alluvial facies characterization and prediction in endorheic … · •a significant volume of...
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ALLUVIAL FACIES CHARACTERIZATION
AND PREDICTION IN ENDORHEIC BASINS:
THE VIEW FROM THE MARGINS
Faculty of Geosciences University of Utrecht
(The Netherlands)
Dario Ventra
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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)
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High-gradient fluvial deposits (Quaternary, Paposo, N Chile)
Channel-fill, detail
(Paleogene, Ollagua Basin,
N Chile)
… AND SOMEWHAT “LESS
CLASSIC” EXAMPLES!
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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???
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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
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• 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
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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
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BASEMENT: Trias, muddy continental to coastal siliciclastics,
unconformably overlain by Middle-Jurassic to Cretaceous marine carbonates
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CASTRALVO-VILLASTAR
PALEODRAINAGE
CENTRAL SEGMENT OF
THE BASIN
PERALEJOS-ALFAMBRA
PALEODRAINAGE
NORTHERN SEGMENT OF
THE BASIN
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• BASIN OUTLINE
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• CONCLUSIONS
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W E
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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
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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 %
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Low architectural
complexity within
channel fills…
Very basic stacking
of vertical and lateral
accretion units….
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MAJORITY OF CHANNELS PRESENT MASSIVE TO POORLY ORGANIZED
CONGLOMERATES AND VERY HIGH CLAY CONTENTS WITHIN MATRIX
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GRAVELLY CHANNEL FILLS MAINLY REPRESENTED BY
HYPERCONCENTRATED FLOWS & POSSIBLY DEBRIS FLOWS
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• BASIN OUTLINE
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• CONCLUSIONS
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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
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CHANNEL ARCHITECTURE
Dominated by poorly-sorted, relatively well-
organized sandy-gravelly sheets, low-relief bars
and abundant coarse-grained lags
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Deposition mainly by tabular bedload sheets and
gravelly splays during major floods
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Occasional matrix-rich, very poorly sorted gravel interbeds…
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… but in general, relatively
good organization:
• GOOD SORTING
• CLAST SUPPORT
• IMBRICATED, FREQUENTLY
OPENWORK FABRICS
• DISTINCTLY BEDDED UNITS
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• BASIN OUTLINE
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• CONCLUSIONS
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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
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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
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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
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• BASIN OUTLINE
• CENTRAL SUBBASIN:
CASTRALVO-VILLASTAR SYSTEM
• NORTHERN SUBBASIN:
PERALEJOS-ALFAMBRA SYSTEM
• COMPARISON AND CONTROLS
• CONCLUSIONS
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• 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
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THANKS TO:
Jan van Dam (University of Utrecht) for field assistance
NWO (National Science Foundation of The Netherlands)
for research grants