Click here to load reader

Colombian Andes-Campbell

  • View
    6

  • Download
    2

Embed Size (px)

DESCRIPTION

Colombian Andes-Campbell

Text of Colombian Andes-Campbell

  • COLOMBIAN ANDES

    COLIN JOHN CAMPBELL Hawksgrove. Hawkswood Lane. Gerrards Cross, Buckinghamshire

    CONTENTS

    A. General data on the segment 705

    B. Subdivision of the segment 714

    C. Data on individual structural zones 715 1. Western Cordillera 715 2. Cauca Basin 716 3. Central Cordillera 717 4. Middle Magdalena subzone 718 5. Upper Magdalena subzone 719 6. Eastern Cordillera subzone 720 7. Garz6n-Quetame Massifsubzone 722 8. Santander Massif subzone 722

    D. References 723

    A. GENERAL DATA ON THE SEGMENT

    1. THE SEGMENT STUDIED

    This article describes the northernmost segment of the Andes marginal to the Pacific, a belt which here gives evidence of orogenic activity throughout most of Phanerozoic time. To the north the three cordillera of Colombia bifurcate, linking both with Central America via the Panama isthmus and with the Venezuelan Andes. Traced southwards into Ecuador the ranges narrow and partly merge.

    Segment: the segment described here has a north-south length of 1000 kin. The margins of the orogenic belt against non-orogenic areas are both narrowly gradational ( to 3 km). The eastern margin is defined by thrusts in most places, but northward splays make it a broader zone in regional terms. Little detailed information is available on the north- west margin, which is taken at the thrust belt, although mild to moderate deformation extends beyond. The width of the orogenic belt thus defined increases from 150 km in the south to 500 km in the north and averages 400 km.

    Zones: the Colombian Andes are described here in terms of eight zones and subzones. The Western Andes (zone 1) are built of metamorphic rocks (?Jurassic) overlain by thick, eugeosynclinal Cretaceous sediments and volcanics. In the Cauca Basin (zone 2) similar rocks are covered by thick, Tertiary continental strata. The Central Andes (zone 3) includes Pre-Cambrian igneous rocks, metamorphosed L. Palaeozoic to Mesozoic rocks, Jurassic-Cretaceous strata and Neogene andesitic volcanics. The M. and U. Magdalena Basins (subzones 4 and 5) con- tain very thick Jurassic and Triassic to Pliocene sequences; major batho- liths were intruded in subzone 5 in early Triassic times. The Eastern Andes include the Eastern Cordillera (subzone 6), the Garz6n-Ouetame Massif (subzone 7) and the Santander Massif (subzone 8). The Eastern Cordillera is built of U. Palaeozoic strata, thick miogeosyn- clinal late Jurassic to Cretaceous strata and continental Tertiary. Subzones 7 and 8 are built dominantly of ?Pre-Cambrian plutonic basement rocks and L. Palaeozoic metamorphic rocks cut by major intrusions; late Palaeozoic and Mesozoic strata occur locally. History: Two or more post Pre-Cambrian orogenies are recognized. Intermittent movements occurred throughout the Phanerozoic and reached climaxes in mid-Palaeozoic, early Triassic, pre-Tithonian, pre-Campanian, pre-U. Eocene, pre-U. Miocene and pre-Pliocene times. The general evolution of the belt involved the progressive westerly migration of sedimentary provinces and tectonic events.

    2. SHAPE OF THE OROGEN IN PLAN

    in The belt continues along strike beyond the selected segment. The Central and Western Andes continue without significant change into Ecuador but the Eastern Andes die out at Moc6a. Northwards the

    at Heriot-Watt University on March 6, 2015http://sp.lyellcollection.org/Downloaded from

  • 706 COLOMBIAN ANDES

    ranges bifurcate and change in direction. 24 General trend: although in gross terms partly arcuate, in fact the Andes consist of relatively straight and structurally uniform segments of up to 1000 km in length separated from one another by complex oroclines. Such oroclines (e.g. at Maracaibo and at Huancabamba--northern Peril) appear related to palaeogeographical gaps in earlier orogenic belts and are in places associated with (?) major wrench faults. Major wrench faults are believed (?) to exert a control on some of the major structural units (e.g. subzone 8).

    I t I I I 79" 77 75 73" ~ ~ 71"

    200km I, I

    SANTA

    Loma de

    -va

    c)

    o

    ....j

    g

    Figure 1. Geological Provinces of Colombia showing the Segment of the Andes under consideration and the structural zones. Structural depressions are stippled.

    3. SURFACE SHAPE OF THE SEGMENT IN ELEVATION

    30 Highest 5% of the ground: 2500 m Average height of: 31 north-west margin of the belt, 100 m; 32 south-east margin, 500 m. 33 Geomorpho- logical surfaces are recognized by summit heights on relict peneplains, but a4 information on lower surfaces--other than in the references listed---is not available. Summit peneplains: little detailed work, see Biirgl (1961, p. 187) and Duran (1964). Reconnaissance surveys indicate an erosion surface based on summit heights at 3000 m near Bogoul and a tilted surface, dipping east at about 8 on the eastern margin of the Central Cordillera, which may be of early Tertiary age since an apparent continuation marks the base of the Tertiary in the adjacent zone 5.

    4. GEOPHYSICAL DATA

    a8-9 Gravity data: Bouguer anomaly map with 20 milligal interval Mapa gravimetrico (1959). See also Case et al. (1969). 41 The segment is in approximate isostatic equilibrium and 42-3 the general gravity field is parallel to the main tectonic and topographic trends.

    45 Regional magnetic data are available and the anomalies are (?) con- cordant with the main tectonic and topographic trends.

    5. PRESENT-DAY ACT IV ITY

    68 The region is currently seismically active (see Ramlrez 1969). Map of epicentres: Ramlrez & DurUm (1957).

    6. T IME RELATIONS

    Pre-Cambrian gneisses are overlain by L. and U. Palaeozoic strata; a late Silurian--early Devonian orogeny can be recognized. Early Triassic movements, which are classified as a separate orogeny ('Hercynian'), are here listed with the long sequence of late Mesozoic- Tertiary movements. See Table 1.

    85 The oldest undeformed rocks belong to the Pliocene Mesa Fro. (Morales et al. 1958; Houten & Travis 1968; Wellman 1970). s3 The youngest deformed rocks are the U. Miocene Real and Honda Fins (references as 85).

    87-9 The initiation of mobility associated with the Mesozoic-Tertiary orogeny: probably at no time during the Phanerozoic has the segment been orogenically dormant. It is difficult to distinguish the last phases of the preceding orogeny, which reached a climax in the L.-M. Triassic, from the first phase (considered to be number 3 in the above list) of the orogeny under consideration here.

    81 The oldest rocks deformed for the first time during the Mesozoic- Tertiary orogeny are: E. Andes-Payand6 Fm (Carn-Nor) (Trumpy 1943).

    at Heriot-Watt University on March 6, 2015http://sp.lyellcollection.org/Downloaded from

  • COLOMBIAN ANDES 707

    Table 1

    MESOZOIC-CENOZOIC PHASES OF MOBIL ITY

    ~1 Phase IV. Andes

    9~ Nature of mobility

    (2. Andes E. Andes

    ~ge

    98 Maximum 94 Minimum g5 Evidence for ages

    15 Uplift

    14 Post- Andean movements

    13 Andean

    12 Proto- Andean

    11 Pre- Tortonian

    I0 Pre-U. Aquitanian

    9 Ere- Miocene 8

    Laramide

    7 Proto- Laramide

    Uplift of Andes

    ?Movement on Santa Marta fault

    Tilting, broad Vulcanism folding, minor faulting

    ?Pleist

    Moderate Onset of folding, fault- vulcanism hag & ?igneous activity

    Post- late Mioc

    Mild deforma- tion involving faulting (L. Magdalena & Sinu-Athintico)

    Local vulcanism

    Tight folding Post- Pre-Plioc & thrusting U. Mioc

    Uplift & erosion

    Mild folding associated with faulting

    Strong, local deformation; ?initiation or accentuation of major wrench faults

    Tilting, broad folding, minor faulting & regional subsidence (especially in the L. Magdalena area)

    Not recognized, ?represented by conglomerates in La Cira Fm

    Post- ?Pre- Torton U. Mioc

    Post- Pre- Burdigal Torton (locally post- Helvet)

    Post-L. Pre-U. Aquit Aquit

    Post-Olig Pre-Mioc

    Igneous activity Mild deforma- Post-M. Pre-U. & ?metamor- tion associated Eoc (locally Eoc phism (also in with faulting within eastern M. Eoc) W. Andes)

    (Sinu area) ?Metamor- Uplift of source folding & phism, not dis- areas faulting finguishable

    from phase 8

    Post- Pre-Eoc & Maestr ?Pre-U.

    Palaeoc

    The wrench fault (if correctly interpreted) shifts the Andean thrust belt & so is late Mioc (i.e. very latest Mioc or Plioc)

    Unconformity beneath unfossiliferous, flat-lying beds (?Plioc) ; note that this phase, in particular, masks the effects of earlier phases

    Unconformity beneath beds considered U. Mioc, although evidence not con- elusive; U. Mioc sediments usually rich in volcanic detritus from ?zone 3

    Minor unconformity beneath fossiliferous Torton, which in many areas rests directly on Burdig (absence of Bulimena carmenensis biozone)

    Unconformity below fossiliferous U. Aquit (C. stainforthi zone in L. Magdalena) in Western Andes

    Absence of Olig in many areas, yet presence of reworked Olig foraminifera in lowest Mioc (Bfirgl 1965)

    W. Andes (Sinu-Atl~mtico) : unconformity below U. Eoc fossiliferous strata; Eastern W. Andes & C. Andes: intrusions dated as 51, 49 & 47 m.y. (U.S.G.S.); also probable age of metamorphism of Cretaceous rocks in zone 3, Santa Marta Massif & Guajira; E. Andes: unconformity becoming smaller to east beneath Mirador & La Paz Fins which are palynologically dated as U. Eoc

    Mild unconformity beneath, or the appearance of elastics in Palaeoc (fossils, including spores)

    at Heriot-Watt University on March 6, 2015http://sp.lyellcollection.org/Downloaded from

  • 708 COLO

Search related