vascular system have been found (Fig. 2b). The grain fraction is generally finer than I and II and consists of calcareous silt and mud. In thin section micritic peloids and radiolarians are abundant. Equidimensional bivalve fragments are either parallel to the bedding surfaces or were accumulated by burrowing organisms. Oscillatory ripples, mostly asymmetrical about 2 - 6 cm high with spacings of 6 -12 cm, characterize the upper part, but current ripples are locally present.

INTERPRETATION

About the origin of HCS many papers have been published and several models have been proposed (see the "oceanographic" or "geologic" scenario and environmental settings reported in Walker 1982; 1984; Monaco 1992; Molina et al.1997 with abundant referencesz therein).
According to the classical model of Aigner (1985) for Triassic shelves, during episodic, very strong storm surges, wave stirring and onshore oriented wind-drift currents would be able to pile up skeletal debris, largely from crinoidal prairies, and accumulate nearshore skeletal banks in shallow-ramps (Aigner 1982; 1985). Water heaved onshore during major storms returns seaward, flowing along the bottom as a sediment-laden unidirectional density current (Fig.3). Close to the bottom falling grains encounter a zone of intense oscillatory sheet flow, which is moulding the bed into symmetric hummocks and swales. Consequently instead of forming Bouma sequences, the density current formed HCS, still under the influence of oscillatory regime (Walker 1982; 1984). Unidirectional bottom currents, together with oscillatory flows produced by the major storms, contributed to determine a wide-spread combined-flow regime. Evidence of oscillatory flows near the sea bottom is found in the characteristic abrasion surfaces on some benthic foraminifera tests, such as Lenticulina in the middle Toarcian interval and winnowed beds. Below the major storm wave base, instead, unidirectional flows are dominant and only calcareous turbidites, showing Bouma BC sequence, were deposited (Fig.3). When HCS deposits are present together with turbidites at depth about the major storm wave base level (Fig.3) the sole marks of the HCS beds are oriented in the same direction as those of turbidites. As reported from pelagic areas of Jurassic in Apennines and Spain, proximal deposits included crinoid ossicles (or other faunas in Spain) imported from nearshore and/or allochthonous bivalves (basal lags); distal deposits, instead, very similar to turbidites (see Monaco, 1996) were progressively fine-grained, mud-dominated and bioturbated, and contain reworked but parautochthonous bivalve faunas (Figs.2,3).

INTERPRETAZIONE

Sull'origine delle HCS tanto si è discusso e ancora si sta discutendo (vedi le citazioni in Walker 1982; 1984; Monaco 1992; Molina et al.1997). Alcuni pensano si tratti di strutture legate a flussi oscillatori legati a particolari regimi di corrente in shelf poco profondi dove si risente dell'azione della base d'onda di tempesta. Esistono comunque una miriade di modelli "oceanografici" e "geologici" estesi a tutti gli ambienti, più o meno dettagliati per spiegare queste strutture, che generalmente non si riescono a osservare mentre si formano attualmente durante un uragano. Il classico modello "geologico" di Aigner (1985) per rampe del Trias prevede un accumulo di banchi a crinoidi strappati e trasportati lungo lo shelf durante le tempeste, sia in senso diretto verso la terraferma, sia verso il mare aperto (correnti di densità unidirezionali cariche di materiale diversificato, Fig.3). In prossimità del fondo, con grande estensione laterale, i granuli incontrano potenti moti oscillatori e verticali innescati nella colonna d'acqua dai fortissimi venti legati alla tempesta (vedi la modellistica "oceanografica" dove si ritrova anche la distinzione tra tifoni, uragani e tempeste invernali a seconda delle latitudini e longitudini). Tali moti creano sul fondo zone circolari/ellittiche con accumulo progressivo di lamine di sedimento (gli hummocks appunto) e zone limitrofe con erosione o trasporto (swales) o winnowing di fango (winnowed beds). Il lag basale sembra venga trasportato per corrente unidirezionale (erosivo) su cui, a causa di un moto progressivamente oscillatorio/unidirezionale (flusso combinato), si viene a depositare la restante sequenza HCS con lamine incrociate a basso angolo. Al calmarsi dell'acme meteorologico su alcuni shelf (o aree pelagiche isolate poco profonde, vedi Giurassico di Spagna e Appennino) si assiste progressivamente al progressivo normalizzarsi dei moti ondosi (ripples simmetrici al top). Il trasporto verso il mare aperto (indicato dalle impronte di corrente alla base degli strati) sembra portare a depositi prossimali caratterizzati da lags erosivi, HCS, fino a depositi distali dominati da fango e lontani dall'area sorgente (Figg.2,3) che spesso sono molto simili a torbiditi nell'aspetto fisico con strutture tipo Bouma o altri caratteri (vedi Monaco, 1996).