van_houten.pdf

7/17/2019 Van_Houten.pdf

1/15

I

I

GeoloSical

S^.:.

/

of Am.rica

Bull.rin,

v.

87,

p.

481-49J,

12 6gs.,

April 7976,Do..

no. 6MO7.

481

Late

Cenozoic volcaniclastic

deposits,

Andean

foredeep,

Colombia

FRANKLYN

B. VAN

HOUTEN

Department

of Geological

and Geopblsical Scien.es, Princeton LJ

iuersiry,

Pti uton, Neu)

lersey

A851A

ABSTRACT

Volcaniclastic

deposits

preserved

in

the upper

Magdalena Valley,

Colombia, were derived from

stratovolcanoes

on Cordillera

Cen-

rral during

and after the lare

Cenozoic

Andean

orogeny. Andesitic

lava clasts

and suites of mineral grains,

commonly mixed with

plutonic

and metamorphic

rock fragments, predominate

in

proxi-

mal

debris flows and in

torrential channel

lenses;

reworked

dacitic

pyroclastic

material

is more

common

in distal debris

flows,

sheeted

overbank

sands,

and

flood-plain

muds.

Earliesr

Colombian

volcanism

is

recorded bv

montmorillonite

in

rhe mid-Cenozorc

Ia

Cira Formation

(1,00b

m).

During

early

phases

of Andean orogeny in

middle and late

Miocene

time

(20

to

10

m.y.

ago),

abundant volcanic

clasts

and volcanic

mineral

grains

accumulated

in

coarse channel

and flood-plain

sediments

of

the

Honda

Group

(3,000

m), along with

a

few

thin sheers

of

reworked

pyroclastic

debris,

The

overlying

conglomeratic Neiva'Formation

(200

m)

records renewed

uplift and

a fresh

supply of lava.

In latest

Miocene

time

(8

to 9 m.y. ago)

the climax

of

explosive activiw

sr.r,amped

rhe Andean foredeep

with debris

tha

accumulared

on

coalescing fans

of

the Giganre

Formation

(750

m).

About 7

to 5

m.y. ago, the 6nal

Andean orogenic phase

de-

fonned

both

Cordillera

Central and its

foredeep.

Subsequent exca-

vation

of the

uplifted

eastem

lowland

was interrupted

at

least

ten

times by local incursions

of

volcanrc

detritus

10 to 75 m

thick.

These deposits

include

polymictic

debris

flows

and

torrential

sedi-

ments

that 6lled valleys

and formed large

fans, and reworked

pumiceous

debris concentrated in

fluvial valley

6ll and in

debris

flows

that

spread

widely

across

the

Magdalena lowland.

Older

de-

posits

are

preserved

in highJevel

mesas,

valley

terfaces,

and deeply

dissected

fans; younger

deposits

form low, relatively

undissecied

fans

and inner terraces

along

major

streams.

Key

words: alluuial

far,

Andean

orogeiy, afldesite,

docite, debris

flow,

explosiue

uol-

canism, molasse,

postorogefiic,

reworked

pumice,

torreltial

de-

Posits,

uolcanist l.

INTRODUCTION

Andesitic

to dacitic volcanism

occurred

along

6,000 km of the

Andes

in

late

Cenozoic

time. In

the

northem

zone

(Pichler

and

Zeil,

1969,

p.

165) at

least

40

cones

on

Cordillera

Central

of

Colombia

(Fig.

1)

lie

along the Romeral fault

zone

(Dolores

megashear)

that

apparently

is the

boundary between

the

continental

South

America

and oceanic Nazca plates (Campbell,

1968;

Barrero and

others,

1969,

p.

177-172;

Case and

others,

1971,

p.2704-2707i

Maltait

and Dinkelman,

1972,

Figs.

5,6).

About

20

of the

great

stratovolcanoes (Hantke

and Parodi,

1965;

Ramirez, 1969,

p.29

-47J

ber*een

lat 2o and

5"30'N are 10

to 20 km

east

of the Romeral fault

zone

and

10 to 20 km

west of

the

Magdalena Valley

border

(Fig.

2). These contributed

consider-

able

volcaniclastic

debris to the

Andean foredeep

(sub-Andean

de-

pression).

In contrast, very

little

ash

was blown eastward

because

the

northeast

trade winds

carried most

of it

westward

as

far

as th

equatorial Pacific

sea floor

(Bowles

and

others, 1973,

2383-2386;

Kowsmann,

1973,

p.

482-484).

An

unusually

complete

record of Andean volcanism

is preserve

in

remnants

of

volcaniclastic

alluvial

fans,

broad Iowland valley

l

and confined valley

fill in the

upper Magdalena

Valley.

These d

posits

yield

signifcaot

information

about the

age,

location,

com

position,

and

nature of

the

late

Cenozoic volcanic

activity. In add

tion,

the

more

extensive

remnants

have

produced

distinctive

so

and

groundwater

conditions important

to Colombian

agriculture

Deformarion

of

western

Colombia

at

the

close

of

the

Mesozo

Era

(proto-Laramide

phase

of

Campbell,

1974a)

ended marin

flooding

of

the

east Andean

geosyncline

and

initiated

a

transiriona

paralic

facies

in the

Cenozoic foredeep

(Fig.3).

Middle Eocen

Laramide

deformation

(Campbell,

1974a,

p.712,

Table l;

ear

Andean

orogeny of Brirgl. 19o-.

lig. l).

especially in

rhe

lre'rer

Andean

regron.

was

accompanied

by uplih and

erocion of

(

ordr

lera

Cenrral and

subsideoce

of its foredeep.

This orogenic

eve

together

with the

middle

to

lare Miocene proto-Andean

phas

(Campbell,

7974a,Table

1)

and its major volcanic

activity in

Co

dillera

Central,

produced

a 5,000- ro

6,000-m

proximal

molass

facies

comprising

five

major alternations

of nonmarine

conglome

ates and 6ner

grained

/detritus

(Fig.3;

Van Houten

and Travis

1958).

Each

megacyclelapparently

records

renewed

teconic

attiv

iry

on the

continental

margin to

the west.

During this developmen

sedimentary provinces and orogenic

events

migrated

progressivel

westward

(Krummenachir,

1973, Fig.

3b;

Campbell, 1974a,

713). ln

latest

Cenozoiotime,

6nal Andean deformation

of Cordi

lera

Central and its foredeep

was

followe{ by

regional

elevario

and consequent degradation

that

have

coniinued to the

present.

Upper

Magdalena Valley

(Figs.

l,

2) is

rhe complexly

faulre

proximal parr

of

rhe

Andean

foredeep,

about 450

km long and

5

km wide. In

order to facilitate reference

ro

geographic

locations

the

oorthern

part

of the

area is

called the Honda Basin,

the middl

part

is the broader

Girardot

Basin, and the

southern

paft

is th

Neiva

Basin. Volcanoes

south

of

Ibague

rlrg,

I

r

jre

in

rhi sourher

volcanic province:

those

norrh of Ibague

are

in

rhe nonhern

ro

canic

province.

Descriptions

of the late orogenic

stratigraphic unit

(Fig.

3) are largely

from

recent

studies

by

Anderson

(1972

Gualanday

Group),

lifellman

(1970;

Honda

Group), and How

(1969,

19741'

Mesa

Group).

The recorded

areel.

disrriburion of

th

late

Cenozoic

volcaniclastic

deposirs tFigs.

4,

5r

is

based

o

geologic

maps

provrded

by the Inremational

Petroleum

Company

Ltd.

(INTERCOL),

and the Instituto

Geologico y

Minas

Naiion

(INGEOMINAS);

these

were

amended

during the study

describe

here.

Potassium-argon

ages were

determined

by Geochro

Laboratories,

using

small

samples

of single

volcanic clasts

(Tabl

1). The

expected

young

ages

of rhese rocks precluded

the

use

o

large

bulk samples,

which might

have

contained

older contaminat

ing

material

in

the

matrix.

Where available,

biotite

and

amphibole

clearly of volcanic

origin, were

separated

for

r nalysis. Plagioclas

feldspar

was used

in

a

few

analyses despite

the

possibility

thit

olde


2/15

F. B. VAN

HOUTEN

CENTNAL

IEgC@IEM

""-'

:: I

de

,\r.jl

1825

, 7il

c.

k"*ll

2550

E I I

E ;

I

,r*"*Tl

llllabadal'5

ll o*u-,* |

uo\DA

1)o

ll****",,,.E9 l';

l

c,

Bi?rc l

ll*'r,,**",n,?

,l

;

ll

itiks:"

5too

?

I .

e

ll

c.

Espa

I

; Y

ll*l#"

?l?H

=

F

ll

*"*

1?so

u

F

t

I

-"*"

r.

qo

tr_

"l

il

l";',"

E

Ie

il

l--i--

tlt

B

I

rs

tl r'"^-

'

Il

*,," ,5a

r

I

ll

'

L\rlva

rr-o

il '

1,

lll=

ll ._""

L6i

E

,

I

llffi """

illi

E

I

il:3i:-::;:

,

l;;

ll-,*

u,oo

'

I

ll :f-

ll

-

-,"

Lroo

t

I

llu*:-"

li?:

,.

I

ll r"t"nt'r*

1125

I

ll,-*l

ll****"

"'"

E

=

I

ll 6&;:;

6

E

"lr2b6

E

':

I

Lo?o

t,

I

ill

-J

r?6{

,I

odn

ffiffi"'

:r::

'l

detrital

feldspars

might

be

included

if

they

were

present

(H w'

Kruger,

1975,

Personal

commun

)'

LATE

OLIGOCENE-EARLY

MIOCENE

VOLCANISM

La Cira

Formation

Errlv to

middle

Cenozoic

molasse

tGualanday

Croup'

Fig

3)

contaiis

no

coeval

volcanic

detrirus.

and

irs

clay

iractlon

ls

nralnl),

kaolinite

and

subordinate

illite.

In

conrrast.

mon(morrilonrte

ano

;;;;;;

montmorilton

ite-illite

are common

tn

the

lrte

6i;;;:;;l;"tlt

Miocene

La

crra

Formarion

v

ellman',

la-0

,p

)16"9-1171). and

tuffaceous

mudstone

occurs

ln

correlatlre

oe_

;;;;'

;; ;;il.;;t;;;Joi

t'r'.topp.

I

e5

r'

300

.km.

to'he

south'

ihis

marked

chanqe

in clay-mineral

contenr

probably.

recoros

lne

beginning

of

Cenoioic

volcanism

in

the northern

Andes

ar'out

zJ

to 30

m.y.

ago.

MIDDLE

TO

LATE MIOCENE

VOLCANISM

Honda

GrouP

The oldesr

imDressive

evidence

of

northern

Andean

volcantsm

".;;"

*'ii;"'fi.;a"

-ctoup

that

accumulated

tn

rhe

broad

i.,-.-J".o'

"bout

20

to l0

m.y.

ago'

This

deposit

lFig

S.r

Table

2)

..rnsists

of

as

much

as

1.000

m of

stream-channel

sandstone

ano

;;;ffiil;

;;;lJ-pi,in

"dtton"

arranged

in

6ning-upward

cvclis.

It has

yielded

fossils

of

a

middle

ro

late

Miocene

vertebrare

iirt.',rt"ii"rirUi,.a

a

tropical

savanna

near rea

level

''1;t;;;h;;;

t-h.;;oup.

grains o{

homblende'

magnetrre'

and

an'

desine ari

common

in

the

rand

fractronl

they

are

also

present

In tne

correlatiue

Real

Formation

as

far as 300

km

to

the

north

(Morales

iiit'i".t.'rsisl

"iJ

,tt"iun"

Formation

in eastern

Ecuador

ii*rt.ii.

"is53,-L.*ii

and

oth.t'.

le56)

Most

of

the

volcanic

ilrsrs

in Honda

conglomerares

are

gray

porphyrltrc.

dacrte

ano

andesite.

with

rare

fragmentr

of

basalr,

rrachyte'

and

rhyollte-

I

ne

amount of uolcanic detritus

and

a

felsic

comPositlon

lncreased

our_

i""l-""oti,i-

.f,t.

La

DoradqFormation

(Table

2t

tn

the

Honda

Bj'sin and

especially

in

rhe

north

Nciva

Basin

ln

conrraqt'

rn

rne

;;;

l.i;";6;;i;,;

felsic

composition

prevailed

as

volcanic

inpur

waned during

La

Dorado"

deposition

ln

rhe

\ubsequent

tectonlc

l:ii';;;;ir;?it"

o''"aving

Villa

Vieia

red

beds.

in

the

Honda

and

Neiva

Basins

was accompanied

by

explosrve

voicenlsm

thar

produced

several

light-gray

conglomeratic

sand

bodle\

a Jew

metres

thick.

as

well

as rhin

beds

of

greenish-gray

benio

ie,Most

.,f

the oebbies

are

very

light

gray

Pumrceous

porphyrltrc

da(rle'

iorn.

"i.

gtry

porphyritic

andesite

leva lTable

l)

ln

general

the

i"iii,

i, ,J.i*t,rirnore

felsic

in the

norrh

Neiva

Basin

No

clasts

;'#iii;;;;;t;;

.,*,

o.."

in the

crrardot

Basin'

and

in

rhe

.lr,it

'ii"ir,

'd".1"-r..r*ul,t'on

ol

rolcanrc

debrir

was minimal'

LATEST MIOCENE VOLCANISM

Mesa

GrouP

lncreased

volcanism.

maior

drfferential

uplift

of Cordillera

Cen-

tral.

and associared

local

deformarion

of

cratonic

blocks

wrthrn

the

a".,.."-f-.aa"p

occurred

during

accumularion

of

rhe

200-

to

iirj-.

...gf".'"t",ic

Neiva

Formltion

(Howe, 1974)

in

the

Neiva

-ii-L,'"n-".t

".

trprl,

p

3rr

crred

rn

'ratrns

rhar

Honda

dtpo


3/15

LATE

CENOZOIC

VOLCANICLASTIC

DEPOSITS,

ANDEAN

FOREDEEP

es.i

augite, together

fragments,

are

com-

=o::

in

volcanic

sandstone;

montmorillonite

predominates

in mud-


4/15

Figure2A.

Indexmap,rvest-cen-

tral Colombia.

Cordilleras

ere out-

lined by

2,500-m and 3,000-m con-

tours. Area

enclosed by dotted

line

is

Cenozoic

deposits

ofupper

Mag-

dalena

Valley;

stipple

=

volcanic

provinces

on Cordillera

Central

(after

Barrero

and

others,

1969).

lnset shows

volcanic

provinces

along

Romeral

fauk zone. Rectan-

gles

enclose areas

shown in

Figures

4

and 5.


5/15

LATE

CENOZOIC

VOLCANICLASTIC

DEPOSITS,

ANDEAN

FOREDEEP

o

q

C)

6

o

b'

o

E

o

&

l

B

COR.DILI,ERA

lDENTAL

(.)

a

l

o

o

&

C.

ORIE}ITAL

6lled the inner Magdalena

Valley,

possibly

as

far north

as

t.\e

with about 20

m

of

cross-strati6ed

volcanic

sand

containing

ab

dant round light-gray

porphyritic

pumice

pebbles,

beds

of

pum

clast

gravel,

and

minor

lenses of

lava-clast

gravel

(Table

2).

B

clasts and sand contain biotite,

green-brown

hornblende, m

pyroxene, and

plagioclase and

quartz

(Table

3).

A

single

radiom

ric age determination on biotite

gave

0.6 t 0.1 m.y.

(Table

1). T

qui

valley

6ll

was dissected as

Rio Magdalena

again cur

to

irs p

ent

level, leaving remnants

as

narrow

low

terraces

along the

r

valley.

Girardot Basin

Ibague

Conglomerate and Breccia. After the Magdalena

land had

been

lowered

nearly to

its

present

level

for

the first ti

the broad re-entrant in

the mounrain front bordering the north

part

of the Girardot Basin had

a rugged surface cur across

piedmont;

but a northwest-trending

structural barrier

iust

wes

Gualanday had

not

yet

been

breiched.

During this

episod

steep-gradient

alluvial fan

accumulated in

the isolated ba

stretching

at

leasr

30 km from

the

vicinity

of

Ibague

to

the bar

(Fig,

4). As aggradation continued,

debris

spread

outward

for

eral

tens

of kilometres along low corridors

to the south and to

north,

where it reached

the southwesternmosr Honda Basin

Porta, 1966,

p.

278-279)

when

its

floor

was several metres

hig

than

it is

now.

Ibague fanglomerate

varies

from

place

to

place,

suggesting

th

may be a composite

of several flood deposits. The cause of th

events is

not known. In

its

proximal

part,

15

to 20 km

southeas

a

volcanic

source,

the fanglomerate

(Table

2) is about 100 m rh

and

composed largely of

yellowish-brown,

very poorly

so

debris-flow breccia with lava

and metamorphic clasts rarely 2

long,

Distal deposits along the

Gualanday barrier

are

50 to 6

thick.

The

upper

part

consists of interbedded,

crudely strati

polymictic.fluvial

and debris-flow

conglomerates several

me

thick.

Clasts are

commonly

15

to 30

cm

in

diameter; a few ar

much

as

1 m long. Rare layers

of

volcanic

sandstone

30 to 50

thick contain

scattered lava clasts and rare pumice pebbles.

Mos

the lava fragments

are

gray

and reddish-brown porphyritic pvr

ene and

pyroxene-hornblende

andesires

(Table

3).

A

sin

radiometric

age

determination

on

plagioclase

gave

the improba

date

of 9.4 t

1.0 m.y.

-

an

age

older than that of the

Giga

Formation

(see

Table

2); this strongly

suggests

contamioation

w

older detrital

feldspars.

Twenty kilometres

southeast

of

Gualanday a small butte

in

middle of

the

Girardot

Basin

(Fig.

4)

preserves

about 10 m of up

Honda

strara

overlain

disconformablv bv

a

lower

several merres

polymictic

conglomerare

and rn

uppei

+io

5 m

of

yellowish-brow

volcanic

debris-flow breccia.

Clasts

of

porphyritic

lava, some

much

as l0

cm

in

diameter,

are

pyroxene-hornblende

varrerie,

those in the Ibague fanglomerate.

Apparently, this

debris

w

:E

crc andesire

containing

abundant

plagioclase,

hypersthene,

and

au-

gire,

and

only a minor

amount of red-browo

hornblende

(Table

3).

Tarqui Sandstone

and

Conglomerate. After

Rio Magdalena

had

established

its

channel

on

part

of the Altamira

surface,

it cut

a deep

r

alley

through the debris

flow, down

to rhe

former level. Then

ex-

plosive volcanism

in

the

middle

of

the southern volcanic province

Fig.

1)

produced

a

fresh

supply

of dacitic

pyroclastic

debris

and

lava

clasr

rhat

spread

northeastward into

the southern

part

of

the

\eiva

Basin

(Fig.5).

These

fluvial

deposits

(Fig.

8;

see

Schmincke,

1974,

Fig. 4-16 for

example

of contrasting air-fall pumice

lapilli)

1a

FiSure 3.

Cenozoic molasse in Andcao foredccp,

uppr

Magdalena Val-

le-v,

Colombia. Hearry line

=

duration oI volcanic

activiry

in

Cordillcra

Ctntral;

X

=

maior and x

=

minot explosivc

episodcs

recordcd in

sedimcn-

tary

deposits. Stipple

=

maior influx

of

conglomcr.tic dctritus.

(,

,v

Ruiz;

CORDILLERA CET{'TR}L

Paleozoic

(metamorphosed)

Cretaceous

ozolc

Figure 28.

Cross section

along line A-B

in

Figure

24.


6/15

486

rrrn\Dorled

eastward

rrom

an

exit

we\t of

San

Luis

when

rhe cen'

irri tr,lrndrl.r"

lo*

land

*a.

abour

l0

m htgher

rhan

ir

is

now All

.rJ", ,r]."r

of

rhe

debr*

flo,n

were remored

before

tpread

of the

strcceedrns San

Lurs

fanglomerate.

San

Lui

Sandstone

an'd

Conglome.ate.

Before the lbague

fan-

elo;..ar.

had

been deeply

drssecred

or

rhe

Gual;nda1

barrrerlrad

Eeen

hre..hed,

erplot'ui

v.rlcani.m

rn the

southernmost

part

of the

HOUTEN

nonhern

vol.anic

Drovince

(Fig.

17

blankered

the

'urroundin

lund

*rth

pyto.lriric

debrit

and

lava. Drarnages

ruere

to

c

ihrt

ui.tu"il"

uncontaminated

volcanic

detrirus

was swep

"",r*"td

into th. Girardot

Basin.

The flood

of

light-gray

rew

ovroclastic

mareri.rl

mantled

rhe

lbague

fan behind

rhe


7/15

LATE

CENOZOIC

VOLCANICLASTIC

DEPOSITS.

ANDEAN

FOREDEEP

through

a

broad exr(

rn

rhe

.trucrur.rl

foorhrlls.

fanning

our a.ro.s

rhelowland,o

to

--m

km

gradienu

rnother20

km,F;gI+,t"Ut"Zy.

In

rhe

narrow

corridor

north

of San

Luis.

rhe

fan"elomerate

,.

1boy1

-0__m

rhick..omprisrng

a lower

bedded,"*o.i"d pr^,..

laprllr

rulf with

srringers

of gray

and reddish-brown

DorDhvflric

lav;

clast:.

a

middle

channeled

poorly

sorted lava_cobble conslom_

erare.

and

an upper

unbedded

pLrmice

lapilli

debri:

flow.

pebb'ies

oi

lava

are

alro

abundanr

in

deposirs

lusr

east

of

San Luir

near

rhe

apex

ol

rhe

outer

[an,

Throughour

mosr

of rt\

disral

fluvral

facres,

=

m

tI

fr: :l:l

[,t.'r

I

E

E

b_.i

FUr-l

l-: -i

L

however,

the

deposit

is

no

more

than

10

m thick

and

cons

Iargely

of

well-bedded

vol.anic

sand

uirh

rcattered

on.nhu

pumice

pebbles

in

bedr

I m

or.o rhrck.

togerher

wirh

rhinner'un

of

concentrared

poorl) rorred

pumice

c]asti

l.

much

as

6 ro

g

cm

drameter.

Near

irr

oLtggs marr;r.

rhe ian

deposrr

r.

onll

a

metres

rhlck.

pumice

clart,

are

no

more

rhan

I

few

cenii;erre5

diamerer.

and rhere

are several

layrrs

ol

ycllowish-gray..,nrmo

loniric

mudsrone

l0 io

l0 (m

rhick.

Borh

lava

and"pumr.c

clarr.

San

Lr.ris

deposits

are predominantly

green-brown

hornblen

Pre-Cretaceous

telrane

Fignre

4.

(Continued).

r,a cblsl

volcanlc

provlnce

ClSante

aolmtion

cuall

fluvlal

mantle

Espl.a1

de 11s-f1ow

mntle

San

Luis dtsral

f1uvi61

fen

Ibague

proximl

fanalo@rete

Lerlda

p ox1ne1

fanglmerste

Ia

Chlna

delrls floe


8/15

488

F. B. VAN

hrorrte dacite

(Table

1),

The

assocrared

sandstone

contains

grains

"i

rr"lr"""r"t

dirrtrrn'dt

of

quartz.

as

well

ar ande''rne

horn-

blende. irroxene.

biotite,

and

magnetire

(Butler'

1940)

-

r..oii"t'

Srna.,on"

and

Conglomirate.

Wtrh

renewed degrada-

,,on,'ii.

t."li.

Jratnage

acro-ss

rhe

lbague

fan

erched

irs

surface

and

hreached

the

Cualanday

barrier.

Afrer

the

canyon

had

reached

;;;r..";,

deoth.

anorher

episode

of explosive

volcanism

in

the

r.ruih".n.ort

part

of

the

norrhern

volcanic

provrnce

{Fig'

l)

.h.,k"d ,h"

uoland

drainages.

This

fresh


9/15

LATE

CENOZOIC VOLCANICLASTIC

DEPOSITS.

ANDEAN

FOREDEEP

48

about 30 m thick, with angular

clasts

of

gray

and reddish-brown

porphyritic

lava and rare fragments

of

metamorphic

rock

as

much

as

30

cm

long.

There are

only

a few thin layers

of

pumice-clast

con-

glomerate. Throughout

most of its exrenr to

the

east,

the

lowland

fill

is

10 to 30

m

of unsorted, round

porphyritic

pumice

clasts

5 to

10

cm in diameter

set

in

a

matrix

of gray volcanic

sand.

The fabric

of

rather uniform, unstratified

layers

generally

less than

a

few

metres thick

is characterisric of

slurry

debris

flows.

ln

the

most dis,

tal deposits, beds

of

small round

pumice pebbles

and

yellowish-

gray

montmorillonite

mudstone

are more common. Espinal

lava

and

pumice

clasts

are

dacite

marked

by abundanr

green-brown

and

red-brown horoblende

and less

common

brown to red-brown

bi

tite

(Table

3). A single

radiometric

age based

on biorire

is 1.-l

t

0

m.y.

(Table

1). With renewed regional

excavation, mrnor cons

quent

streams began

to

etch the Espinal surface, Rio

Coello c

through

the sheet down to its

former

levei,

and Rio

Nlagdalena

a

justed

to

its

present course along the

east side

of

thc

Girardor

Basi

The

three large fans

in

the

Girardot

Basin

form

some

of

the

mo

exrensive agricultuial tracts in

the upper Magdalena Valley. B

cause it

is

low, relatively undissected,

and

easy

ro

till,

the

Espin

surface

is

the

most favorable. The

bouldery,

dissected Ibague frn

the

least

favorable.

Fi9lure

5,

(Cont;nued),

\c

=

fI

n

F]IT,3

litrii:1

il

ffi

e

La PIat

Altamira

debrls

floq

Paicol debrls flou

Pre-cretaceou6

terrane

volcenic

provlnce

clgante

Eomatlon

Tarqui

fluvtal va1le, f

t11

laez torrentlal

va1ley

fill

r

t


10/15

490

F. B.

VAN HOUTEN

Honda

Basin

Lerida Conslomerate,

Breccia,

and Sandstone.

Early

in

rhe

ex-

.rration

of so"rthern

Honda

Basin, a

rugged

rurface

cut on

Honda

deoosits

was

as

much as

l0O

m above

the

Present

lowland

along

ihi

^oun,"in

front

and

l0

ro

20

m hrgher

in

the

mrddle

of

the val-

Iey.

Ar

thrs srage,

explosive

volcanism

in the

vicinity

of

Pico Ruiz

IFis- lJ Droducid

pvroclasric

debris

rhat

was swept

easrward

as

tar

,t

'iio

Vaea"t.n".'tr

6lled

rhe

rrregular

lowland

topography

with

as

much

as-50

m

of

gray volcanic sand

and

lighr-gray

porphyritic

oumice

pebblesl

rhis

forms

rhe

lower

part ot the

Lerida

fan, espe-

lirll,

ni.th

of

Rio

Recro

(Fig.

4,

Table 2i

de

Porta. 1956'

P'

2?5'-278).

Pumice

clasts

in

thi

lower

Lerida

deposit

are augite-

hvpersthene

andesite

(Table

3t.

A

single

age

determination

on

piagioclase

gave 4.3 t 0.3

m.y.

(Table

I

l.

'

l-o116117;nirhe

brret

explosive

event'

several

successive

floods

of

volcanic-rich

sand

and

gravel

and debris"flow

breccia

consrructed

the composite

steep-gradient

(22

m/km)

upper

Part

of the

fan lTable

2t both

no-nh

and

south

of

Rio Recio

On

its

nor

"in

marein.

rhe

fanglomerare

abuned

against

an old

rcarp

secred

C'igante

srrat-a

whose

toP

ts still

a few

tens of

merres

than

the Lerida

sudace.

Proximal

deposits

of

the upper

Lerida

fanglomerate

expo

Rio Recio

canyon

consist

of

as

much

as 50

m

of

poly

debrir-flow

breicia

wirh

some

clasts

2

m

Iong

These

deposits

an

rrresular

surface

cut acro\s

the

sott

lower

Lerida

sandsro

locally'on

high

knobs of

Honda

depostrs

Along the

norther

sin. distal

deposirs

consrst

of

a

lower

l0 ro

lS

m

of

gray

lan

ini b."..ir.

wirh clasrs

generally

les'

rhan 5

cm tn

diameter'

uooer few

metres

ol coar.e

fluvial

conglomerare

and

mrn

canic

randstone

lying on

an

rrregular

surface

cut

across

the

"nJ

to."tt"

into'

hiih-sra

ndrng-

remnants

of

Honda

'tra

southern

marqin

of the

fan is a

rarher

rimilar

l5-

to 20-m

'e

of

*r,

,ot.rii. sand5tone

and

breccia.

but

the

upper

20 m

ior"thitn

lob.

consrsrs

of

a

very

coar\e

polymictrc

conglo

,nJ b."..ia

*hor. surface

forms

the

highesr

part

of

t

he Lerid

Most

ol the

upper

Lerrda

volcanic

clasrs

are

dark'gr

reddish-hrown

porphvriric

two-pyroxene

andesire

lata

Ifa

like a toneue oi

laua

orerlving

J

uolcanic debris

flow

in

the

n..io

V"tT.y

(Fig.4r. A few

li-ght-gray

porphyritic

pumice

also

occur

in more

distal

deposrrs.

but

therr

pauctry

sugg

working

of

rhe

lower

Leridi

pumiceous deposits

that

lay

eartern"slope

of

Cordillera

Central,

Plagioclase

in

a

clast

tr

uppermost'part

oI rhe

Lerida

fanglomerate

gave a radiome

of

1.1

t 0.1

m.y.

(Table 1).

lilh"n,.glon"i

e.or,on

prevarled agarn.

Rio

Recio

cur a

d

l"u

berweei

rhe

rwo Lerida

lobe.

down

ro

Present

Ievel.

a

..in.eou"n, slream5

etched

the

fan

surface,

This

rugged

iaphy,'tog.th..

wirh

bouldery

rextures.

makes

the Lerida

relativelv difficult

to

till.

Aboui

I0

km south

of

rhe

couthern margin of

rhe

Lerida

f

small hieh-level

remnants

ot

volcanic breccia

along

Rio

L

lie on

tr-uncated

Honda

deposirs

about

100

m above

the

ioJand

floor

(Fig.

a).

The 5-

to

10-m-thick

breccia consists

mras:T'J -AFrc{N

ACFS OF L.A1E

CB,IOZOIC

V0lrlA}l1C'- -STia

DEPISTIS

rr,091

0.1168

0.768

),221

a.69-a

0.7i1

4lrr1

3- 20

t

6e.clft.

Labo.aLorles

sanFle .mbers.

+ Esrhted

percent; 3

=

bl;tiie,

H

=

hontl..de'

P

=

pv'oxene'

0

=

olhers

Baolosenic

coiponerl.

TARIT 2,

DISJRIBU]'ION

A}ID

DI}IB{SIONS

OF I'A1E

CEN]ZOIC

TI]T'A'IICIAS 1C IEF'SII:

.1\l

dPss

Dlstal

S'zc

'' 'rfa('

-i;";.;,;'

-

ht

r's

d

sroD'

,j'

"il'

"4.

.'

o'

s

'roe

'd/Ya

T,r.{E i.

FnINalral

P}ix.r8Y

T

n.:l'u1r,

Arroal,llll:

rri

rlr'$ll:_

)'1

:r

i

'n'

.]trI.: rl.

]kI,;':,1]

c1g&re

z-2721)

608,

I'H,

50

0ua1i

a-272rr 90H, 5P,

23,

l0

Ouali

F-2?2\

Praioc14e

Paez

F-272a

Pl.gloclase

Esplnar

v2726

6tn,

)aH,

I

TL,aAue

F-2727

Plagloelse

lErlda,

upper

F 2728 Plagloclase

lErida, 1ore.

'-2729

PlagL..las

Paleot

8-2?30

Bl.t1ie

TaxqDl

a-21)r

8oB,

t>d,5A

,

r_.rr, L ]t

.t

: a.

q

ar.ai,

..

a.a

'

t

)

a.a6i,

1-a)i

..a

I

a.a

a.alt,

a,a:?

L.:

: a..

r.:2i,a.rl8 a 4.2

a.oq ,

a.a6l

:.c:

l a

a.ll:,

-1.01t

1.l

r

.1

a.aa1,

a.x6i

q

l:

o,l

0.081,

a.034

1,7

i n.2

0.01i

0,022

0.5

1

0,1

2.6

1r.l

0,5

60

?,

15

llo-15

M

5

..

15

N?52a2,

s50\52'

st0525

,s

xg

xE

:E

50

rO

8-10

\2a

I

..

M

1.4

N

t.3

3.7

0.6

35

40

27

3a

30

50

3'11

6-7

.

21r-10

tzad,lr

la rava

turr.srn.r.

^i-br.fr,,i

M

[1

S

8.5

754

E

Nore:

Posltlon

of

postorcgenlc deposlts

n

EtE

co1@

1'FUe' a

'tloologlc

Elatlo.

o.1,

,1th1.

each

rN-..rth

M=hdddle.S

ir--';;;t.

ir

=

niddre,

s

-

south'

+' thtluddt

basln

fi,-

=

N-r-^

^r

-

crrard.t. Ho

=

Honda

\e.

Nelva

Ct

- clrturdot,

ofteli-brcwrad

75+

100

(Ho)

75+

150

(r{e)

T5+

100

{no)

7t+

150

(Ne)

C1&rtr-

fm1c.,

13va

I

Horla

rm1..

i=

",alo.

i"a

r

= mlnor

phases

or

r'de&

oEsenv


11/15

LATE CENOZOIC

VOLCANICLASTIC

DEPOSITS,,{NDEAN

FOREDEEP

491

of

dark-gray

and

reddish-bro*n

porphyritic

andesite

clasts

rarely

as

moch

as

30

cm

long.

Common

phenocrysts

are andesine,

hyper-

sthene,

and

augite;

colorless

garnet

rs

rare. These

remnants

record

a

debris

flow

that

spread

eastward

in a broad

valley at

about

the

same

elevation

as

ihe

surface beneath

the Lerida fanglomerate

and

about

50 to

75 m

above

the

northern

distal

tongue of

the lbague

fan.

Guali

Sandstone

and Conglomerate.

Afier

the

main

streams

in

the

Honda

Basin

had

cut

to their

present

level,

explosive

volcanisrn

in the

northern

part of the

northeln

volcanic

province

(Fig.

1)

eruDted

Dvroclas;rc

debris

aod

lara thar

mantled

rhe

'urrounding

Co;dill.;a

(

entral.

Detrrrus

trom this erent

.prerd

easlward

over

an

irresular

slopine

(10

m'kmt lerratn

rn the norlhern

half of

rhe

Honda"Ba'in

iuafi

'ed'menr'

6lled br^ad

valle]'

cur

rn Mesa and

Honda

strata

as

much

as 75

m above

their floors.

It

extended

a

few

kilomerres

east

of

rhe

presenl

Rio

Magdalena

charnel

and

as

far

north

as La

Dorado

1fig.4:

Table 2r.

where

ir is 6 to

-

m thick'

Ficure 6. Cieante

Formation

A, Characlerirtic

outcrop.

'howing

Iens of

darlivello*

irh-"erav

fluvial

randstone

iupper

right'

ii much

rs

s

m thick

and oarallel-bed--ded

debrir-flow

deposit..

Large''cale

bedding

i\

Produced

hv inierlaverine

of dark-srav

mudflows

and

lar a'clast lahan

and

light-gray

.i..ted oumici-cla

van_houten.pdf

Documents