EVOLUTION OF BUNTSANDSTEIN FLUVIAL SEDIMENTA TION IN...
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EVOLUTION OF BUNTSANDSTEIN FLUVIAL SEDIMENTA TION IN THE NORTHWEST IBERIAN RANGES (CENTRAL SPAIN)I,2 AMPARO RAMOS, ALFONSO SOPEÑA, AND MARTA PEREZ-ARLUCEA Departamento de Estratigrafía Instituto de Geología Económica U.CM. - CS.I.C. Facultad de Ciencias Geológicas Universidad Complutense 28040 Madrid, Spain ABSTRAer: The Buntsandstein sediments in central Spain (Permian and Triassic) are mainly continental redbeds that form the base of a sequence that passes up into siliciclastic and carbonate tidal sediments, the Muschelkalk facies. Buntsandstein sedimentation began with conglomeratic and sandy Iluvial facies whose detailed sedimentological analysis is the main subject ofthis work. Massive sheets are the most important conglomerate facies; they were formed by deposition as longitudinal bars. Channel-fill conglomerates are the second most important facies. Lateral accretion deposits also occur in the conglomerates; they evolved from longitudinal bars because of decreasing flow or because the bars had reached a certain height. Sandstones are composed of eleven different facies. Large Iinguoid or transverse bars were the main bedforms in this environment. These bedforms may be related downstream to large, oblique troughs developed when the tabular bedform grew to a certain size or when a tabular bedform reached another one that acted as an obstacle. Compound and composite compound bars consisting of large sets of trough cross-bedding with scoured, wavy bases are also recognized in the sandstone facies. Two major cycles occur in the complete conglomerate sequence which correspond to different physiographic models. The lower cycle was mainly formed by channels and bars tha! were smaller than those in the upper cycIe. The upper cycle is more extensive than the lower one and has characteristics that indicate a higher stability. Paleocurrents for both cycles have a north-northeasterly trend. The vertical evolution oC these sediments is clearly related to the different fault systems that controlled their distribution. The paleogeography of the sandstone facies, together with paleocurrents (perpendicular to the conglomerate paleoccurents) indicate deposition from a separate dispersal system that extensively overlies the previous one. This unit, which can be followed extensively over most of the lberian Ranges and even outside the lberian Ranges, represents the growth of a ftuvial braidplain over a large area of Iberia during the Lower Triassic. INTRODUCTION Sedimentation ofthe Alpine cyc1e in central Spain (Fig. 1) began with the Buntsandstein facies. The facies consists of redbeds that rest unconformably on a basement of Hercynian metamorphic or Early Permian sedimentary rocks and varíes in thickness from 70 m to more than 1,000 m (Fig. 2). This varíation was produced by relief formed at the beginning of Buntsandstein sedimentation as a result of tectonic movement in the basement. The Buntsandstein redbeds form the base of a sequence that passes up into silicic1astic and carbonate tidal sedi- ments (Fig. 2), the Muschelkalk facies. The carbonate sediments mark the western extremity ofthe Tethys Sea, and they decrease in thickness westward with a corre- sponding increase in siliciclastic beds towards the western margin of the basin. The Keuper facies, consisting of mudstone and evaporítes, extensively overlie the Mu- schelkalk. Buntsandstein sedimentation began with conglomer- atic and sandy fluvial facies (Fig. 2). The Hoz del Gallo Conglomerate consists of quartzitic conglomerates (Fig. 2, HGC), with very little sandy matríx and sorne thin sandstone beds. Thickness is quite irregular with up to 150 m in the Arandilla section (Fig. 2). The overlying Rillo de Gallo Sandstone (Fig. 2, RGS) covers not only most of this area but also most of the lberían Range. It is comprísed of medium -grained red sandstone with sorne I Manuscript received 20 lune 1985; revised 14 April 1986. 2 This work has been financed by the C.A.LC.Y.T. and C.S.I.c. (Proj- ect No. 452) and is incIuded in the projects No. 4 and 106 ofthe l.G.c.P. (UNESCO). conglomerate beds. The Riba de Santiuste Upper Con- glomerate (Fig. 2, RSUC) consists of purple quartzitic conglomerates and is a local unit related to the active border of the basin to the west. The detailed sedimen- tological analysis ofthese three units is the main focus of this work. The overlying Prados bed (Fig. 2, PB), consists ofthin- bedded, micaceous sandstones and mudstones. Pedoge- netic carbonatges and trace fossils occur commonly in this unit. The Rio Arandilla Sandstone (Fig. 2, RAS) overlies the previous one. It consists of red sandstones with sorne thin-bedded mudstones. The Prados bed has been deposited by a system of greater sinuosity than the RiBo de Gallo Sandstone. The Rio Arandilla Sandstone represents low-sinuosity streams that have been reacti- vated by tectonic movements. The Upper Buntsandstein (Rillo Mudstone and Sand- stone, Fig. 2, RMS) consists oftrough cross-bedded chan- nel sandstones with well-preserved, fine-grained over- bank deposits containing sorne well-developed "caliche" beds. Bioturbation is common. These sediments were probably deposited in an alluvial system that was more stable and had streams ofhigher sinuosity (Ramos 1979) than the Lower Buntsandstein. There is evidence that the upper part of this unit evolved from a distal alluvial en- vironment that passed into a tidal area. This tidal influ- ence marks the beginning ofthe Tethys transgression over this area of western Europe. The Cercadillo Sandstone and Mudstone (Fig. 2, CSM) consists of sandstone and mudstone with sorne conglom- erate beds. They are the lateral equivalent to the RMS unit (Fig. 2) and were deposited as a braided alluvial system in the westem border of the basin. JOURNAL OF SEDIMENTAllY PEntOLOGY, VOL. 56, No. 6, NOVEMBER, 1986, P. 862-875 Copyright © 1986, The Society of Economic Paleontologists and Mineralogi.ts 0022-4472/86/0056-8621$03.00
Transcript of EVOLUTION OF BUNTSANDSTEIN FLUVIAL SEDIMENTA TION IN...
EVOLUTION OF BUNTSANDSTEIN FLUVIAL SEDIMENTA TION IN THE NORTHWEST IBERIAN RANGES (CENTRAL SPAIN)I,2
AMPARO RAMOS, ALFONSO SOPEÑA, AND MARTA PEREZ-ARLUCEA Departamento de Estratigrafía
Instituto de Geología Económica U.CM. - CS.I.C.
Facultad de Ciencias Geológicas Universidad Complutense
28040 Madrid, Spain
ABSTRAer: The Buntsandstein sediments in central Spain (Permian and Triassic) are mainly continental redbeds that form the base of a sequence that passes up into siliciclastic and carbonate tidal sediments, the Muschelkalk facies. Buntsandstein sedimentation began with conglomeratic and sandy Iluvial facies whose detailed sedimentological analysis is the main subject ofthis work.
Massive sheets are the most important conglomerate facies; they were formed by deposition as longitudinal bars. Channel-fill conglomerates are the second most important facies. Lateral accretion deposits also occur in the conglomerates; they evolved from longitudinal bars because of decreasing flow or because the bars had reached a certain height. Sandstones are composed of eleven different facies. Large Iinguoid or transverse bars were the main bedforms in this environment. These bedforms may be related downstream to large, oblique troughs developed when the tabular bedform grew to a certain size or when a tabular bedform reached another one that acted as an obstacle. Compound and composite compound bars consisting of large sets of trough cross-bedding with scoured, wavy bases are also recognized in the sandstone facies.
Two major cycles occur in the complete conglomerate sequence which correspond to different physiographic models. The lower cycle was mainly formed by channels and bars tha! were smaller than those in the upper cycIe. The upper cycle is more extensive than the lower one and has characteristics that indicate a higher stability. Paleocurrents for both cycles have a north-northeasterly trend. The vertical evolution oC these sediments is clearly related to the different fault systems that controlled their distribution.
The paleogeography of the sandstone facies, together with paleocurrents (perpendicular to the conglomerate paleoccurents) indicate deposition from a separate dispersal system that extensively overlies the previous one. This unit, which can be followed extensively over most of the lberian Ranges and even outside the lberian Ranges, represents the growth of a ftuvial braidplain over a large area of Iberia during the Lower Triassic.
INTRODUCTION
Sedimentation ofthe Alpine cyc1e in central Spain (Fig. 1) began with the Buntsandstein facies. The facies consists of redbeds that rest unconformably on a basement of Hercynian metamorphic or Early Permian sedimentary rocks and varíes in thickness from 70 m to more than 1,000 m (Fig. 2). This varíation was produced by relief formed at the beginning of Buntsandstein sedimentation as a result of tectonic movement in the basement.
The Buntsandstein redbeds form the base of a sequence that passes up into silicic1astic and carbonate tidal sediments (Fig. 2), the Muschelkalk facies. The carbonate sediments mark the western extremity ofthe Tethys Sea, and they decrease in thickness westward with a corresponding increase in siliciclastic beds towards the western margin of the basin. The Keuper facies, consisting of mudstone and evaporítes, extensively overlie the Muschelkalk.
Buntsandstein sedimentation began with conglomeratic and sandy fluvial facies (Fig. 2). The Hoz del Gallo Conglomerate consists of quartzitic conglomerates (Fig. 2, HGC), with very little sandy matríx and sorne thin sandstone beds. Thickness is quite irregular with up to 150 m in the Arandilla section (Fig. 2). The overlying Rillo de Gallo Sandstone (Fig. 2, RGS) covers not only most of this area but also most of the lberían Range. It is comprísed of medium -grained red sandstone with sorne
I Manuscript received 20 lune 1985; revised 14 April 1986. 2 This work has been financed by the C.A.LC.Y.T. and C.S.I.c. (Proj
ect No. 452) and is incIuded in the projects No. 4 and 106 ofthe l.G.c.P. (UNESCO).
conglomerate beds. The Riba de Santiuste Upper Conglomerate (Fig. 2, RSUC) consists of purple quartzitic conglomerates and is a local unit related to the active border of the basin to the west. The detailed sedimentological analysis ofthese three units is the main focus of this work.
The overlying Prados bed (Fig. 2, PB), consists ofthinbedded, micaceous sandstones and mudstones. Pedogenetic carbonatges and trace fossils occur commonly in this unit. The Rio Arandilla Sandstone (Fig. 2, RAS) overlies the previous one. It consists of red sandstones with sorne thin-bedded mudstones. The Prados bed has been deposited by a system of greater sinuosity than the RiBo de Gallo Sandstone. The Rio Arandilla Sandstone represents low-sinuosity streams that have been reactivated by tectonic movements.
The Upper Buntsandstein (Rillo Mudstone and Sandstone, Fig. 2, RMS) consists oftrough cross-bedded channel sandstones with well-preserved, fine-grained overbank deposits containing sorne well-developed "caliche" beds. Bioturbation is common. These sediments were probably deposited in an alluvial system that was more stable and had streams ofhigher sinuosity (Ramos 1979) than the Lower Buntsandstein. There is evidence that the upper part of this unit evolved from a distal alluvial environment that passed into a tidal area. This tidal influence marks the beginning ofthe Tethys transgression over this area of western Europe.
The Cercadillo Sandstone and Mudstone (Fig. 2, CSM) consists of sandstone and mudstone with sorne conglomerate beds. They are the lateral equivalent to the RMS unit (Fig. 2) and were deposited as a braided alluvial system in the westem border of the basin.
JOURNAL OF SEDIMENTAllY PEntOLOGY, VOL. 56, No. 6, NOVEMBER, 1986, P. 862-875 Copyright © 1986, The Society of Economic Paleontologists and Mineralogi.ts 0022-4472/86/0056-8621$03.00
