alluvial facies characterization and prediction in endorheic … · •a significant volume of...

ALLUVIAL FACIES CHARACTERIZATION

AND PREDICTION IN ENDORHEIC BASINS:

THE VIEW FROM THE MARGINS

Faculty of Geosciences University of Utrecht

(The Netherlands)

Dario Ventra

EXAMPLES OF “CLASSIC” FLUVIAL

FACIES AND ARCHITECTURES IN

THE ROCK RECORD…

Image courtesy Sophie Leleu (University of Aberdeen, UK)

Image courtesy Adrian Hartley

(University of Aberdeen, UK)

High-gradient fluvial deposits (Quaternary, Paposo, N Chile)

Channel-fill, detail

(Paleogene, Ollagua Basin,

N Chile)

… AND SOMEWHAT “LESS

CLASSIC” EXAMPLES!

ARE WE OVERLOOKING SOMETHING?

• A significant volume of fluvial sediments in tectonically active

basins consists of poorly understood facies associations,

deposited by ephemeral alluvial systems different from the most

commonly studied long-range, low-gradient rivers with protracted

discharge

• Essential distinction between the two kinds of fluvial systems is

the ratio of sediment load to available discharge during significant

hydrological events (mainly related to: 1) poor catchment maturity

and integration; 2) local hydrology and regional climate

• How does this influence our prediction of :

… facies character and distributions?

… subsurface patterns of fluid flow??

… geohazard in flood-prone highland settings???

LET’S LOOK AT SOME EXAMPLES FROM THE TERTIARY

TERUEL BASIN (CENTRAL SPAIN)…

• BASIN OUTLINE

• CENTRAL SUBBASIN:

CASTRALVO-VILLASTAR SYSTEM

• NORTHERN SUBBASIN:

PERALEJOS-ALFAMBRA SYSTEM

• COMPARISON AND CONTROLS

• CONCLUSIONS

• System of three interlinked

half-grabens (early Neogene

extension over folded Mesozoic

basement Iberian Range)

• developed in a semiarid

climate for most of its history

TERUEL BASIN

• internally drained until Early-

Middle Pleistocene

CLOSED BASIN FOR MOST OF ITS HISTORY

• Fully aggradational architecture (forced rise of alluvial

base-level in endorheic setting)

• Essentially complete preservation of stratigraphic column

BASEMENT: Trias, muddy continental to coastal siliciclastics,

unconformably overlain by Middle-Jurassic to Cretaceous marine carbonates

CASTRALVO-VILLASTAR

PALEODRAINAGE

CENTRAL SEGMENT OF

THE BASIN

PERALEJOS-ALFAMBRA

PALEODRAINAGE

NORTHERN SEGMENT OF

THE BASIN

• BASIN OUTLINE






• CONCLUSIONS

ABRUPT APPEARANCE AND DISAPPEARANCE

fluvial system spans a narrow stratigraphic interval (~70-80 m) within late Miocene

deposits (phase of active basin subsidence and high accommodation)

~ 1.5 Km

Best 3D exposures: 5 x 5 km outcrops near Villastar

GENERAL ARCHITECTURE

Poorly connected, coarse-grained, sheet- to ribbon-like channel fills

within dominant aggrading muddy overbank

Approx. volume ratio channel / overbank facies ~ 15-25 %

Low architectural

complexity within

channel fills…

Very basic stacking

of vertical and lateral

accretion units….

MAJORITY OF CHANNELS PRESENT MASSIVE TO POORLY ORGANIZED

CONGLOMERATES AND VERY HIGH CLAY CONTENTS WITHIN MATRIX

GRAVELLY CHANNEL FILLS MAINLY REPRESENTED BY

HYPERCONCENTRATED FLOWS & POSSIBLY DEBRIS FLOWS

• BASIN OUTLINE






• CONCLUSIONS

SYSTEM ARCHITECTURE

• Developed in the Pleistocene (open outlet to basinal drainage and

reduced accommodation) within a broad, locally incised valley

• Sheet-like, shallow, sandy-conglomeratic channel fills within

muddy sandy overbank

• Good vertical and lateral connectivity (approx. volume ratio

channel / overbank fines ~ 40-60%)

• Low clay % both within overbank and conglomerate matrix

CHANNEL ARCHITECTURE

Dominated by poorly-sorted, relatively well-

organized sandy-gravelly sheets, low-relief bars

and abundant coarse-grained lags

Deposition mainly by tabular bedload sheets and

gravelly splays during major floods

Occasional matrix-rich, very poorly sorted gravel interbeds…

… but in general, relatively

good organization:

• GOOD SORTING

• CLAST SUPPORT

• IMBRICATED, FREQUENTLY

OPENWORK FABRICS

• DISTINCTLY BEDDED UNITS

• BASIN OUTLINE






• CONCLUSIONS

HYDROLOGICAL CONTROLS

EPHEMERAL DISCHARGE

(VERY POOR FACIES & ARCHITECTURAL ORGANIZATION)

CONSTANT SEMI-ARID CLIMATE REGIME

(FROM MIOCENE TILL PRESENT DAY)

+

ACTIVE BASEMENT TECTONICS

=

SHORT-LIVED HYDROLOGICAL TRIGGERS + SMALL

AND POORLY INTEGRATED CATCHMENTS

ORIGIN & TIMING IN BASIN’S HISTORY

FORCED BASE-LEVEL RISE IN

TOPOGRAPHICALLY CONFINED BASIN

NEOGENE ACTIVE TECTONIC PHASE

(SUBSIDENCE + SEDIMENT SUPPLY)

PREVALENT OVERBANK & POOR

CHANNEL CONNECTIVITY

DEGRADATION &

REDUCED ACCOMMODATION

QUATERNARY TRANSITION FROM

ENDORHEIC TO EXORHEIC

(INCISION & ALLUVIAL BYPASS )

HIGHER CONNECTIVITY

CHANNEL BODIES

BASEMENT SOURCES

MUD-DOMINATED SYSTEM

(MAINLY HYPERCONCENTRATED FLOWS)

GENERALLY VERY POOR POROSITY &

PERMEABILITY

CLAY-POOR, COARSE-GRAINED SYSTEM

(CLEAR WATERFLOWS, NEWTONIAN)

GENERALLY VERY HIGH PERMEABILITY

TRIAS CLAYSTONES

JURA MARLSTONES,

LIME- AND

DOLOSTONES

JURA LIME- AND

DOLOSTONESOLIGOCENE ALLUVIALS

• BASIN OUTLINE






• CONCLUSIONS

• HIGH VOLUMES OF COARSE-GRAINED FACIES IN THE TERUEL

BASIN REPRESENTED BY EPHEMERAL, HIGH-GRADIENT FLUVIAL

SYSTEMS DEVELOPED ALONG BASIN MARGINS FROM POORLY

INTEGRATED CATCHMENTS

• IN SPITE OF SOME NOTABLE HYDROLOGICAL DIFFERENCES (SEE

FACIES & ARCHITECTURES) WITH “CLASSIC” FLUVIAL EXAMPLES,

THESE SYSTEMS, USUALLY OVERLOOKED IN THE LITERATURE, CAN

BE ANALYZED AND COMPARED ACCORDING TO THE SAME

PRINCIPLES

• RELATIVELY SMALL RANGE & STRICT DEPENDENCE FROM

ADJACENT SOURCE AREAS AT BASIN MARGINS FAVOR

PREDICTIVITY OF FACIES AND LITHOLOGY IN A BASIN-ANALYSIS

CONTEXT

CONCLUSIONS

THANKS TO:

Jan van Dam (University of Utrecht) for field assistance

NWO (National Science Foundation of The Netherlands)

for research grants

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