Evidence of Brackish Lagoons in the Pliocene of Irpinia (Southern Ltaly)
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Bollettino della Società Paleontologica Italiana 37 (1), 1998 ISSN 0375-7633 89-98 2 pls. Modena, Luglio 1998 Evidence of brackish lagoons in the Pliocene of Irpinia (Southern ltaly) Diana BARRA Gioacchino BONADUCE Sabatino CIARCIA Dipartimento di Paleontologia Dipartimento di Scienze della Terra Università di Napoli Federico II Università di Napoli Federico II KEY WORDS - Ostracods, Palaeoecology, Systematics, Lagoonal deposits, Pliocene, Southern Apennines. S UMMARY - The sedimentary section outcropping in the locality S. Andrea near Montecalvo Irpino has indicated the presence ofpaleo- environmental characters which are referred to as a marine basin margin evolving to the regressional phase. This succession is constituted by coastal sandy sediments which end at the top with silty-clays lagoonal sediments. The analysis ofthe sedimentary facies and the studied ostracod fauna allowed the defìnition of4 facies assemblages (Al-A4) each of which is characteristic ofdifferent environmental conditions: upper shoreface - foreshore, coastaf eolic dunes, washover fan and brackish lagoon. As far as the ostracodes are concerned, two new species are established and illustrateci, Euxinocythere (Maeotocythere) nasseri n. sp. and Leptocythere lagunaris n. sp. from brackish environments. RIASSUNTO - [Evidenza di lagune salmastre nel Pliocene dell'Irpinia (Italia meridionale)] - La successione sedimentaria esposta in località S. Andrea presso Montecalvo Irpino ha evidenziato la presenza di caratteri paleoambientali riferibili al margine di un bacino marino in evoluzione regressiva. Tale successione è costituita da depositi sabbioso-costieri e termina in alto con sedimenti argilloso-si/tosi di ambiente lagunare. L'analisi delle facies sedimentarie e l'esame delle ostracofaune hanno consentito di differenziare quattro associazioni di facies (AJ-A4) carat- teristiche di altrettante condizioni ambientali: marino di bassa profondità, eolico di duna costiera, ventaglio di washover e lagunare. Per quanto riguarda le ostracofaune, 2 nuove specie sono state descritte ed illustrate da sedimenti di ambiente lagunare. Euxinocythere (Maeotocythere) nasseri n. sp. e Leptocythere lagunaris n. sp. INTRODUCTION evolurion from sands to the top conglomerates in the Unità di Ariano in the Ariano area and in Baronia is A shon sedimentary succession of Pliocene age, locally marked by thin clayey layers rich in an oligo- located NW of Ariano Irpino (Southern Apennines) typic microfauna indicative of a hypohaline lagoonal has been studied from the sedimentological and environment and by thin ash layers. Volcanoclastic palaeoecological point of view. The section examined layers in the same sedimentary cycle were found in and the samples studied indicate the presence of shal- the Paternopoli conglomerates (Chiocchini, 1969), in low marine sedimentation evolving from the upper the conglomerates of the middle River Ufìta Valley shoreface to a brackish lagoon, with a continuous (Chiocchini et al., 1971) and in the sands of Anzano regressive trend. di Puglia (Guerrera & Veneri, 1989). The sediments outcropping in the area have been Finally, Amore et al. (1997) split imo two distinct attributed to the Early - Middle Pliocene by Dessau sedimentary cycles the Pliocene sediments outcrop- (1953) and subsequently by D'Argenio et al. (1973) ping in Eastern Irpinia; taking into consideration the consequent to the presence of an infraPliocene sedi- lateral continuity of lower cycle layers with the pre- mentary cycle (Unità di Ariano). Previously, the sur- sent section, it is possible to attribute them to the top veyors of the Sheet 174 ''Ariano Irpino" (ltalian pan of the Early Pliocene (Biozone M Pl 4a). Geological Survey, 1963) fìrst attributed these sedi- ments to the Pliocene, and later doubtfully to the SEDIMENTARY FACIES ANALYSIS Middle - Late Pliocene (Iacobacci & Martelli, 1967). Recently, Chiocchini et al. (1992) dated to the The sediments examined belong to a short Early - Middle Pliocene (G. puncticulata and G. sequence (about 9 m thick) outcropping in a quarry aemiliana Zones) the sediments of the Ariano Irpino along the left slope of the River Miscano, S. Andrea Basin between the Ufìta and Miscano rivers, in agree- locality, near Montecalvo Irpino (Text-fìg. 1). The ment with Cantalamessa et al. (1988) who had lithology consists mainly of sands and subordinately accomplished a preliminary study of the sandy sedi- of pelites with some pebbles. The stratifìcation dips ments occurring between Ariano Irpino and about 20°W Four sedimentary facies associations have Montecalvo Irpino. been recognized, whose description and interpretation Ciarcia & Torre (1996) observed that the gradual are schematically reported below (Text-fìg. 2). 90 D. BARRA et alii m .__ ____=<x::uJ):>'" _ - - A4 8 bfie h g g A3 6 MCS MC4 :: .. ::. :: :. :·.'.::.:: .. ... : . :. : MC3 Texr-fìg. 1 - Skerch-map showing the areai distribution of rhe ---......:. .. :...:-i;" MC2 Lower to Middle Pliocene sediments of the Unità di Ariano (pattern) and location of rhe srudied sec- tion (asterisk) (modifìed from S.G.I., F° 174, e Ariano Irpino, 1:100000). d e A2 4 d e Al - Upper shoreface - jòreshore deposits The basai parr of the section is constituted by about 3.5 m of yellowish, poorly cemented, quarrz- b lithic sands with medium to thin well-classed grain- size. The boundaries of the different sedimentary bodies are rarely evident; the parallel laminations are 2 not always visible, while the wedge-shaped one with a Al megaripple foreset, sometimes with opposite dipping, is noted. The foreset packages are truncated and sometimes separated from the following event by a shell debris and small pebbles. The symmetrical rip- ples are rather rare. A gray day layer, slighdy silty and about 15 cm o thick, occurs in the lower parr of this interval and shows a lateral continuity. The boundaries with the underlying and overlying sands are abrupt. Sedimentary structures and shell remains are absent, sand D pebble while a microfaunal assemblage is present. Upward, frequent decimetric layers with aligned pebbles, shell detritus and mud clasts occur. The pebbles, of maxi- clay and volcanic mum diameter of 1O cm, are of sedimentary origin, r-=:::====l polygenic, and rounded to discoidal in shape. b:::"===:i silt ashes Few thin and silty day lenses of limited extension Texr-fìg. 2 - Stratigraphic column of the srudied succession wirh (1-2 m), gray-brown in colour, are also present. They rhe position of rhe samples examined for rhe osrracod contain abundant vegetai remains of subaerial origin fauna and rhe idenrifìed sedimenrary facies (a = rip- with some well-preserved leaves of arboreous plants. ples, b = leaves, c = rhizolirhes, d =cross concave lam- inarion, e = pulmonate gastropods, f = gypsum crys- The features reporred above allows for their attri- tals, g = cross lamination, h = burrows, i = bivalves). bution to a marine sedimentation influenced by waves from upper shoreface to foreshore. The oppo- site foreset lamination of the megaripples (similar to the herringbone cross-bedding structures) can be longshore currents and the effect of waves in the surf explained with a litoral migration due to variations in zone (McCubbin, 1982). The symmetry of the rip- the energy conditions, and to the subsequent trunca- ples' shape is a clear indication of an energy regime tion by erosion during storms, in a zone at the dominated by wave action (Komar, 1974). boundary between the foreshore and the shoreface The aligned pebbles have probably been accumu- (Clifton et al, 1971) or to the interaction between lated in peaks of litoral drift during shorr inrervals of EVIDENCE OF BRACKISH LAGOONS IN THE PLIOCENE OF IRPINIA 91 higher energy; the strongly elaborated clasts should are transitional to a proper lagoon. The generally be correlated to more extended conglomeratic bodies massive appearance of the sandy sediments is possibly which were accumulated in a contiguous coastal braid indicative of flow conditions analogue to the grain plain (Ciarcia & Torre, 1996). The accumulation of flow noted in the beach backwash by Clifton (1969). vegetal remains of subaerial origin occurred sporadi- The occasionally found, poorly defined clinostratifì- cally in local conditions of low energy indicating a cations of the foresets could be referred to a dose continental area. The prevailing deepening of microdelta (Schwartz, 1982). the foreset packages suggests ( Clifton et al., 1971) the The day layers point out to low energy phases westward position of the land. alternating with storm activity. The water salinity was undergoing strong variations up to saturation condi- A2 - Coastal eolian dunes tions and salt precipitation, as testified by the saline crusts and gypsum crystals. Light coloured sand, fine grain-sized, well-sorted, of lithic composition, generally poorly cemented, A4 - Lagoonal deposits sometimes with arenaceous layers. The trough-cross bedding at medium-large scale (lengths over 10 m) Clay and marly day, silty day, dark in color, yel- occurs with foresets deepening around NE - SW in lowish by weathering, without sedimentary struc- some cases more than 30°. The sands are barren, and mres, with badly defined thin stratifìcations. These there is the sporadic occurrence of rhizocretions and clays correspond to the top of the sequence having a scattered shells of gastropods. thickness of about 1 m. These sedimentary bodies in the area show a The lagoonal pelites are separated from the under- thickness at times greater than 1O m and