Late Oligocene-Early Miocene syntectonic fluvial sedimentation in the Aragonese Pyrenean domain ofthe Ebro Basin:facies mnodels and structural controis CONCHA AIffNA5 and GONZALO PARDO Area de Estratigrafía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Plaza San Francisco s/n., 50.009, Zaragoza (Spain) Fax: 976-76 10 88, E-mail: [email protected] gonzalo.pardo®msf.unizar.es
RESUMEN
Se estudia un sector del margen Norte de la Cuenca del Ebro (Fig. 1>, cuyo relleno molásico Oligoceno superior-Mioceno inferior (Formación Un- castillo> corresponde mayoritariamente a un extenso sistema fluvial: el siste- ma de Luna, que drenaba la Unidad de Garvarnie y que emergía en la cuen- ca al Oeste de las Sierras Exteriores Aragonesas. Este sistema colectaba pe- queños abanicos aluviales o marginales adosados a las Sierras Exteriores. Al Este, fuera del ámbito estudiado, el sistema fluvial de Huesca también cons- titula un importante colector. En la Formación Uncastilo se diferencian tres unidades tectosedimenta- rias (Figs. 2, 3 y 4>, cuya evolución estuvo controlada por la tectónica que afectaba a las áreas fuente. El estudio de las diferentes litofacies reconocidas, de sus relaciones laterales y de su distribución areal a lo largo del tiempo
Cuadernos de Geología Ibérica, nOn,. 21, pp. 277-296. Servicio de Publicaciones. Universidad Complutense, Madrid, 1996. 278 Concha Arenas and Gonzalo Pardo mentación en los surcos sinclinales situados al Norte de los anticlinales com- pensó el levantamiento de éstos, suavizando la pendiente y originando así re- llanos morfológicos dentro de la pendiente general del abanico. En estas áre- as de baj a pendiente se desarrollaron cursos arenosos de alta sinuosidad en sectores proximales del sistema, a tan sólo 5,5 km del margen actual de la cuenca.
Palabras clave: Sedimentación fluvial, Tectónica sinsedimentaria, Forma- ción Uncastillo, Neógeno, Cuenca del Ebro.
ABSTRACT
This contribution deals with a part of the northern margin of the Ebro Basin (Fig. 1). The Upper Oligocene-Lower Miocene molassic infilling in that part
¡ INTRODUCTION The Uncastillo Formation (Soler and Puigdefábregas, 1970) comprises alluvial and fluvial sediments deposited in tibe northern-central part of tibe Ebro Basin during the Late Oligocene-Eairly Miocene. During tbis period, tibe principal configuration of the Sierras Exteriores Aragonesas took place and the Ebro Basin started its last stage of evolution as the southem foreland ba- sin of the Pyrenean Ranges, while the older moíassic Jaca Basin became an area of erosion. The Uneastillo Formationor the equivalent Sariñena Formation (Quiran- tes, 1978) mostly corresponds to deposition in two large (>2x1 0~ km2 each> terminal fluvial systems (Fig. 1): the Huesca and Luna fluvial systems 13- - - ~-.7;’ - -2z47§c§-§ 4-722 7 — ., — — —. —HUEScA FLUVIAL — 0 ‘-4 — — — — - %- — 7 ZARAGOZA CRIDA 25 kni [3 1 [32 [33 WIW [3s Fig. 1—General location map with indication of tite two large fluvial systems defined by Hirst and Nichols (1986). 1: Miocene. 2: Oligocene. 3: Paleocene-Focene. 4: Mesozoic. 5: Paleozoic. Fig. 1—Situación general del área de estudio, con indicación de los dos grandes sistemas finvia- les definidos porHirst y Nichols (1986). 1: Mioceno 2: Oligoceno. 3: Paleoceno-Eoceno. 4: Me- sozoico. 5: Paleozoico. 280 ConchaArenas and Gonzalo Pardo cient Pyrenean Eocene-Oligocene basins (Jaca Basin for the Luna system>, tibe Sierras Interiores and Axial Zone of the Pyrenees. The apices of these flu- vial systems were placed in stmctural lows: the westem end of the Sierras Ex- teriores for the Luna system and the area between Mediano and Boltaña an- ticlines br the Huesca system (1-Iirst and Nichols, op. citj. Botib fluvial sys- 2> alluvial or marginal fans tems enclosed several small (generally < 15 km directly fed from the Sierras Exteriores. At present, tibeir proximal, conglo- meratic facies are exposed as tower bodies called mallos. Reeent studies on these fluvial systems have analysed geometry aspcc-ts of sand bodies and their environmental controls (Hirst, 1983; Nicibols, 1984; 1-Iirst and Nichols, 1986; Friend, Hirst and Nicibols, 1986; Nichols, 1987a and 1989; Friend, 1989). Some authors have also studied the relationships between sedimentation and tectonies: Nichols (1 987b) pointed out tibat syn- depositional folding affectcd tibe marginal fan sediments (AgOero fan), but he considered tibat tibe Luna system was not affected by syndepositional defor- mation and estimated that basin margin deformation to the west of tibe Sie- rras Exteriores and folding within the basin (í STRATIGRAPHIC AND TECTONIC FRAMEWORK The Uncastillo Formation lies unconformably on Mesozoic and older Tertiary formations tibat constitute tibe Sierras Exteriores Aragonesas, but to the west of Fuencalderas Fig. 2—Geological map of the study area. S.F.: San Felices. A-A- and B-B : geological cross-sec- tions of Fig. 3. Vertebrate localities: LG: La Galocha, SJ: San Juan. Fig 2.—Mapa geológico del área estudiada. SP.: San Felices. A-A’y B-Wseñalan las trazas de los cortes geológicos de la Fig. 3. Yacimientos de vertelicados: LO: La Galocha, Rl: Sari Juan. 282 Concha Arenas and Gonzalo Pardo o 0 Late Oligocene-EarlyMiocene syntectonic fluvial sedi¡nentation. - - 283 Biel-Gallipienzo, afíer Soler and Puigdefábregas, 1970). In tUs area, tibe Campodarbe and Uncastillo successions ibave an apparent conformity reía- tionship and tibe boundary between both is given by a shaw granulometric change (passage from sandstones and mudstones to conglomerates), while paleocurrcnt directions change from E-W to N-s (Puigdefabregas, 1975> In the Uncastillo Formation, Arenas (1993> and Arenas and Pardo (1 994a) eharacterized three allostratigrapibie or tectosedimentary units (T.S.U.> with a cyclic tendency (Figs. 2, 3 and 4; See also Fig. 2 of Arenas and Pardo, 1 994a, for more stratigraphic information>. Ml of them have a lower fining-upward megasequence, wibich records a retrogradationál stage of tibe marginal and fluvial fans, followed by an upper coarsening-upward megase- quence, which corresponds to a progradational stage of the fans. UnU US is more complcx, as its upper part consists of two megasequences (U32, coarse- ning, and UY, fining-coarsening> separated by a relative granulometrie ma- ximum, whicib was used for correlation tibroughout the study area and betwcen tibis and southward lacustrmne areas of the basin (Arenas, 1993>. Boundaries between units U1, U2 and U3 are sedimentarybreaks recognized as cibanges from coarsening to fiing-upward in tibe sequential tendency. To fue west of Fuencalderas these boundaries are conformities within cumulati- ve wedge systems located along the basin margin and along the Fuenealderas and Uncastillo folds witbin the basin. Tibe latter are located 5 to 8 km south of the basin margin (Figs. 2 and 3). To tibe east of Fuencalderas, boundaries between units are syntectonic intraformational unconformities witlún the margina] alluviail fan deposits (Fig. 3B>. Dating of units U), U2 and U3 (Arenas, 1993> is based on vertebrate lo- calities (San Juan and La Galocha, from Alvarez Siena el aL, 1990> found in megasequences U3’+U32 (Fig. 2), on reinterpretation of magnetostratigra- phic data from 1 logan (1993> and on correlation of those units witib otiber well-dated arcas of the Ebro Basin (Arenas, 1993>. The dating proposed is: Ul: Upper Oligocene (up to Agenian, MNI>; U2: Agenian (MNI -MN2); U3: Agenian (MN2 or Zone Y> to Lower Aragonian (Lower Miocene, MN4>. Al- tiuough the top of unit U3 is not represented in tibe studied alluvial series, it is known in tibe lacustrine sequences located southward, wibere tibe equivalent unit Nl is entirely represented. During those time intervals, tectonie activity alfected tibe dralnage basin of tibe Luna system, the Sierras Exteriores and tibe Ebro Basin. Sequential ten- dencies of T.S.U. were controlled by topography variations in the sediment source area (Gavarnie unit>; according to Nicibols (1 987a>, ffiese variations could be due to out-of-sequence thmsting. The marginal fan deposits show syndepositional folding and syntectonic intraformational unconformities re- Jated to tectonie uplifí in tibe Sierras Exteriores (Nichols, 198714. Tibe for- mation of alí these marginal fans was not simultaneous, but evolved from east to west, and was related to the setting of successive tibrust sibeets that compo- 284 Concha Arenas and Gonzalo Pardo ‘ / o OOQO ca,»~odarbe F¿fM$t¡on -7~--- — — — 3 ~j ~YZ~[js[36 ~-7 -8 Fig. ‘L—Chronostratigraphic schemeshowing lithofacies chauges aid stratigraphic relationships of the Uncastillo Forination from west to east, parallelto the basin margin. Vertical axis is not at real tirne-scale, bntarbitrarily referred to thickness inthe Luesia aren (Puig Moné and Hañón region), whcre Ul + U2 + 1J3 = 1600 mthick eninimum). L: Local zones and MN: Mein ro- nes. Zones and ages froin Calvo el al (1993). 1: Stratigraphic lacuna. 2: Sandstoncs and muds- tones (Campodarbe Formation). 3: Ltlcsia-t3qe conglomerates ±sandstones. 4: Mallo-type con- glomerates ±sandstones. 5: Sandstones and mndstones. 6: Mndstones and sandstones. 7: Sedi- mentary break: Unconformity and corí-elative conformity. 8: Megasequence boundary. Fig. 4—Esquema cronoestratigráfico EsLe-Oeste mostrando los cambios de litofacies y las reía- oones estratigráficas de la Formación Uncastillo paralelamente al margen de la cuenca. La esca- lo vertical no es una escala de tiempo, sino referida a la potencia en el área de Luesia se the complex soutib-Pyrenean tibrust front (Arenas, 1993; Arenas and Par- do, 1994b; Millári, 1996) (See Fig. 7 below). Witibin tibe basin, the Fuencal- deras and lJncastillo folds are syndepositional structures that were active at least duiing U2 and U3 sedimentation (Fig. 3). Unit US crops out extensively throughout the study area; tibe sedimentological analysis of its deposits aflows folding influente on fluvial system development to be evaluated. Apart from tibe tibree units inentioned, diere are some localized outcrops of tibe Uncastillo Formation placed diredtiy on ffie Sierras Exteriores. Dating Late Oligocene-Early Miocenesyntectonicfluvial sedimentation. - - 285 is not known yet. These are Peña del Sol conglomerates (mainiy derived from the Jaca Basin> and monogenetic breccias (formed from tibe Sierras Exterio- res> (Fig. 2>. Both lic unconformably on megasequence U33 strata and then constitute younger allostratigrapibie units, but the eronostratigraphie reía- tionships between Peña del Sol conglomerates and breccias are unknown. ALLUVIAL AND FLUVIAL FACIES MODELS In tibe study aiea, tibe UncastiJio Formation is composed of several lidio- facies (mappable facies associations defined by their lithology, texture, depo- sit geometry and sedimentary structures>. Each one represents tibe sedimen- tation in a specific sector of an alluvial or fluvial environment. The following lithofacies have been distinguished: * Conglomerate lithofacies (>85% conglomerates and -<15% sandsto- nes±mudstones>.According to tibe texture and internal organization two ty- pes are distinguished: — Mallo-type conglomerates (MC>: they constitute localized, reddish, to- wer-shaped bodies (ma¿’bs) 200 to 400 m bigh and 0.5 to 4-5 km2 in outcrop area, composed of thick tabular bodies, commonly witib crude stratification. Tbose have clast-supported textures with dominant angular to pooriy roun- ded, mainly calcareous clasts. — Fluvial, Luesia-type conglomerates (LC): they form large, brown and gray masses, up fo 7 km long and tens of km wide in outcrop surface. Very well rounded clasts, mainly of Campodarbe sandstones and minor limestones and black chert, constitute clast-supported, lenticular and tabular deposits. * Sandstone and conglomerate litibofacies (SC)(40-80% sandstones, 20- 60% conglomerates and up to 30% mudstones). Based upon the conglomera- tes, two types are differcntiated: Sandstone aud mallo-type conglomerate lithofacies (SMC), organized as metre to decametre, fiat base, tabular sequences. Tibeses can be either co- arsening or fining-upward, or cyclic coarsening-fining-upward in tendendy. Sandstone and Luesia-type conglomerate litbofacies (SLC), organized as metre to decametre, gentiy channeled base, tabular sequences. Tibese are mainly fining-upward in tendency. * Sandstone and mudstone litibofacies (SM) (40-80% sandstones aud 20- 60% mudstoncs). * Mudstone and sandstone litibofacies (MS) (60-90% mudstones and 10- 400/o sandstones; locally, up lo 10% limestones>. SM aríd MS litibofacies occupy a large pafl. of tibe study area. Both have si- milar sandstone deposits: — Sheets made of lenticular bodies — Sheet aud lenticular deposits composed of lateral accretion bodies 286 Concha Arenas ami Gonzalo Pardo — Isolated lenticular or ribbon deposits Sheets formed of laminar bodies Ml of them can be simple or multistorey and can occur in both SM and MS lithofacies, being the main difference between both the thiekness and fre- quency of occurrence of such deposits. * Mudstone Iithofacies (M) (>80% mudstones and 10w percentages of sandstones, limestones and locally gypsum). Sandstones mainly constitute tbin sheet bodies. Lateral relationslviips (Figs. 3 and 4) and areal distributions of these litibo- facies through time wilI allow fo characterize tibe facies models for the studied successions. Figure 5 shows two examples of lithofacies distribution: Fig. SA represenís a retrogradation stage and Fig. SB a progradation stage of the flu- vial and marginal fans. These relationsibips show the existence of two maln lithofacies associations (Fig. 6A and B): 1> MC —~ SMC —-* 2) LC —* SLC —* SM -4 MS —4 XI, referable to tibe Luna fluvial system These two systems coexisted during units Ul, U2 and U3, but tibeir enti- re development is recognized only in unit U3. For the older Ul and U2, li- thofacies distributions are exposed only parallel to the basin margin, but are coherentwith the areal relationsibips observed for unit U3. The maln charac- teristies of tibe two models proposed are: 1) Tibefirst associaUon represents short ~mdsmall alluvial fans (Fig. 6A) in whicib three sectors existed: = Proximal sector: Most deposits of this sector (lithofacies MC) originated from unconfined flows (sheet floods>, which in some cases gaye rise to lobe coarsening-up sequences. Lithofacies MC may also filí paleovalleys wibere massive deposits produced by high energy confined flows are present; there, Fig. 5—Lithufacies distrihution for the base (A) and top (B) of megasequence 1132 (Agenian (Y2>-lkamblian (Z)>. Lithofacics: 1: Luesia-type conglomerates. 2: Mallo-type congloincrates. 3: Sundstones and Luesia-type conglomerates. 4: Sandstones aud malJo-type cunglomerates; 6: Sandstones and mndstones. 6: Mudstones and sandstones. 7: Muds-tones. 8: Sandstones, muds- tones and limestones. 9: Mudstones and gypsum. F: Fnenc,qideras. Fig. 5—Mapas de distribución de litofacies para la base (A) y el techo (B> de la megasecuencia 1132 (Ageniense (Y2)-Rambuiense (Z)>. Litofacies: 1: Conglomerados tipo Luesia. 2: Conglome- rados tipo maflo. 3: Areniscas y conglomerados tipo Luesia. 4: Areniscas y conglomerados tipo mallo. 5: Areniscas y Jutitas. 6: Lutitas y areniscas. 7: Lutitas. 8: Areniscas, hititas y calizas. 9: Lutitas y yesos. E: Fuencalderas. Late Oligocene-Early Miocene syntectonicfluvial sediinentation. - 287 A A/erie - 4 Luna —c-—~~~~ - - - EMa ...~— AI.,Uv 15 km B ______1? Unces tilo A’- 9 be 4 $Eje a Afaudtvar 12 3 4 5 6 7 8 9 288 Concha Arenas ami Gonzalo Pardo turbulent mass flow processes could also exist. Locally, talus monogenetie breccias are found. = Middle sector: Lithofacies SMC and SM form tliis sector. SMC mosfly corresponds to bbc sequences and minor fluvial braided system deposits. Channel and bar braided systems, locally 10w sixmosity channeis aud sheet flows are recorded by lithofacies SM, which aLo displays overbank sedi- ments. = Dista? sector: Litibofacies MS is dominant upstream wibile M extensively occurs downstreaim. Extense flood plains with sibeet flows and rare sand rib- bons originated in tlviis sector. Alluvial fans witib this association were up to 15 km long, as in die case of Aniés fan (NE Ayerbe>, although shorter dimensions were common (5 km) because middle and distal sectors usually combined in larger fluvial systems Fig. 6.—Sedimentary facies models: A: Marginal fan association. B: Fluvial fan association (Luna system)- Fig. 6—Modelos de facies: A: Asociación de abanicos marginales. B: Asociación de abanicos fin- ~iales (sistema de Luna>. Late Oligocene-Early Miocene s-yntectonicfluvial sedbnentation. - - 289 290 Concha Arenas ami Gonzalo Pardo = Proximal-middle sector The existing lithofacies are SLC and SM. Con- glomeratic and sandy cibannels and bar braided systems, in places with two distinct topographic lcvels, were predominat. F-Lowever, nortib of Fuencalde- ras and Uncastillo anticlines, and in the second case close to proximal con- glomerates, sandy mcandering deposits are present. The syndepositional folds within the basin represent the boundary of this sector. = Middle-distal sector Typically, Ibis is represented by lithofaeies SM, with development of low and high sinuosity rivers, epibemeral or witib limited lateral migration. Sinuosity cibanges through space and time indicate chan- ging slopes and/or important diseharge variations. Flood plain deposits be- come tbickcr and spread downstream grading mb lithofacies MS. = Dista? sector (lithofacies MS ímd M): Sheet flow sand deposits (splay deposits> and rare, low or high sinuosity, siballow channels are found within wide mud flood plains, wliich downstream are almost entirely formed of mudstones with din lacustrine, sulpibate or carbonate intercalations. The fluvial systems with such sectors were 40 to 60 km long, xvith radial paleocurrent patterns (Nichols, 1987a; Jupp et al., 1987>. Alí the scctors are wel] represented and lithofacies changes are gradual. Proximal sectors are characterized by braided systems, while proximal-middle and middle-distal sectors show a great variety of fluvial styles. These features refer to alluvial systems widi dominant fluvial processes or «ibigh transport efficiency fans» af- ter Colombo <1989) or «fluvial fansa after maz Molina eta?. (1985>. Dralna- ge area surface (including tibe Jaca Basin, Sierras Interiores and part of dic Axial Zone> in dic Gavamie unit is not possible to be determined exactly, but it was extremely larger dan those of tibe marginal fans. PALEOGEOGRAPHY AND TECTONIC CONTROLS ON SEI)IMENTARY FEATURES As indicated aboye, the Luna system resuited from coalescence of two dif- ferent fluvial fans: the Uncastillo fan, to the xvest, and the Luesia fan, to tibe east (Figs. 4, 6B and 7), tibe apices of wibich were located to the northwest of Selva and nortb of Puig Moné points. Luesia fan apex remained at tibe same place dirough time, but Uncastillo fan apex moved eastward, and at tibe time 2 it was placed to dic nortL¡ of of sedimentation of units U2 and UY + U3 Cruz poiní. For megasequence U33 tbere is not sedimentary record of con- glomerates of die Uncastillo fan. Tbis may be due to later erosion, but it is not probable as geomorphological features of thc area do not reflecí it. Are- nas (1993) suggested that during megasequence U33 deposition, de drainage basin of tibe Uncastillo fan was captured by the maln flow of die Luesia fan: tbe Luna system consisted tiben of a single apex (Fig. 7>. At that time, a new rejuvenation pLise of tibe source aren topograpiby, caused by increasing tee- Late Oligocene-Early Miocene syntectonicfluvial sedúnentation. -- 291 tonic activity, gaye risc to progradation of proximal sectors of the fluvial sys- tem. However, the capture eventcannot be attributed to any particular struc- ture of the Gavarnie unit. Figure SA shows for megasequence U32 dic coexistence of two fluvial con- glomerate bodies with three discrete mallo-typc conglomerate outcrops (Aguero, Murillo and Riglos marginal fans). Downstream, fluvial conglome- rate litibofacies are fringcd by sandstone and mudstone lidiofacies, the out- crops of wbich sibow an elongated shape or flattcning against the margin. Thus, proximal-middlc and middle-distal sectors tended to elongate paralící to dc basin margin. This fact determined that the marginal or short fans combined in dic larger Luna fluvial systcm in arcas ncar dic basin margin. This particular distribution could ibave been present during dic wibole Uncastillo Formation deposition, but its presence can only be reported for units U2 and U3 (Fig. 7>: That distribution proves that a part of the Luna system diseharge was driven parallel or quasi-parallcl to dic adjaccnt mar- gin, as shown by de WNW-ESE oriented, tltck sandy channels that are found near dic basin margin (cg., Bid and Agúcro-Murillo arcas). Such well-developed longitudinally-flowing rivers suggest that a pail of the basin margin was underfilled. That part corresponded to tibe region adjacent to the Sierras Exteriones structural high. Locally-increased subsidence, caused by dirust sibeet stacking, could lead to such situation, but syndepositional foldingxvithin tibe Ebro basin, at least from unit U2 sedimentation, is a more simple explanation (Figs. 6 and 7): the uplift of die Uncastillo and Fuencal- deras anticlines lcd to channel part of the fluvial diseharge along the syncli- rial subsident zones. Tiben, deep, mostly low sinuosity cibannels flowed axial- oriented enlarging dic middle sectors of the Luna system parállel to the ba- sin margin. Sorne other sedimentary features of de Luna system can also be attriibu- ted fo dic dcvelopmcnt of syndepositional folds in de Ebro Basin: 1) The limited basinward deve]opment of dic conglomerate and sandsto- nc and conglomerate litibofacies (up to 6-7 km), in contrast to tibe wibole length of tibe system (about 40 to 60 lun long). More extended proximal scc- tors should be expected from a fluvial system that transported clasts up to 75 cm in diametre. 2> Tibe shaw transition in proximal sectors from braided gravel deposits to amaJgamated, lateral aceretion sandstones deposits of about 5 m tbick. Tliis is common to the north of dic anticlines, where lateral aceretion sands- tone deposits are found 5.5 km south of dic basin margin, as in the case north of Luesia village (Arenas, 1993). These features were diirectly related to the local change in de general alluvial fan slope caused by dic anticline developmcnt. Ncvcrtheless, dic Un- castillo and Fuencalderas anticlines did not constitute a restrictive barrier, as the Luna system net is essentially radial. Within tibis situation, two principal 292 ConchaArenas ami Gonzalo Pardo Late 011gocene-Early Miocene syntectonlc fluvial sedimentation.- - 293 structural re-entrants existed in relation to tibe antielines: the unfolded arcas located i> betwcen tibe two anticline-synclinc palrs, south of Bid village, aud u> east of de Fuenealderas folds, north of Ayerbc. An iimportant portion of the Luna system discharge emerged across dic former structural 10w, feeding an active sector of tibe system that flowcd southward. Trtmk channeis coñid reach arcas far from de proximal sectors, and at maximum progradation pibases dat paft of tibe fluvial system (middle-distal sector> extended to re- gions as far as south of Luna village, as evidenced by the extension of lidio- facies SM (elongation of SM lithofacies outcrops toward tibe soudcast of de Luna system, Fig. 5B>. In dic second structural re-entrant, north of Ayerbc, emergent channcls became unconfined due to a local lowcring in de fan slo- pe. Tibe expanded flows originated an arca dominated by sheet sand deposits. Such is the case of Condiio-Ayerbe region (Arenas, 1993). In contrast, in thc area of Uncastillo village (Fig. 5B), alluvial fan progradation phases did not cause an enlargement of middlc-distal sectors toward thc south, as no struc- tural re-entrants existed diere. Apart from these facts, on the crests of the syndepositional anticlines, and particularly on the Uncastillo anticline, vertical aceretion is recorded and overbank deposits also formed diere. Deep cibannel incision is not an extensi- ve feature of particular zones of this fold. Thus, tibe existence of an anteccdcnt fluvial net cannot be argued to explain dc fluvial growth basinward. More- over, some lateral accretion bodies are found on the flexure arcas of the Un- castillo anticline (Arenas, 1993>. Uncastillo anticline uplift rate scems to ibave been compensated by vertical accretion rate in the adjacent northern syneli- na] region, which received thc greatest part of the diseharge of dic Luna sys- tem. Then, northern flanksof dic anticlines acted as benclí terraces witbin thc general fan siope. In contrast, soutibem anticline flanks had bigiber gradient than de fiat nortbern ones, and greater dcvelopment of sandy bodies with 1k- Lle lateral migration and moderate incision are common features. in odier xvords, general slope across dic Uncastillo anticline area was basinward, with very gentle slope in the northern flank and slightly ibigher slope in the sou- thern flank (Fig. 6). South of the anticlines, middle-distal sectors of tibe system possessed cha- racteristies of a dfflnent nct of mostiy laterálly stablc channeis, as considered by Nichols (1989). Ijnconfined flow deposits are typical features of distal see- tors. Avulsion was a common autocyclic process in both seclnrs, and frequent changes in fluvial arcibitecture Lhrough time have been descri bed. Base level 2 (A) and U22 (B). base of megasequence Fig. 7—Paleogeographic reconstructions for iints U1 J32 (6) and middle pad of inegasequence 113(D) - Yig. 7—Reconstrucciones paleogeugráficas de Las unidades U12 (A) y 112~ (13). base dc la mega- secuencia i..}3~ (6) y parte central de la megasecuencia U3~ (D) - 294 Concha Arenas ami Gonzalo Pardo ebanges in the adjacent central lacustrine systems werc controlled by climatie cycles (Arenas, 1993; Arenas and Pardo, in press>, but deir relationships with de changing fluvial architecture is still under study. Concerning the origin of the Uncastillo and Fuenealderas folds, recentiy Teixelí and García-Sansegundo (1995> ibave proposed dat dic existence of a buried thrust under dic Sierras Exteriores affeeting de Tertiary succession of the Ebro Basin may have originated detached folds witbin de basin. Synde- positional deformation in de studied series in the present contribution sug- gests that sucb folds were active at Ieast duringthe deposition of units U2 and U3 CONCLUSIONS In the study aiea, the Uncastillo Formation resulted mostly from deposi- tion in the terminal, Luna fluvial system. Sinee its existence, Rs evolution fo- ]lowed successive progradation-retrograd ation stages controlled by tecto- nies in the drainage basin (Jaca Basin, Sierras Interiores and Axial Zone>. West of the Sierras Exteriores Aragonesas, dirust sibeets are not present and syndepositional deformation along the basin margin (progrcssivc unconfor- mity of Biel-Gallipienzo> does not explain such progradation-retrograda- tions stages. Progradational and retrogradational stages, and probably lacustrine base level fluctuations too, may have caused fluvial arelttecture variations through time. These variations were described by Nichols (1989) for middle sectors of the fan. Nevertheless, fluvial net distribution did not change essentially drough time, and its development was greafly controlled by Agenian-Ram- blian syndepositional folding witbin the Ebro Basin. Tibe most relevant con- sequences were the deviation of a paft of tibe discharge parallel to tbe basin margin and the formation of liigh sinuosity rivers in proximal sectors of the fluvial system. ACKNOWLEDGEMENTS This contribution ‘vas financed by projects P1389-0342 and P1393-0580 ofthe DGICYT, and supported by a scholarship from ihe Gobiernode Aragón. The reviewers are thanked for their suggestions andcosmnents, which belped to improve the inanuscrspt. Late Oligocene-Early Miocene s-yntectonicfluvial sedimentation. - 295 REFERENCES AlVAREZ SIERRA, M. A., DAAMS, R., LAcOMBA, 1. 1.., LÓPEZMARTÍNEZ, N., VAN DER MEU- LEN, A. 1., SESÉ, C. & DE VisEn, 1. (1990), «Palentology and biostratigraphy (mi- cromammáls) of tibe continental Oligocene-Miocene deposits of the North-Central Ebro Basin (Huesca, Spain)», Scripta Geologica, 94, 1-77. ARENAS, C. (1993>, «Sedimentología y paleogeografía del Terciario del margen pire- naico y sector central de la Cuenca del Ebro (zona aragonesa occidental)», Tesis doctoral (inéd.), Univ. of Zaragoza., 858 Pp. — & PARDO, G. (1994a), «Estratigrafía del margen septentrional aragonés de la Cuen- ca del Ebro (sector Uncastillo-Bolea)», II Congreso del Grupo Español Terciario, Ahstract, 31-34, llaca (Huesca). & PARDO, G. (1994b), «Paleogeografía del intervalo Ageniense superior-Arago- niense inferior (U.TS. U3-Nl) en el sector noroccidental aragonés de la Cuenca del Ebro», II Congreso del Grupo Español Terciario, Simposio (I.G.CP. 324), 231-234, Jaca (Huesca). — & PARDO, G. (in press), «Neogene lacustrine deposits of tibe north-central Ebro Ba- sin, nortbeastern Spain». In E. Gierlowski & K. ICelIs (eds.), Global Geological Re- cord oflake Basins, 2, Cambridge Univ. Press. CALVO, J. p. & 24 co-autbors (1993), «Up-to-date Spanish continental Neogene synthesis and paeloclimatic interpretation», Rey. Soc. Geol. España, 6 (3-4), 29- 40. CoLoxino, E. (1989), «Abanicos aluviales». In Sedimentología 1, A. Arche (coord.), Co- lección Nuevas tendencias, 11, C.S.I.C., 143-218. Díaz MOLINA, M., f3UsTnÁo, A., CAPOTE, R. & LÓI’E-z MARTÍNEZ, N. (1985), «Wet fluvial fans of tibe Loranca Basin (central Spain), channel models and distal bioturbated gypsum with cherb>, Excursion guidebook 6th European Meeting 1. A. 5., Inst. Est. flor., 149-185 (Lérida). FRIENO (1989), «Space andtime análysis of river systems, illustrated by Miocene sys- tems of the Northern Ebro Basin lii Aragón (Spain)», Rey. Soc. Geol. España, 2, 55-64. HIRSí-, 1. P. P. t NIcíIoLs, G. 1. (1986), «Sandstone-body structure and river process in the Ebro Basin of Aragón, Spain». Cuadernos de Geología Ibérica, 10, 9-30. HIRSi-, 4. P. P. (1983), «Oligo-Miocene alluvial systems in the northern Ebro Basin, Huesca Province, Spain», Unpublished Ph. D. Thesis, Univ. of Carnbridge, 247 pr’. — & Ní<:í IOLS, G. 4. (1986>, «Thrust tectonic controls on Miocene aluvial distribution patterns, southern Pyrenees>, $pec. Pubis. mt. Ass. Sediment., 8, 247-258. 1lOGAN, P. 4. (1993), «Geochronologic, tectonie, asid stratigraphic evolution of the Southwest Pyrenean Forelasid Basin, Northem Spain>~, Unpublished Ph. D. The- sis, Univ. of Southern California, 235 pp. Jcpr, P. E., S~uun, B. D., Nícííots, Ql. & HmsT, 4. P. P. (1987), «Statistical Estima- tion of the Apex of a Sediment Distribution System from Paleocurrent Data», Ma- Ihematical Geology, 19 (4), 319-333. M¡wta, II. (1996), «Estructura y cinemática del frente de cabalgamiento surpirenai- co en las Sierras Exteriores Aragonesas», Tesis doctoral (inéd.), Univ. of Zarago- za, 330 pp. 296 Concha Arenas ami Gonzalo Pardo NlcFíoLs, O. J. (1984), «Thrust tectonies and alluvial sedimentation, Aragón, Spain», Unpublished Ph. D. Thesis, Univ. of Cambridgc, 243 Pp. — <1987a>, «Structural controls on fluvial distributary systems —the Luna Systcm, Northern Spain», Soc. Econ. Paleontol. Minera?., Spee. Pubí., 39, 269-277. — (1987b>, «Syntectonic alluvial fan sedimentation, southern Pyrenees», Geological Magazine, 124 (2), 121-133. (1989), «Structural and sedimentological evolution of pail of the west central Spa- nish Pyreneesin the Late Tertiary»,f Geol. Soc. London, 146, 851-857. PUIODEFÁBREGA5, C. (1975), «La sedimentación molásica en la cuenca de Jaca», Piri- fleos, 104 (núm. extr.), 188 Pp. QunouNTrzs, 1 (1978), Estudio sedimentológico y estratigráfico del Terciario continental de Los Monegros, Inst. Femando El Católico, Dip. Prov. Zaragoza, 200 Pp. SOLER, Nl. & PLIODEFÁRECAS, C. (1970>, «Lfneas generales de la geologíadel Alto Ara- gón Occidental», Pirineos, 96, 5-20. TEIXELL, A. & GARcÍA-SANSEOLJNOO, J. (1995), «Estructura del sector central de la Cuenca de Jaca (Pirineos meridionales)», Re-e. Soc. Ceo?. España, 8 (3), 215-228. Manuscrito recibido: 25-4-1996 Manuscrito aceptado: 8-7-1996