The Upper Permian Boniches Conglomerates Formation: evolution from alluvial fan to fluvial system environments and accompanying tectonic and climatic controls in the southeast Iberian Ranges, central Spain Jose L6pez-G6mez *, Alfredo Arche Instituto de Geologia Economica, CSIC-UCM, Facultad de Geologia, Universidad Complutense, Madrid, Spain 28040 Abstract The Boniches Conglomerates Fm. (Upper Permian) represents the first episode of sedimentation in the southeast border margin of the Iberian Basin, central Spain. The Iberian Basin commenced development as a rift basin running mainly NW-SE during the Early Permian and underwent its first general extensional period during the Late Permian. The Boniches Fm. was deposited as a small segment (less than 100 km long and 9-10 km wide) of the Iberian Basin that extended in a northeast direction during this period. Detailed mapping and palaeocurrent analysis have shown that the Boniches Fm. is of a curvilinear, wedge-shaped morphology. Sediments close to the southwest main faulted border are of a maximum thickness of 230 m. Clear lateral variation in thickness controlled by the activity of secondary transfer faults perpendicular to the main NW-SE axis may be observed. This network of faults permitted the formation of blocks, limited laterally by the transfer faults. These blocks experienced subsidence and rotation against the elevated southwest footwall block which resulted in the isolation of reduced sub-basins with different sedimentary characteristics. Based on facies and facies association, the Boniches Fm. is thought to have evolved vertically from two main alluvial fan systems in the lowest part of the formation, into fluvial braided systems flowing southeast. As expected, no flow orientations towards the footwall block are observed during the first episodes of the refill. This may be explained by erosion, or simply a lack of sedimentation due to the gentle slope. The change from alluvial fan to fluvial environments is interpreted as a relatively sudden tectonic reorganization of the whole segment favouring new longitudinal fluvial drainage from a transfer zone located to the northwest. Three different reorganization episodes, clearly marked in the Boniches Fm. as 'main boundary surfaces' (MBS) separate four members subdivided into levels (3-4 m thick). Each level consists 4-5 of fining-upward successions less than m thick. 1 Te ctonics and climatic factors clearly controlled sedimentation. Tectonic activity was probably of greatest importance and was conditioned by the movement of the main NW-SE and NE-SW transfer faults. The role of climate in ancient conglomerate sediments has not been well established. Palaeogeographical, mineralogical, sedimentological and clast analyses suggest that the Boniches Fm. was deposited under humid conditions by running water. The differentiation between 'proper' alluvial fans and fluvial environments during the transition period of the Boniches Fm. is not clear since, in many recent alluvial fans, braided fluvial systems are associated with fans. fluvial; fans; tectonic control; climatic control; rifts; Permian Keywords: sediments seen in ancient records were in fact de­ 1. Introduction posited as a fan, or deposited as a fluvial system The Boniches Conglomerates Fm. was initially related to that fan. The first studies on alluvial fans described by L6pez-G6mez and Arche (1985) and and their processes date back to the past century was interpreted as mainly consisting of alluvial fan and the beginning of the present (e.g. Drew, 1873; deposits. Its present geographical extension is very McGee, 1897; Knopf, 1918). Recently, this contro­ reduced, cropping out basically at five outcrops re­ versy has been well discussed (Lecce, 1990; Blair lated to anticline cores located in the south of the and McPherson, 1994a,b) and some interpretations present Iberian Range, eastern Spain (Fig. 1). The of classical examples, such as those of Hooke (1967); sediments are of the early Thiiringian (Late Permi­ Bull (1972), Boothroyd and Nummedal (1978), Mi­ an) period, as dated by pollen and spore associa­ all (1981) and Nemec and Steel (1984, 1988) have tion (Doubinger et aI., 1990). The sediments always been questioned. lay unconformably on the Ordovician quartzites and slates or, locally, on the Tabarreiia Breccias Fm., Tectonic and palaeogeographical setting or on Stephanian (Upper Carboniferous) sandstones 2. and coals (L6pez-G6mez and Arche, 1993) (Fig. 2). The Boniches Fm. shows an unconformable base The scope of the present study includes inter­ on the Ordovician-Silurian basement, Stephanian pretation of the sedimentary environments of the sandstones or Autunian breccias. It represents the Boniches Fm. and the effects of tectonics and cli­ beginning of a sedimentary cycle that started dur­ mate during sedimentation. The study is based on ing the first stages of the evolution of the Iberian detailed mapping, sedimentary facies and mineralog­ Basin (Arche and L6pez-G6mez, 1996). At the end ical analyses of data related to the general and local of the Carboniferous period, the Iberian microplate tectonic evolution of the basin in which the Boniches was part of the Hercynian or Variscan Belt, a wide Fm. was deposited. This basin represents only a linear structure affected by intense deformation and segment of the larger intracontinental rift basin, the magmatism from the southern Appalachians to the Iberian Basin, that evolved in the southern border of Bohemian Massif. This structure emerged due to Pangea until invasion by the Tethys sea during the the collision between the following different plates: Middle Triassic period (Sopeiia et aI., 1988; Arche Laurentia, Fennoscandia, Africa and several south­ and L6pez-G6mez, 1996). ern European microplates (Dewey and Burke, 1973; The difficulties encountered during the study in­ Ziegler, 1988). clude the detailed estimation of the age of the sed­ The present-day Iberian Ranges started develop­ iments required to establish sedimentation and sub­ ment as a rift basin during the Early Perrnian and sidence rates. Another difficulty related to climate, experienced the first extensional periods during the consists of the general lack of palaeoclimatological Late Permian (Fig. 3). The present orientation of the indicators present in conglomerates. It is important Iberian Basin probably originated due to the field of to emphasize that alluvial fan deposits in ancient stress created during the opposing movements of two settings have formed the subject of very few investi­ transcurrent fault systems (Pyrenees-Biscay Gulf to gations in comparison to those of more recent origin. the north and Gibraltar-Chedabucto to the south) dur­ Studies on fans developed under humid conditions ing the Permian (Arthaud and Matte, 1977; Arche and are of an even rarer nature. L6pez-G6mez, 1996) (Fig. 4A). This stress created To date there is much controversy as to whether movements along a series of NW -SE normal faults MONTALBAN o .. GUADALAJARA I - PYRENEES 2 . IBERIAN MASSIF 3 - DUERO BASIN o 50Km - IBERIAN RANGES I r 4 5 - EBRO BASIN - TAJO BASIN - _ 6 � PERMIAN AND _ / • PALEOZOICN � 7 - CATALONIAN RANGES � TRIASSIC ROCKS BASEME T - BETIC RANGES / 8 - STUDY - GUADALQUIVIR BASIN 0 S0ME OF THE MAIN FAULT S \- -t AREA- 9 Fig. General location of the study area. I. at the end of the Permian (Fig. 4A), allowing the de Cuenca fault represented the main southwestern development of the rift systems, probably associated border for this southeast part of the Iberian Basin with localized contemporary volcanism and granitiza­ (Fig. 4B), defining the drainage pattern inside thein­ tion episodes in the Iberian Basin (Arche and L6pez­ terior sub-basins. The small basin (less than 100 km G6mez, 1996). Permian basins, associated with old long), in which the Boniches Fm. was deposited, was Hercynian chains, developed in central Europe in a conditioned by the movement of different subsidiary similar manner (Menard and Molnar, 1988). interior blocks and also limited by curvilinear minor The Iberian intracratonic rift basin initially faults (less than 15 long) (Fig. 4B) similar to km evolved into different, subdivided segments or asym­ those described by Ebinger et al. (1984, 1987) and metric sub-basins (Fig. 4) (Arche and L6pez-G6mez, Frostick and Reid (1987). 1996) similar to that described for the recent evo­ lution of the Great Eastern African Rift (Versfeld 3. The sediments: lithology,sedimentary and Rosendahl, 1989). The present study focuses characteristics and their interpretation on the first stages of the evolution and infill of one of these segments (Fig. 4A). Based on detailed The Boniches Fm. currently covers an area 72 mapping and palaeocurrent analysis, a curvilinear long (NW-SE) by 9 wide (NE-SW). It km km wedge-shaped morphology of the segment was ob­ is composed of two main outcrops (Boniches and served. This segment was separated from similar Henarejos) and two smaller ones (Talayuelas and neighbouring basins by transfer areas. The Serrania Chelva) sedimentologically associated with the latter NW SE ( m.y.) ® ® © ® © ® 208 JURASSIC I 2 2I I I 209.5 sd- ? I ; 4 r--../""\ /\ /\ 1\ GYPSI�A�IUM u . __ - -' 223.4 •• H- DOLO MITE Ko 239.5 0:: MARL 241 An t- Se r==l 245 l:=:j MUOSTON E .... - - I ..-:.o� �-_--:-:- � .�:'3.' . �-:-:: . ..', ' z Th · SANDSTONED i . ... I __ . � 256.1 1 _1 J . 2 CONGLOMERATEl:o:c:1 •• �. BRECCIA�. .., �290 -a� - -I- � rJf [1 J]1 TJ]] NOT ID SI r [[] 0:: SEDIMENTATION <[ I U BA SE MEN T NOT RED PRESENTED - TABARREHA,