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

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- TABARREHA, 2- BON/CHES, 3-ALCOTAS, 4-CANIZAR FORMATIONS: 1 5-ES LlDA, MARINES, LANDETE, 9-CANETE 6- 7- 8-MAS,

- KEUPER FACIES, - COAL AND SANDSTONES •• •

A-VALDEMECA, B-BONICHES, C-HENAREJOS, LOCATIONS: D-TALAYUELAS, E-CHELVA, EL MOLlNAR F- Fig. 2. Stratigraphical location of the Boniches Conglomerates Fm. in the general stratigraphical framework of the southern part of the

Iberian Ranges. See Fig. for geographical points. = Stephanian, Au = Autunian, a = 'Saxonian', = Thiiringian, = Scytian, I St S Th Se An = Anisian, = Ladinian, Ka = Karnian, No = Norian, = Rhaetian. La Rh

(Fig. 5). The sedimentological study was based on and locally infilled with Autunian sediments (debris ten detailed logs (Fig.6) plus detailed mapping of flow sediments of the Tabarreiia Fm.) (LOpez-G6mez the whole outcrop area and the major and minor and Archei 1994) (Fig. 7) or Stephanian sandstones faults of the basin. with coal (Fig. 2). The palaeorelief and the style of Sedimentation of the Boniches Fm. was condi­ the synsedimentary tectonics allowed an important tioned by a marked Ordovician-Silurian palaeorelief lateral thickness change in the sediments ranging composed of valleys incised on quartzites and slates, from 230 m in the central part of the basin (Henare- UPPER PERMIAN mainly of quartzite clasts, but also contain a reduced proportion of slates ( occurring in layers close < 1%) to the basement. The matrix is mainly composed of arkoses of medium grain size. The conglomerates are clast-supported, with the exception of the upper­ most parts of the formation. In this region they are supported by the matrix. Quartzite clasts are mainly subrounded although subangular and round-shaped specimens also appear in the lower and upper parts of the formation, re­ spectively. The largest clasts measure up to 37 cm CONTINENTAL towards the upper part of the formation. Mean values 4NS range between 5 and 13 cm in most of the layers (Fig. 9). Only a few angular quartzite blocks from Fig. Tentative reconstruction of the palaeogeographical loca­ 3. the basement exceed 60 cm. Fig. 9 shows the lack tion of the Iberian Basin during the Upper Permian. = Iberian lB of abrupt change in clast size between layers in the Basin, = Hercynian Massif. = Ebro High. White zones HM EH represent elevated areas. evolution of the formation. Fig. lO shows the mineralogical composition of the sandstone which consists mainly of phyllosili­ jos area) to less than 20 m in the northwest and cates, quartz, haematite and feldspars. The clay is southeast ends (Figs. 5 and 6). composed of illite, pyrophylite and kaolinite. The general mineralogical composition shows that there Lithology is a marked change at the top of the lower third 3.1. of the standard log (log A, Fig. 6). This shows a The Boniches Fm. is composed of packets 3-4 m reduction in the quartz and pyrophyllite content and thick, separated by thin sandstone layers less than 0.6 an increase in phyllosilicate and illite levels as far m thick (Fig. 8). The conglomerates are composed as the upper third of the log. Here, kaolinite shows

' 41 l'\:c/ STUDIED SEGMENT

40' �

PLATE ��AFRICAN CANETE 2 - 3 - LANDETE HENAREJOS IBERIAN MICROPLATE DURING 4 - TALAYUELAS 5 - THE PERMIAN - LOWER TRIASSIC CHELVA 6 - Fig. 4. (A) The Iberian Rift Basin of the Iberian microplate during the Permian-Early Triassic. Location of the study segment. (B) Sketch of the present day main NW-SE faults and the transversal NE-SW secondary faults of the Iberian Basin. The segment indicated in (A) corresponds approximately to the studied area in (B). vALDEMECA MEMBERS

-LOGS " SANDY CONGLOMERATES (S C) \ 0 \ 8- ffi b. PALEOCURRENTS - \ "I EB ffi UPPER CONGLOMERATES-2(UC-2) b. , UPPER CONGLOMERATES (UC-1) / C•• T' ) " - I NE ffi @ LOWER CONGLOMERATES (L C)

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Fm. 1°;",,°01o "0 BO�ICHES THURINGIAN � TABARRENA Fm. � AUTUNIAN (? ) 1:'.·:,:',:',<:-:1STEPHANlAN

",t< ~ I;'" • �."a "-\ BASEMENT PALEOZOIC C SERRANIA DE CUENCA S.C.M.F MASTER FAULT 11. �F" Fig. 5. Mapping of the Boniches Conglomerates Fm. showing main and secondary faults. Palaeocurrents (mean) are indicated in the outcrops for the different members into which the Boniches Fm. is subdivided. See Fig. 2 for lithology and location of the Tabarrefia Fm. and Stephanian sediments. See Fig. 6 for the location of logs A (1-4) to More detail about palaeocurrents are included in Fig. 6. J. A B c o E F

CASTILLO TORMOS CERRO EL CHORRERO BARRANCO _DE DEL REY DE ILINoZ CABEZAS DEL LOBO HENAREJOS

C. s. C. C. s. s. 100 '\ 80 - 20 �'._ s. C. " ...... 40 U. C. U. C. ,,--­ C. " - 2 2 C . .... LI. - 2 :::: " - s. 60 C. o -;00:· BO a�"":'· s. c. LI. - 2 20 C. " -' U. C.-I U. -1 40 60 ...... - C. --cc:-- LI. C. I L 20 Tab. Fm. 40 -- u. C. U.C.- 2 (Autunlanl Tab. 0 •• (AutunlanlFm. �.0. -­ -- � 20 o.o: t' Tab. m � F . I L C. (Autunlanl . · -;;>i3VA •• _ U.C.-l • :. <1,

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20 s. c. �� - s.C. . ..." � " s. c. .�o·o •• ...... 60 40 o o�o 0'''0'. -.. s. C. 0> 00° � " -. U.C.- o - U. C. e 2 r'o'" U. C. 2. - 2 ---. � 40 0°00 •c;, " 0_

--- eSt phanion 20 C. o LI. -1 __ C.-I � PRESENT LI. ISOPACHS (metres) MAIN FAULTS � I. 0 I � OUTCROPS - I � I I/"'I

Fig. 6. Logs showing vertical evolution of the palaeocurrent trends and isopachs for the Boniches Fm. Members: = Lower Conglomerates. = Upper L.C U.C-J Conglomerates-I, = Upper Conglomerates-2, = Sandy Conglomerates. See Figs. 1 and 5 for more detailed geographical location. Arrows represent individual U.C-2 S.C palaeocurrent measurements. Fig. 7. Unconformity between the Boniches (Upper Permian) and Tabarreiia (Autunian?) Fms. south of Boniches village (BC) (TB) 2 km (see Fig. 5 for location). Note-book in the centre of the picture measures 18 cm.

Fig. 8. General view of the Boniches Conglomerate Fm. in the Castillo del Rey section (see Fig. 6 fo r location). The fo rmation consists of conglomerate packets m thick) separated by thin (less than 0.6 m thick) sandstone layers. The picture was taken fro m south to (3-4 north. The width of the road is 6.5 m. a clear and progressive decrease and pyrophyllite 4. Depositional environment interpretation disappears. Many of the textural and sedimentary structural Facies description features of the Boniches Fm. described above have 3.2. been previously described for both modem (Bull, Few sedimentary structures have been observed 1964; Ore, 1964; Boothroyd and Ashley, 1975; in the Boniches Fm. Low- and high-angle planar Hein and Walker, 1977) and ancient (McGowen and cross-stratifications are most abundant with a re­ Groat, 1971; Bull, 1972; Larsen and Steel, 1978; duced number of trough cross-stratification, imbri­ Bluck, 1980; Middleton and Trujillo, 1984) allu­ cations and reactivation surfaces. Planar laminations vial fan-braided stream settings. Recently, Blair and and current ripples appear in the intercalated sand­ McPherson (1994a) have described what they con­ stones. sider to be basic characteristics to differentiate allu­ A total of 238 palaeocurrents have been measured vial fan deposits from those of fluvial origin. These (Fig. 6) mainly from imbrications (13%), cross-bed­ criteria are based on sedimentary processes, fac ies ding (61 %), channels and scours (17%) and parting associations, hydraulic processes and morphology. lineation (9%). A clear N45°-65°E direction towards The present study of the sedimentary processes in the base of the formation, evolving sharply upwards the Boniches Fm. is based on the differentiation and to N 80° -900E and then to N 130°-1750S towards interpretation of associations of the facies mentioned the top of the formation (Figs. 5 and 6) may be above. observed. The lithology, sedimentary structures and palaeo­ Facies associations 4.1. currents differentiate four members: Lower (LC), Upper-l (UC-I), Upper-2 (UC-2) and Sandy (SC) Six vertical facies associations were observed in conglomerates from bottom to top respectively the Boniches Fm. (I-VI). These appear vertically (Fig. 11). Each one is separated from the follow­ distributed between the formation members and are ing member by a clearly defined maj or boundary generally incompletely preserved. Fig. 13 summa­ surface (MBS): MBS-l, 2 and 3 from bottom to top rizes these facies associations and their main charac­ respectively. The MBS do not only separate these teristics. members but also indicate an overall, gentle change in the dip of each member. Depositionai environments 4.2. The packets of each member are composed of different thinning-upwards sequences, 24 to 52 cm According to facies and facies association anal­ thick. Coarsening-upwards and normal-reverse to ysis, the Boniches Fm. shows basic sedimentary thinning-upwards sequences are, in general, very characteristics that could be classed as having been scarce (Fig. 9). The lower members sometimes dis­ deposited within alluvial fan and braided fluvial sys­ appear laterally or wedge out due to marked palae­ tems. Alluvial fan systems have been observed in orelief (Fig. 6). However, when present, these show the lower part of the LC member and in the upper the same general sedimentary characteristics to the parts of the LC member deposits. In contrast, braided area under study. fluvial systems are seen in the upper LC and UC-l, The different facies and facies associations are UC-2 and SC members. In view of the change in the defined from the main sedimentary characteristics general sedimentary characteristics observed above and lithologies of each member. The terminology and below the MBS-I level (Fig. 14), the transition used to define and interpret these facies is similar to from the alluvial fan to the fluvial environment is that used by Miall ( 1977, 1978) and Rust (1979). The thought to have occurred in a relatively short period main sedimentary characteristics of the facies in the of time. Boniches Fm., including fabric, sorting, distribution Each MBS (1, 2 and 3) represents general changes and sequence type are summarized in Table and in sedimentary style that include palaeocurrents I Fig. (Figs. 6 and 7), clast fe atures (Fig. 9), petrologic 12. MEAN LARGEST MEAN LARGEST L o L o S 10 15 20 25 S 10 20 E E I I I E SEO 1 1 I I I E SEQ I 1 M V LITHO· M V LITHO· Q E TYPE 0 2 40 Q t TYPE 0 2� L i F I 4�

- 80 4 40 8 2 -. r " " . 1 ..... '- 3 \ " -. \ r \ ...... 4 2 f\ " '-. 19 \ 3 " '-. "'-.. i '\\. " j 7 2 A '" " . <::::� ,. 2 1 ', ./ 18 -.::; . MBS" , ') . 4 , 'l '��- - \ , � -" 35 3 \ 75 - ' � 5 1\ -'" '-- " '" . '-. 4 . 6 2 , � . . I > . . . " -.... 17 3 0 1 \ . / . -...... " - ./ I. A \ ' '" . 1\ \ . 9 ' ...... / 13 ''" ...... 12 I 1\ '\ ' 7 " , . I 6 " .... . " 1\ \ ' . " . i 5 "- ".-.... \ 30- 10 '. 70 "- . !\ \ 16 4 I 9 . . ' . /' 8 ""C" .....- -- ", 2 " 7 7 /' i\ I.... . 1/ / \ ...... i 5 6 \ I I.: V / ./ 3 " . .---- 4 . "i 2 � '-. . " -..... 15 . 3 ::::=:. . , � -- / 2 " ./. - " ...... - 5 '" '-- 25 . 65 . . 4 L.., ...... i 3 \ . ,/ 14 I 2 L::: , � ) \ \ � '. I . ' ! , ..!\ ...... -.. 6 ...... 9 /\ 1. . i \. 5 . � ',0,',0: ...... 8 \ /\ \ . ) 13 3 '\ \ \ I '-. 20- � \ 60- 2 .... -. i 7 ' " '. , . ! 4 6 '\ / c: 5 '\ ';--.. , 4 " -...... j 5 '-. I i 2 " . . ; ...... " 4 !\/\ \'''.. I ..... \ 12 \\ '-"""'" " \ 3 ...... 5 "' 2 f � I 15 - /\ . 55- 4 . i '; " '-.... '\ , I / \ I . 3 . ' "- 3 9 ...... , '-..... \ <:: .� .' 2 '- \ -. A '\" . ' . -- I ' ./ 5 - . 4 '-' i ...... , 11 '- " " '\ , 3 \; . '0 " \" -. . '\ 2 -c:; : ,/ 9 :s ...... " - , - I 10 - 8 " 50 , \ .e5' \. \ // 3 ' 7 . \ � , 2 . 6 /\ " . , 10 " 2 '\ " f :--. i 5 /\ \ , � '. . 4 ' /\ "- >�\ - 4 .,./. f \. ' . ." ' 3 - \ 2/\ ....:. \ /' '\ . / 9 j I , 2 00 >'1 '::;::' /\ "\ "-. ../ ., 5 " 5- ! 5 �'0" -. 4 , ./ "�I \ 9 . \. \ \ 4 ...... 8 /\ '-. \ \ 7 � 3 \ ' \ " . 1 6 \ � '. 2 ') - . 8 5 � i V ' . i 4 '""" ;--. i I j \ , 3 . ! '''. _\ . 0 \ 40 ' SYNTHETIC BULK CLAY MINERALOGY LOG FROM __ __MINERALOGY _ . ----A- '- --- '------'�'-----'-- � ------CASTILLO DEL REY 1- --1 1 2 --I 2 01 . 1 1 1

50 100% 0 50 100% 0 50 100% '-'--'--'--L.....J...... --'---'--'- ....J I. . I I I I , , , I , • I

•

• . . . • • • • • • . . . . . • • . . . . . E . • ... • • .. . • . • • .�'" '" .. .. PHYLLOSLICATES A T HEMATITE :: • D QU R Z § §!Hl KAotMTE �MVOsrONEs � CONGLOMERATES � FELDSPARS W PYROPHYLUTE § n..UTE DSANDSTONES �BRECCIAS

Fig. Bulk and clay mineralogical vertical evolution of the Boniches Fm. including part of the lower and upper units. Synthetic log 10. from Castillo del Rey (see Figs. 5 and 6 for geographical location). and clay mineralogy content (Fig. 10) and sedimen­ per stage plane beds (R0e, 1987) bearing forms that tary structures (Fig. indicate episodes of cut-and-fill in shallow scours 12). A set containing vertical sequences of facies (Smith, 1990). Gt may indicate gradual channel filling. In general, the As previously stated, facies Gms and were Cm absence of planar cross-stratified conglomerates, ac­ mainly included in the LC member and were very companied by sets smaller than 1.5 m, indicates close to one of the main synsedimentary faults that flow was considerably more shallow in facies (Figs. 5 and 15, I). These facies probably defined association in comparison to facies association the fan perimeter and the influence of proximal lIT These are clearly separated by MBS-I (Fig. 13). braided systems generated by the migration of lon­ n. A new increase in energy was detected above the gitudinal bars which indicate a high-energy fluvial MBS-2 since facies association contained sets system (Fig. 16). Palaeocurrents were to the north­ IV with thicker facies in addition to the reappear­ east, perpendicular to the cited main fault. Gp ance of facies (facies association V). Finally, the An increase in deposits, interpreted to repre­ Gm MBS-3 separates facies association VI at the top of sent transverse or linguoid bars, and the increas­ the Boniches Fm., where sets composed of and ing amounts of coarse-grained sand towards the Gp facies are of least thickness with a very gentle top of the Boniches Fm., suggest vertical change Ct angle (sometimes with transition into horizontally from a proximal to a more distal alluvial setting. bedded units). Furthermore, there is a high content UC-I, UC-2 and SC members clearly correspond to sandstone indicated by the St and Sp facies which an abrupt change in palaeocurrents (Fig. IS, Ill). of suggests that deposition is near the transition to up- Palaeocurrents at the base of the LC member could

Fig. Clast vertical evolution in the Boniches Fm. including largest and mean (in cm) for the levels and elementary sequences 9. differentiated in the Castillo del Rey outcrop (see Fig. 6A for location). No represented lithology in the log (in while) indicates conglomerates and dolled levels indicate sandstones. Sequence type is fining-upward when arrow points upward and coarsening-upward when arrow points downward. SE

.. -.a e o [40 m 1,.,,1 ::>; 2o0

40 60 oI 20 I I , _._ ��- . '--- TA8ARRENA Fm Fig. 11. General view and architectural reconstruction of the Boniches Fm. in the Castillo del Rey outcrop (see Fig. 5A for geographical location). Wider lines indicate major bounding surfaces (MBS) that separate members composed by different levels represented by numbers in this figure. Each of these levels is composed of centimetric sequences as shown on the right side of the figure (see this detail also in Fig. 9). The Tabarrefia Fm. appears unconformably below on the left side. Table 1 Summary of the main sedimentary characteristics of the facies observed in the Boniches Fm

Facies General characteristics Characteristics in the Boniches Fm. Interpretation Refs.

Massive or crudely Appears in the LC member. Grading Mass flows. Deposition from Miall (1978). Rust Gms horizontal stratification. almost absent. highly concentrated sediments (1979). Allen (1982). matrix to clast-supported with very small dispersion. Todd (1989). conglomerates.

Massive or crudely Appears in the LC and UC-2 Longitudinal bars and diffuse Steel and Thompson Gm bedded. bimodal to members. Show good vertical gravel sheets. Presence of (1983). Nemec and Steel polimodal clast-supported organization. Normal grading in waning flows and flashy (1984). conglomerate almost every bed. hydrological regime.

Through cross-stratified Although not very abundant it Channel fill. Migration of Rust (1979). DeCelles et GI conglomerate appears all along the Fm. Troughs up transverse bars with curved al. (1991). Miall (1985) to few metres wide. crest lines

Planar cross-stratified Very common in the whole of the Linguoid or transverse bars. Whiting et al. (1988). Gp conglomerate. Fm. Sets can be followed for more Falling stage modification of Alien (1983). Miall than 45 m. Commonly overlain by the downstream margin of (1978). Smith (1990). sandstones longitudinal bars. Rust (1978).

Trough cross-stratified Appears in UC-2 and SC members. Dune migration. Miall (1978). Alien SI sandstone. Constitute cosets up to 80 cm thick (1983). R0e (1987). and cosets up to 45 cm thick. Smith (1990). Southard (1971 ).

Planar-tabular Appears only in the SC member. Linguoid bars and dunes Miall (1978) Miall Sp cross-stratified sandstone. Moderate sorting migration. (1996). Bluck (1971).

Horizontally stratified Individual horizontally stratified sets Planar bed flow in upper Alien (1968). Harms et Sh sandstone. are less than 8 cm thick. Lateral regime al. (1982). R0e (1987). extent is on the order of 8 m.

IJ) o-W I- FEATURES z!!> IJ) w" !!! 0::0:: o:: W ��§o:: u CONGLOMERA TE S SANDSTONES CLASTS SEQUENCE o::� .. ::;» LL u" "'uW::;)Wo:: 0 o::::> w...J � U) z MEMBER .. ::::>w � Mds. Mxs. Morpr Mt. Mth. ...Jo- z...J"

Le, Lower Conglomerates, UC-I: Upper Conglomerates-I, UC-2: Upper Conglomerates-2, SC: Sandy Conglomerates, S - Scarce, F - Frequent, A - Abundant, - Not found, \\\Planar cross. strat, """" Through cross. strat, Reactivation, '2... Massive, ��'Illmbrication, Ripples, Parting lineation, Paleocurrent, Mds- Medium size, Mxs- Maximum size, ODD __ ff; bf.. Clast Morphology (An-Angular, Sa- Subangular, Sr- Subrounded, R- Rounded); Mt- Main Sequence Type,

Finning- upward, \l Coarsening- upward. Coar sening-Finning- upward); Mth- Medium thickness ( 6 � Fig. 12. Main sedimentological features of the conglomerates and sandstones and main features of the clasts. sequences and palaeocurrents of the Boniches Fm. for each of the four differentiated members. For more details of palaeocurrents see also Figs. 5 and 6. MAIN FAcrES ASSOCIA TrONS SOME MAIN PARTIAL GENERAL IN THE BONICHES Fm. FEATURES INTERPRET ATION INTERPRET A TION REFERENCES MEMBERS

Deposition near the Kraus (1984). Miall. m Very low angle cross- transition to upper stage Medium to distal alluvial (1996). � D. SI 5p : lamination. High percentage of plane beds. Cut and -fill in braided system reaches. Roe (1987). � '-, Gp sandstones. shallow scours. Dunes, Smith (1990), o �;,.. D. " ". i,l linguoid-transverse bars and Southard (1971) Mb$- :3 11&1@ sand waves. N ';'1 0 � . ® Flat or irregular erosional Cut banks due to migrating Medium to proximal Miall (1985), 1&1 3m SI 5p surfaces between bar sets. channels. Mid-channel bars alluvial braided system Steel, and Im :: �':"":l Q:� GmD. � (1983), mo� D. Gp General sedimentary structures with cross-stratified bar reaches (evidence of Thompson o -0- features indicating an increase platform deposits caped by reactivation with respect to Bluck (1971). (1984) \�::-o� of energy with respect to Ill. horizontally bedded supra- 1II). Kraus �! D. Gp )i o bar gravels. 8 MBS-2 Planar-crass-stratification. angle Migration of transverse bars Proximal alluvial braided Rust (1978), @)3 .. �SI 5h Ib of dip smaller than those of with curved crest lines. Flow (1985, 1996), GI 11 system reaches to very Miall (1984), 1&10:o:� D. Gp and IV. Planar-cross- depth relatively shallow at distal fan. Kraus stratification is frequently the top of the sequences in Steel, and GI I �� absent at the top of the comparison with Thompson (1983). :::J ..J Gp 11. o /::, sequences. !! �-� MBS-l I 8 4m Sequences of gravels Deposition in channels Distal fan to proximal Rust (1978), 5h SI V) subdivided by erosion surfaces. during normal and wanning alluvial braided system Decelles, and others 1&1 GI I @ D. Planar-tabular cross-stratified now stages. Filling of minor reaches. (1991), Massari � Gp sets up to 2 10cm in thickness. channels that could debouch (1983). Boothroyd, Q: ' 1&1 F oresets dips up to 35 . into pools. Superimposed and Ashley (1975). 2; o longitudinal bar deposits. ..J D.'V(;) I I!> o Gp I Z Textural inmaturity. Grading Transport unable to winnow Low-viscosity debris or Middleton, and I o (.) CD and internal sedimentary fines and sorting clasts. mass-now deposits in distal Trujillo (1984), Bull I structures are almost absent. Acti ve bed load system. areas of alluvial fans (1964.1972), Miall a: m �. 5h 1&1 2 (1970, 1985), D. Gm Diffuse gravel sheets. Sediments forming broad associated with water-laid � �oo sheet-like bodies. High-water deposits of a braided Nemec. and Filipov o d- Gm. ..J 083 stages and waning-now nuvial origin with flow (1989). Nemec, and o conditions. perpendicular to the fans. Postma (1993).

Fig. 13. Sketch showing the main facies associations differentiated for each of the four members of the Boniches Fm. and their interpretation. Symbols of the sequences

taken from Miall (1978). = major boundary surfaces (see also Fig. 11). References: Bluck (197 1). Boothroyd and Ashley (1975). Bull (1964, 1972), Deeelles et al. MBS (1991), Kraus (1984). Massari (1983). Miall (1970, 1985. 1996), Middleton and Trujillo (1984). Nemec and ili ov (1989), Nemec and Postma (1993). R� (1987), Rust F p (1978). Smith (1990), Steel and Thompson (1983). Southard (1971). I

BASEMENT

UC-1 / LC INTERRUPTION . TIME WITHOUT SEDIMENTATION Fig. 14. Hypothetical cross-section of the study basin showing the relationship between longitudinal faults and LC and LC-l members. During the time of interruption of the sedimentation, reorganization in the basin took place favouring the evolution from alluvial fan to fluvial system environments perpendicular to the fans. The hypothetical cross-section represents the area around Barranco del Lobo (see Fig. 5 for location).

still be clearly controlled by the Ordovician base­ vial systems. The present study investigates the char­ ment where the Boniches Fm. pinches out against the acteristics of the LC member by detailed mapping and older palaeovalleys (Figs. 16 and 17). This resembles by structural and sediment dispersion analysis. examples described by Trujillo et al. (1983) in the The sediments of the LC member related to the Upper Proterozoic of Central Arizona. When palaeo­ alluvial fan system, originated in the active fault valleys were overstepped, palaeocurrents spread and front located in the SW (Fig. and These 15, I 11). turned toward the east and southeast (Fig. 15, and were dispersed more or less perpendicularly to that 11 III), coinciding with the transition stage between the margin and showed a clear reduction in energy and distal alluvial fan and the fluvial system. thickness during their flow towards the northeast. Lithological differences between alluvial fan sed­ Eventually they disappeared within a radius of less iments from palaeovalleys and from those outside than 9 (Figs. 15 and 16) in a similar way to km 5, the valley corresponding to the more mature fluvial that described by Anstey (1966) in Pakistan and in sediments, were clearly marked by the pyrophyllite the United States, and Blair (1987) in the Jurassic­ content (Fig. 10). This mineral was found to be abun­ Cretaceous transit of the Chiapas region, Mexico. dant in the basement and therefore served to indicate The sediment dispersion showed two main char­ the distance from the source area. acteristics. Each of the two main outcrop groups (Boniches and Henarejos) showed a particular area Morphological characteristics of origin, although both these areas come from the 5. same scarp-fault of curvilinear disposition (Fig. 5). Many of the morphological characteristics of allu­ The sediment dispersion for both groups also shows vial fans in ancient sediments are very difficultto rec­ how the palaeocurrents disperse in a fan-like man­ ognize. However, these may represent fundamental ner, with a general northeast trend, for the lower criteria in the differentiation of alluvial fans from flu- part of the LC member. Mapping, log distribution IT]

� '\ Os '",-"'I c: - N ( � � '--'- ---

<- --...... � --- [ill � [NJ 0/ \ \�';y tf \ \'x: \

� c: �7 /� � (�y/ .�.

ALPINE COMPRESSION

0102030 PALEOCURRENT EROSION PALEOZOIC . � AND J FAULT PRESENT OUTCROPS ./ LINEAMENT I S':.�, I E N Km tJ � _ BAS ME T

o CONGLOMERATES . . : ... SANDSTONES SERRANlA DE CUENCA LINEAMENT UL . FA T 0 0go S.C.L.F.

A, B, C 1- 2 CANETE, 4-TALAYUELAS, S-CHELVA - SUCCESSIVE FAULTS VALDEMECA, - 3-LANDETE, Fig. 15. Hypothetical evolution, in different stages, of the infill of the basin in relation with the general tectonic evolution of the basin. Secondary NE-SW faults separate the two main alluvial fan systems (Boniches and Henarejos) in stage An elevated transfer zone 11. towards the north-northwest represented the provenance area for the fluvial sediments perpendicular to the fan systems in stage Stage Ill. represents the present situation of the Boniches Fm. outcrops after alpine compression and erosion. IV and joined by an imaginary line between the upper and lower points for both systems (Boniches and Henarejos), the resulting cross-profiles display clear semiconical forms with an irregular base, caused by palaeorelief infill (Fig. 17) and restricted radial length. This is the consequence of the transfer of sed­ iment-charged flows from an upland drainage basin to the fan site through a point source located where FAN the feeder channel intersects the mountain front. - I The curvilinear border described above is com­ plex and was composed of two subparallel lines of _ DISTRIBUTARY CHANNELS faults (Figs. 5, 15 and 16), clearly shown in the mapping and log distributions. The fault synsedi­ "- BRAIDED FL�VIAL SYSTEM mentary activity created blocks linked by the faults that conditioned the thickness of the sediments, con­ NW SE centrating a major accumulation basinward rather CD ------® than towards the border (Fig. 15, A similar pro­ (Aprox) 11). :om cess can be deduced from the repeated back-faulting I of the basin margin as described by Steel and Wilson (1975) in the North Minch Basin. The facies association analysis outlined above would suggest that the change or transit from an alluvial fan environment to a generalized fluvial one was produced in the MBS-l, that is, between the LC and UC-1 members. However, morphology, 0------@ sediment thickness and dispersion, palaeocurrents and mineralogical analysis indicate that this change cl 0 may have occurred in the lower part of the LC , ,

\ member. This point will be discussed later. I , I \ 6. Controls over general alluvial evolution In a field study of small alluvial fans in northwest England, Wells and Harvey (1987) showed that the - - t 2, 3 4 CROSS - SECTIONS , LC - LOWER CONGLOMERATES MEMBER spatial variation of the type of depositiona1 process Fig. 16. Hypothetical representation of the Boniches fan system was controlled by variables other than tectonism or during the Lower Conglomerate (LC) member sedimentation climatic change. Climatic and tectonic factors are with parallel and perpendicular (3-4) cross-sections to the (1-2) the two most important factors causing changes in principal scarp. erosional energy. This was shown by Lecce (1990) with the reintroduction of the dimension of time in and palaeocurrents (Figs. 5 and 6) differentiate two alluvial fan research. Although a steady-state bal­ main alluvial fan systems (Boniches and Henarejos) ance exists between form and process, if erosional (Fig. 5) with associated minor systems. energy changes through time, then form must also If we reconstruct the original position of these change (Hack and Goodlet, 1960). In spite of these systems before the more recent alpine movements controls over non-marine rifts, alluvial fans may be (Fig. 15, I-IV), it is easy to observe the curvilinear important. The details of how this sedimentary en­ fault border that defined both fan head systems on vironment and its facies react to changes in the rate the southwest side. If each log is then located (Fig. 6) and type of tectonic movement and climate are still N /

B TIME LINE �-. PALEOCURRENTS

W BASEMENT 1°.°0\ CONGLOMERATES

CD@LE VELS CJ SANOSTONES

SSE NNW

Fig. 17. Hypothetical longitudinal cross-section of the northern half of the study basin during the sedimentation of the member Le showing the relationship between deposits, palaeorelief and time-lines. poorly understood (Frostick and Steel, 1993; Ritter Semi-arid alluvial fans characteristically have et aI., 1995). abundant debris-flow deposits and few systematic sorting patterns (Bull, 1972). Very few debris-flow Climatic factors deposits (Gms facies) have been observed in the 6. 1. Boniches Fm. despite the fact that the bed-rock The climate of the study area during the depo­ lithology and the morphology of the drainage basin sition of the Boniches Fm. was considered to be suggest that conditions are favourable. On the other generally humid. This theory is supported by the hand, the relative frequency of facies Gp in sets aver­ lack of significant indicators of arid environments in aging m in thickness and their high preservation in 1 the sediments, such as the presence of desert varnish the conglomerate association of the Boniches Fm. is on pebbles (Lustig, 1965), the absence of calcic soils an indication of consistently high discharge in a hu­ and evaporites and the scarce amounts of potassium mid climate. This brings to mind Rust's description feldspar, biotite and ferric oxide are suggestive cri­ of the Devonian Malbaie Conglomerates Fm. (Rust, teria of non-arid conditions (Fig. 10). In contrast, 1984) in Gaspe, Canada. kaolinite is relatively abundant which could repre­ There are no marked changes in the evolution of sent kaolinitic saprolites of the Hercynian basement clast size between consecutive sequences within the in a humid or seasonally humid source area. The same member in the Boniches Fm. (Fig. 9). This internal sedimentary structures of the conglomerates may indicate that catastrophic events related to arid also indicate transport and deposition by running or semi-arid areas without vegetative cover associ­ water along the Boniches Fm. ated with significant reactivations of the fan head affecting clast size, were not common or did not take regions provides a continental supply of fresh debris place. The Iberian Basin was located near the Equa­ from steep drainage basins (McPherson and Hirst, tor during the Permian according to palaeomagnetic 1972; Lecce, 1990). and palaeogeographic data (Ziegler, 1988). Today, Mapping of the Boniches Fm. provides clear a whole range of climates, from tropical rainforests evidence that tectonism controlled sedimentation, to arid deserts are found near the Equator. In the as previously indicated. Palaeocurrents suggest two Boniches Fm., the absence of distinctive criteria of stages with different source areas. The first from hot-arid climates such as mass-flow deposits, de­ the southwest, during the alluvial fan stage, and the flection surfaces, dreikanters and evaporites (Miall, second from the northwest, when the drainage was 1996, chapter 12) and the exclusive presence of run­ established parallel to the main axis of the basin ning-water deposits indicate that it was deposited in (Figs. 14 and 15). Although the change in flow direc­ a humid climate in the equatorial zone. tion started during the Le sedimentation, the MBS- I Lecce (1990) considers that climatic change influ­ was identified with the definitivechange and with the ences the development of alluvial fans by inducing new reorganization of the basin. This general change variability in the magnitude and frequency of fluvial could be related to the appearance of a possible processes which alter alluvial fan features. Lustig transfer zone towards the northwest, where sedimen­ ( 1965 ) suggested that fan aggradation occurs re­ tation disappeared against the outcropping basement gardless of climatic regime. Possibly, the complex in a topographically high area (Fig. 15, Ill). This has response of drainage systems may not always be been described in many recent sedimentation studies related to external influences (Schumm, 1977). How­ (Ebinger et aI., 1987; Ebinger, 1989a,b; Morley et ever, it is clear that sedimentation, at least, may be aI., 1990; Gibbs, 1990; Frostick and Reid, 1990; Fro­ conditioned by vegetation which is closely related to stick and Steel, 1993). This transfer zone separated climate. If climatic changes are produced during a two sub-basins of the Iberian Basin and also limited particular time interval, this could produce sufficient sedimentation in the Boniches Fm. (Figs. 3, 5 and water discharge to initiate fan aggradation and may 15). There was another, almost symmetrical, high be repeated periodically. This is clearly seen under area located towards the southeast that definitely iso­ arid or semi-arid conditions and climatic changes lated the basin under study during this first stage in exert a strong influence on the nature of fan se­ the evolution of the Iberian rift. quences, as Frostick and Reid (1989) reported for The general geometry of the basin was longitudi­ the Pleistocene Dead Sea fans. However, under hu­ nal, approximately 75 long and 20-26 km wide, km mid conditions, as in the case of the Boniches Fm., wedge-shaped with a regionally curvilinear complex climatic factors could have affected the sediments border fault and stratal packages successively onlap­ in general, favouring a constant supply and a better ping either basement rocks or older sediments. This selection of the clasts. was demonstrated by the thickness of the sediments It should taken into account that several diffi­ in relation to the disposition of main fa ults (Figs. 5 be culties may hinder the analysis of climatic control in and 15) and the angular unconformities between this type of sediment. These include the general lack the MBS that involve tectonic tilting during infill of indicators typical to coarse-grained sediments, a (Fig. 18). Similar sizes for tilted sub-basins have lack of knowledge about the timing of ancient fan been described by Rogers and Rosendahl (1989). deposits (Ritter et aI., 1995) and also the possible These MBS could also represent hiatuses in deposi­ overlapping of effects produced by tectonics and tion (as shown in Fig. 14) which may be attributed to those of climatic origin. movements of the faulted margins. The influence of a combined change effect is also possible but difficult to distinguish (Frostick et aI., 1991). 6.2. Te ctonic fa ctors The distribution of sediments also show that Although alluvial fans may form in areas where preservation potential is particularly high in the cen­ tectonic uplift is not an important factor, they are tral part of the border-fault segment in the Henarejos especially prominent where the uplift of mountain area (Fig. 5). This may show similarity to certain �-­ ------

-----

Fig. 18. Castillo del Rey section. Prolongation towards the southwestern border (the main border of the basin during the Boniches Fm. sedimentation that was geographically located very close, as shown in Figs. 5 and 6) of the major boundary surfaces (MBS)and members and The progressive unconformities indicate possible block rotation during the sedimentation. (LC. VC- I. VC-2 SC).

Eastern African Rift examples (Ebinger, 1989a) that hanging-wall blocks, parallel to those comprising the decrease towards the northwest and southeast, that transfer zones (Figs. and 17). These internal faults 5 is, against the transfer zones. The interior part of the controlled the infill of the basin, firstly isolating the basin also shows that there are at least two more two main alluvial fan systems described above and NE-SW faults that cross both the foot-wall and later overlapping both systems in a unique transver- sal fluvial system (Fig. 15, Finally the faults re-elevation of the basinward part of the block, cre­ Ill). caused the widening of the basin area. ating a new slope which dips against the main border During this infill, the first stage corresponds to fault. The unit members of the Boniches Fm. show the LC member sedimentation, during which the divergent internal strata patterns, indicating contin­ Henarejos area probably had a more active faulted ued faulting and rotation accompanying deposition. border with a greater accumulation of fan sediments. During the first stage, when alluvial fans were de­ During the syn-rift stage, high rates of subsidence veloped, rotation may have been more intense and create space faster than it can be filled and coarse led to the deposition of wedge-shaped units, while sediments are trapped and stored nearer to the fault interbedded general tabular units of later stages were scarps, as N!iSuvedt et al. (1995) pointed out in their deposited during periods of general tectonic subsi­ study of the northern North Sea rift basins. The dence and minor rotation. more elevated areas of the interior part of the basin The separation of the effects of extension from corresponded to the southeast extremes. These were those of rotation on sediments is difficult for many probably never covered by fans, and only crossed reasons. These include the fact that both rates can by late transversal fluvial systems at the time when vary during the syn-rift stage or due to the overlap­ the rate of sediment supply resulted in over-fill­ ping of these effects. However, in the Boniches Fm., ing of the deeper parts of the basin. This caused the decrease in rotation in favour of extension, could coarse clastic sediments to spill over into neighbour­ represent the beginning of the transition from the ing parts when the transverse drainage system was alluvial fan to the axial fluvial systems as the latter better established. This situation appeared during the cover almost the whole of the study basin homo­ sedimentation of the UC-l, just above the MBS-l, geneously, while sedimentation of the LC member when most of the palaeorelief of the central part of does not permit the progradation of fans more than the basin was probably over-stepped (Figs. 11 and 2-3 basinward (Figs. 5 and 15, II-III). km 15). This created a new situation which is also well In spite of petrological and sedimentological sim­ marked by palaeocurrent data and a reduction in the ilarities between the LC member and the UC-I and content of unstable pyrophillite above the MBS-l UC-2 members, we interpret the first one as alluvial (Fig. 10) which indicates that the provenance of sed­ fan deposits because of its palaeocurrents trending iments could be more distant source areas already normal to the Basin Boundary Fault and the presence linked to the longitudinal fluvial system. of the slate pebbles derived from a nearby source Mapping clearly shows the curvilinear main fault area and the latter two as fluvial deposits by the border of the segment under study (Figs. 5 and 6). sudden change in palaeocurrents indicating longi­ This may suggest a listric development in depth and tudinal transport and the absence of slate pebbles. concave basin ward morphology (Frostick and Reid, The alluvial fan and fluvial deposits are separated 1987). The vertical movement of the hanging-wall by a regional erosive surface (MBS-l), indicating individual blocks of this border fault could include a widening of the basin and a change of regional rotation with simultaneous subsidence. Rotation is slope after a period of tectonic activity. There is no evident in the parts of the block closest to the main interbedding or transition zone among the two types border fault of the basin as in the Boniches area. of sediments. This idea is supported by the progressive unconfor­ Sedimentation of the LC member was different in mities discussed previously and the fact that it is the two main alluvial fan systems described above. possible to observe how the Tabarrefia Fm. appears In the Henarejos sub-basin area (Figs. 5 and 6), sed­ unconformably below the Boniches Fm. with a clear imentation was faster than in the remaining areas. wedge-shaped distribution (Figs. 7 and 16). This resulted in an almost triple rate of sedimenta­ Block rotation during crusta1 extension is not un­ tion in comparison to that of the Boniches system. common in the Triassic syn-rift European basins, Although, after LC member sedimentation, sediment before a general lateral progradation of the sedi­ expansion was possible, it was always in an en­ ments to the whole width of the basin (Frostick and dorheic basin comparable to the examples described Steel, 1993; NjiSttvedt et al., 1995). Rotation includes by Frostick and Reid (1987). During extension, the Fig. 19. Roots in the uppermost part of the Tabarreiia Fm. (unconformably below the Boniches Fm.) that could indicate the angle of the slope in which the Boniches Fm was deposited (see text) . The largest root is 95 cm long. The compass in the circle is 12 cm long. basin grew both wider and longer as the basin­ from the northeast border were initially deposited, bounding faults lengthened and displacement accu­ but rapidly eroded due to the upward movement of mulated. The change in the rate of increase in basin the basinward part of the block during rotation. volume is positive (Schlische, 1991). During the first stage of sedimentation of the In spite of the fact that during the first stages of Boniches Fm., the slope could be approximately sedimentation of the Boniches Fm., a plane dipping estimated. Some root levels were observed just at towards the main SW-faulted border of the basin the top of the Tabarrefia Fm., located unconformably was created by combination of rotation and exten­ below the Boniches Fm. (Figs. 2 and 13). These sion, palaeocurrents indicate northeastern rather than roots grew oblique to the Tabarrefia Fm. stratification southwestern flow. The latter might be expected if and just prior to the Boniches Fm. sedimentation one considers African models (Frostick and Reid, (Fig. 19) since the conglomerates cut them. Thus, 1987). Only the first strata in the Castillo del Rey a perpendicular plane to the roots would indicate log, indicated south-southwestern flow (Fig. 6A), but the original surface in which the alluvial fans were since it is a unique and isolated level, this may not deposited and since the dip of the conglomerate be indicative. There are several possible reasons for levels is known, the original slope may be estimated the lack of palaeocurrents indicating southwestern as being 3 degrees. This is considered low with transport at this stage. A simple one could be that respect to recent alluvial fan examples (Blair and these sediments indicating southwestern flow were McPherson, 1994a,b) and would probably not be not deposited, due to a possible very gentle slope, enough to transport clasts larger than 10 cm (Costa, or deposited in a very reduced proportion without 1983) (Fig. 9). reaching the southwestern border. In the latter situa­ The new high, created by the relative movement tion, vegetation covering the slope could also retain (elevation-subsidence) of the footwall block, was or disperse the movement of clasts (Hughton, 1989). probably more important for sediment records in Another argument could be that these sediments the first stage of sedimentation. This created space faster than it could be infilled and trapped coarse respect to the type of main border fault movement, sediments stored nearer to the main southwest fault i.e. the relationship between the footwall and hang­ scarps. As a consequence of the rotation, the change ing-wail block movements. There were at least two in the rate of increase of the volume of the basin was main faults related to the footwall that conditioned probably negative during uniform extension (Schlis­ sedimentation during the first stages of the sedimen­ che, 1991). In a later extension of the basin due tation of the Boniches Fm. Fig. 9 shows that the to fault growth, alluvial sediments progressively levels that constitute each member (Le, UC- l, UC-2 covered pre-rift rocks of the hanging-wall block and SC) were also composed of minor sequences when their basinward extension was restricted, as less than 70 cm thick, of a mainly fining-upwards described by Blair and Bilodeau (1988). In contrast, tendency. There was no clear vertical progressive the most highly progradational levels within clastic change in mean grain size in the vertical evolution wedges correspond to times of minimum tectonic of the formation. Only level 17 showed an increase activity along the basin margins (Frostick and Steel, in the mean of the sequences and largest pebble 1993). This could correspond to the upper part of size, coinciding with sheet-flood deposits, as previ­ the LC member in the Boniches Fm., probably also ously suggested. Two arguments could justify this coinciding with the overlapping of the fluvial system regular evolution: a lack of strong movements in the drainage over the fans. A rift model in which axial source area and/or abundant and permanent rain with drainage dominates and drainage across the ramp is intense vegetative cover. subsidiary, was described by Leeder and Gawthorpe The general decrease of bed thickness from base (1987) in ancient sediments and by Hunt and Mabey to top (with exception of levels 7, 15 and 17), the (1966) and Hooke (1972) in recent alluvial fan sys­ homogeneous petrology of clasts (more than 99% tems. quartzite with only some slate pebbles close to the Sediment thickness (Fig. 6) and estimated reduced basement, in level 1), a vertical evolution of facies rate of subsidence for this stage and basin (Arche associations indicating an evolving fluvial environ­ and L6pez-G6mez, 1996) could indicate a general ment and the fining-upward tendency of most of the decrease in the infillof the basin probably also linked minor observed sequences, all point to a reduction of to reduced border fault movements in comparison to the source area relief and/or scarp retreat (Heward, other classical Permo-Triassic rift basins (Frostick 1978). This retreat or formation a second floor of et aI., 1988, 1991; Glennie, 1990; N0ttvedt et aI., fault, cutting back towards the footwall as exten­ 1995). sion proceeds, could be a result of the unloading of Based on a compilation of data from publica­ the footwall, probably largely gravity-driven (Gibbs, tions including fan area, fan slope and drainage 1984). The particularly thicker beds, largest pebble area, Blair and McPherson (l994b) described certain size increase and facies of level 17 (Fig. 9) Gm morphological characteristics in active fan systems. could indicate faulting with a history of progres­ Following these parameters it may be estimated that sively greater vertical movements (Steel and Wilson, if the Boniches fan system covered about 6-9 km2 1975; Gloppen and Steel, 1981) in a reduced period as deduced from mapping (Fig. 5), and if the esti­ of time. mated slope was less than 3 degrees as quoted above, the drainage area would be about 7-9 As the km2 • 7. Henarej os fan system was larger than the Boniches Discussion andconclusions system, the total estimated drainage area could be The stratigraphical and sedimentological analysis about 20-25 This dimension fits well with the of the Boniches Fm. allow the interpretation of its km2 . ones compiled by Heward (1978) for recent humid sediments as deposits that evolved from two main fa ns. alluvial fan transverse systems progressively to a Differentiation between minor and major se­ longitudinal braided fluvial system environment. quences and the general clast evolution of the whole Sedimentation of a reduced segment of the Iberian of the Boniches Fm. (Fig. 9) gives insight into the Rift Basin occurred at the end of the Permian in an development of major fans and fluvial systems with asymmetrical half graben where extension and/or ro- tation configured different sedimentary geometries ifications (i.e. development of braided distributary during the first stages of refill of the basin. The channels on top of sheet flood deposits) have elimi­ progressive evolution from fan sedimentation to the nated almost all evidence that sheetflooding was the axial fluvial braided system was related to the dif­ dominant depositional process responsible for fan ferent stages and styles of the basin, where fault­ aggradation. This was reported by Blair (1987) for controlled subsidence dominated basin geometry and the Roaring River Fan. It is not uncommon to find facies distribution. comments in the literature such as "fluvial deposi­ Climatic factors also controlled sedimentation, tion dominates below the intersection point" (Hooke, favouring a constant supply and better sorting of 1967), taking into account that Bull (1963, 1968) the clast. However, it is difficult to determine its considered that the alluvial fan radiates downslope effects on the sediments, especially since these were precisely from that particular point. Similarly, Hoppe juxtaposed with those of tectonic origin and also and Ekman (1964) pointed out that "braided streams due to the lack of palaeoclimate indicators found in are known to function on alluvial fan surfaces" coarse-grained sediments. and later on, Bluck (1967) added that "this braided If an alluvial fan consists of stream deposits with a stream can be seen to grade upstream into alluvial surface which forms a segment of a cone that radiates fan deposits". More recently, Selby (1994) stated downslope from the point where the stream emerges that "alluvial fan processes are increasingly fluvial from the mountain area (Bull, 1963, 1968), it is easy in nature as the depth and regularity of flow increase to understand that differentiationof this environment away from source". from one that is strictly fluvial, is very difficult in Blair and McPherson (1994a) have proposed a ancient sediments. This is particularly true if the distinction between alluvial fans and rivers based on sediments were deposited under humid conditions. morphology, hydraulic processes, sedimentary pro­ The present study discusses the evolution from al­ cesses and facies assemblages. Our study of the luvial fan deposits to braided fluvial systems. From Boniches Fm. allows us to say that there is no real the mineralogical, morphological, palaeocurrent and distinction between the sediments of humid allu­ tectonic points of view, the transition basically co­ vial fans and braided river systems, a point of view incided with the MBS-l, affecting the fan stage of sustained by authors previously mentioned, because practically the whole LC member. However, from there is not an intrinsic, exclusive process to one or the sedimentological (facies analysis) point of view other depositional system that causes characteristic it is possible to locate the alluvial fan stage as being sediments; this is one of the main conclusions of this restricted to the lower part of the LC member only. work. Although several recent studies (see review in Blair We consider that the only valid differentiation and McPherson, 1994b) suggest that alluvial fans are criteria among those cited by the previous authors in a naturally unique phenomenon readily distinguish­ ancient records are: (1) morphology, very difficult to able from other sedimentary environments including reconstruct; (2) palaeocurrents dispersion, large for gravel-bed rivers, we consider that some difficulties alluvial fans and small for braided fluvial systems; are still unresolved. and (3) orientation of the palaeocurrents with respect Miall (1977, 1996) pointed out that there is noth­ to the axis of the basin, transverse for most of the ing unique about the depositional processes of al­ alluvial fans and longitudinal for the braided river luvial fans; only the geometry of the fan and its systems, because of the different nature of the basin deposits are distinctive. In this sense, it is easy to boundary faults on one or the other depositional understand how in the literature many interpretations system. of fluvial or alluvial fan environments became jux­ We feel that in ancient sediments in which there taposed (Bluck, 1967; Hooke, 1967; Harvey, 1989; is an upward evolution from alluvial fan deposits Postma, 1990 and Miall, 1992). This is true es­ to braided fluvial deposits, palaeocurrents could in­ pecially when braided-stream processes overlap al­ dicate when a fluvial system perpendicular to the luvial fan deposits in ancient sediments maintaining fan is clearly established. This was the case of the the same palaeocurrent trend or when postflood mod- UC-l member with respect to the LC member in the Boniches Fm. However, dispute over the fluvial response to episodic tectonism. Geology 16, 517-520. Blair, McPherson, J.G., 1994a. Alluvial fans and their nat­ or fan nature of deposits continues to exist when in T, fact the two systems were most likely superimposed ural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. in the same current trend, as seen in the upper part Sediment. Petrol. 3, 450-489. of the LC member of the formation. This is true Blair, McPherson, J.G., 994b. Alluvial fan processes and T, I especially if we consider that downslope from the forms. In: Abrahams, A.D., Parsons. AJ. (Eds.), Geomorphol­ intersection point, the development of the proper fan ogy of Desert Environments. Chapman and Hall, London, pp. (Bull, 1968) may coincide with the local aggradation 354-402. Bluck, 1967. Deposition of some Upper Old Red Sandstone and loss in competence in a lateral flow expansion. 8.1., conglomerates in the Clyde area: A study in the significance This represents the direct physical sedimentary cause of bedding. Scott. J. Geol. 3, 139-167. of primary braiding (Ashmore, 1991). Bluck. BJ., 1971. Sedimentation in the meandering River En­ drick. Scot!. J. Geol. 7. 93-138. Bluck. BJ., 1980. Structure, generation and preservation of up­ ward finning braided stream cycles in the Old Red Sandstone Acknowledgements of Scotland. R. Soc. Edinburgh Trans. 71, 29-44. Boothroyd, J.c., Ashley, G.A .. 1975. Processes. bar morphology, We thank Modesto Escudero for printing the fig­ and sedimentary structures on braided outwash fans, northeast ures. This paper is a contribution to Projects PB92- Gulf of Alaska. in: Jopling, A.V., McDonald. B.C. (Eds.). 0041 and PB 95-0084 financed by the DGICYT, Glaciofluvial and Glaciolacustrine Sedimentation. Soc. Econ. Paleontol. Mineral. Spec. Publ. 23, 192-222. Ministry of Education Research, Spain. We sincerely Boothroyd. J.c., Nummedal, D .• 1978. Proglacial braided out­ thank A.D. Miall and the reviewers L. Frostick and wash: a model for humid alluvial fan deposits. In: Miall. 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