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
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• CONCLUSIONS
W E
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
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• 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
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• 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
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• 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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