University of Parma. FACIES AND FACIES ASSOCIATIONS. SANDBODY GEOMETRY. AND STACKING PATTERNS. EGU Jean Bap]ste Lamarck Medal Lecture ...
High-resolution facies and sequence stratigraphic analysis of fluvio-deltaic systems in tectonically-active basins Emiliano Mutti University of Parma EGU Jean Bap9ste Lamarck Medal Lecture, Wien 2012
FACIES AND FACIES ASSOCIATIONS
SANDBODY GEOMETRY AND STACKING PATTERNS
Drainage basin
THE FLUVIAL SYSTEM AND ITS THREE BASIC ZONES (SCHUMM, 1977)
Patagonia
Transfer zone
Zone 1
TRANSFER ZONE
Patagonia
Patagonia
Deposi9onal zone
Zone 2 Zone 3
The model of Schumm (1977)
Cameroon
THE PROBLEM (1) • Fluvio-deltaic sedimentation is rarely viewed as a whole. Sedimentologists traditionally consider separately the two different aspects of the problem and have thus developed a fluvial and a deltaic sedimentology. • Fluvial sedimentology deals essentially with river processes and deposits, focusing on types of channel and bar and architectural elements that can be recognized in both modern and ancient fluvial deposits. These subjects are dealt with in great detail in classic work by Miall (2006, with references therein). • Deltaic sedimentology is mostly based on large modern deltas with the classic tripartite classification in fluvial-, tideand wave- dominated deltas stemming from the fundamental papers of Coleman and Wright (1975) and Wright (1985). An updated review of these concepts supplemented by some ancient examples has been provided by Bhattacharya (2006).
THE PROBLEM (2) • In 1992 Milliman and Sivitsky suggested that in small mountainous rivers of tectonically-active margins high-elevation drainage basins, high-gradient transfer zones, limited extent of flood-basins and coastal plains, and proximity to shoreline favour an abundant sediment flux to the basin primarily through floods. • As documented by Mutti et al. (1996, 2000), most ancient fluviodeltaic systems of tectonically-controlled basins fed by small “mountainous” rivers are in fact dominated by facies and facies associations resulting by flood-related processes. • These flood-dominated fluvio-deltaic systems do not fit sedimentological models currently available for fluvial and deltaic sedimentation, thus raising a series of new problems concerning facies types and their analysis, facies associations, sequence stratigraphy and terminology.
THE APPROACH • The problem is faced herein from a very simple sedimentological approach mostly based on outcrop observations framed within local and regional stratigraphic constrains. Most of the presentation is thus based on what we can observe in outcrop studies and on what we can infer from vertical and lateral facies relationships, i.e., back to the basic principle of stratigraphic analysis.
• The problem is mainly restricted to marine delta-front settings where river outflows enter seawater and fluvial and marine processes combine to form facies and facies associations extending from the exit of fluvial channels to low-gradient shelfal regions. Narrow and steep shelves, where fluvial channels may act directly as feeder channels of adjacent turbidite systems, are omitted from the discussion.
The lateral transi9on between fluvio-‐deltaic and turbidite systems in the Eocene of the south-‐central Pyrenean foreland basin (from Mu[ et al., 1988)
Fluvio-‐deltaic systems (marine and con9nental )
Foredeep turbidite systems
GRADED BEDS ARE UBIQUITOUS IN MARINE AND CONTINENTAL DEPOSITS
Turbidite sandstone lobe: Graded sandstone bed
Delta-‐front sandstone lobe: Graded sandstone bed with HCS
Fluvial deposit: Graded flood unit
In tectonically-‐acHve basins verHcally and longitudinally graded beds are ubiquitous. WHY ?
Clear-‐water stage
Catastrophic vs “normal” floods
River in flood
Sand lobe deposited by a modern flood
Types of deposiHonal systems for increasing flood magnitude
EXPOSURES OF DELTA-‐FRONT STRATA IN THE EOCENE OF THE SOUTH-‐CENTRAL PYRENEES
FIGOLS GROUP
SABINANIGO DELTA
CASTIGALEU GROUP
SANTA LIESTRA GROUP
Detailed stra9graphic cross-‐sec9on of the Sabinanigo Sandstone (fluvio-‐deltaic system), south-‐central Pyrenees
GEOLOGIC MAP OF THE JACA GROUP IN THE SABINANIGO AREA (SOUTH-CENTRAL PYRENEES)
From Mu[ et al., 2009 (original data from Bongiorni, Cabellani, Co[, Marchi and Mu[, 1998-‐1999)
THE BASIC ELEMENTS OF FLOOD-DOMINATED FLUVIO-DELTAIC SYSTEMS IN MARINE DEPOSITIONAL ZONES (RIVER DELTAS)
EXAMPLES OF MOUTH-‐BAR DEPOSITS
A
B
Offlapping component slightly toward viewer
A – Typical deposi9onal mouth bar in which the bulk of the sand is deposited on the bar crest with shalying out of individual sandstone beds along the bar slope (Atares delta, south-‐central Pyrenees, see also Puigdefabregas et al., 1975). Note the transi9onal contact with the underlying prodeltaic mudstones.
B – Mouth-‐bar deposit characterized by a basal
erosional surface (red arrows) resul9ng from mul9ple scouring (Cas9ssent Group, south-‐central Pyrenees)
C – Mouth-‐bar deposit largely deposi9onal and partly reworked by 9dal ac9on. Note the sharp and slightly erosive basal contact (red line).
C Offlapping component
Marine flooding surface at the top of each sandstone bar
Tabular geometry
Sandstone lobes from the Jurassic Bardas Blancas FormaHon, Neuquen Basin, ArgenHna
Basal shell debris
Grading and HCS
Tabular geometry of flood-‐generated shelfal sandstone lobes A
B
Tabular sandstone lobes and intervening shelfal and prodeltaic mudstones. Individual lobe packets have lateral extent up to several km. A – Eocene Figols Group, south-‐ central Pyrenees B – Pleistocene Aliano Group, Val d’Agri, southern Apennines,
The basic elements of flood-‐dominated fluvio-‐deltaic systems in marine deposi9onal zones (river deltas)
DELTA-SLOPE DEPOSITS (Eocene Castissent Group, south-central Pyrenees) Delta Front
Delta Slope B
A A –Spectacular exposure of delta-slope mudstones and thin-bedded sandstonesoverlain by prograding flood-dominated delta-front sandstones (red arrow)
B – Close-up of delta-slope deposits showing alternating packets of mudstones and thinbedded sandstones. Mass-transportt units (MT) containing abundant shelf material are common (red arrow)
C - Detail of silt/mud couplets and thin starved ripples (arrows). Mudstone is light gray. Deposition is thought to be dominated by river plumes and dilute hyperpycnal flows exiting river mouths. These details are rarely seen in outcrop.
C
Sediment flux to the sea as related to flood magnitude • The degree to which fluvial floods increase sediment flux to the sea is expressed by the amount of sand that can bypass channel-‐exits through hyperpycnal flows thus forming sandstone lobes in adjacent shelfal regions. • A broadly intergrada9onal spectrum exists from systems in which most of the sediment load is trapped in channel exit regions (poorly-‐efficient systems) to systems in which most of the sediment load (including gravel) is carried seaward in adjacent shelfal regions (highly-‐efficient systems). • In highly-‐efficient systems, channelized features generated by successive flood events extend from alluvial to marine regions probably offering an explana9on for the origin of many submarine “incised valleys” lacking evidence for normal fluvial erosion.
Detailed stra9graphic cross-‐sec9on of the Eocene Sabinanigo Sandstone (fluvio-‐deltaic system), south-‐central Pyrenees
Examples of delta-‐front “incised-‐valley fills” (some are indicated by arrows) eroded by channelized hyperpycnal flows exi9ng distributary fluvial channels. These broadly channelized features have widths up to 1000-‐1500 m and depths up to 10-‐15 m. Their final infill is largely represented by channel-‐exit and channel-‐lobe transi9on facies and facies associa9ons.
From Mu[ et al., 2009 (original data from Bongiorni, Cabellani, Co[, Marchi and Mu[, 1998-‐1999; interpreta9on added )
In poorly-efficient systems (A), sand is trapped in channel-exit regions; in highly-efficient systems (B), sand bypasses channel-exits and is mainly deposited in adjacent deeper-water regions Bar crest
prodelta Bar slope
POORLY EFFICIENT SYSTEM Thick and coarse-‐grained, graded and tabular sandstone beds (sandstone lobe) deposited by hyperpycnal flows that bypassed the channel-‐exit and occur at the base of a slightly younger and finer-‐grained offlapping mouth-‐bar deposit (MB)
Floods die out at channel exit (river mouth) forming thin sandstone beds deposited by dilute suspensions and shalying out along the bar slope into the adjacent prodelta mudstones. The sediment load of the river ouolow is en9rely trapped at channel-‐exit Mouth-‐bar sandstones
Sandstone lobe
A
HIGHLY EFFICIENT SYSTEM
B
ELEMENTARY DEPOSITIONAL SEQUENCES (EDSs): THE BUILDING BLOCK OF SEQUENCE STRATIGRAPHY
sequence
STACKING PATTERN OF DELTA-FRONT FACIES AND FACIES ASSOCIATIONS AS OBSERVED IN OUTCROP Late Midde Eocene Sabinanigo Sdst
MB
Mouth-bar sandstone Bar-toe mudstone
SL
Delta-front sst lobe
Lower Eocene Figols Group
MB
Mouth-bar sandstone Bar-toe mudstone
SL
Delta-front sst lobe
Sequence boundaries (SB, red arrows) are marked by the sharp basal contact of sst lobes. Transgressive surfaces (TS) are marked by thin and bioturbated sst facies, locally replaced by carbonates or sst reworked by tidal action
SEISMIC SCALE VS OUTCROP SCALE THE DIAGRAM COMPARES SEISMIC-SCALE SYSTEMS TRACTS WITH SEQUENCES FACIES AND FACIES ASSOCIATIONS OBSERVED AT OUTCROP SCALE From the smallest to the largest, depositional sequences show a similar stacking pattern related to their fractal nature
SEISMIC STRATAL PATTERN
OUTCROP EXPRESSION
OF A LONG-LIVED (3rd order) DEPOSITIONAL SEQUENCE
OF SHORT-LIVED DEPOSITIONAL SEQUENCES SL HIGHSTAND + FALLING STAGE
SB TS
TS MB
PARASEQUENCE
PROGRADING LST DELTA SLOPE FAN BASIN FLOOR FAN SB
SL
DEEP WATER
EDS
TRANSGRESSIVE
SB
TS: transgressive surface
TURBIDITES
MB
EDS: elementary depositional sequence SHALLOW WATER
POSAMENTIER and VAIL (1988)
SL
MUTTI (1989, 1990) MUTTI et al. (1994, 1999, 2000)
EDS
TS
SB: sequence boundary
SB
Decreasing sediment flux to the sea
Flood magnitude
meter to decameter scale
ACCOMMODATION (SUBSIDENCE)
RELATIONS BETWEEN SEDIMENT FLUX, FLOOD MAGNITUDE AND FACIES TYPES
STAGES OF GROWTH OF A FLUVIO-‐DELTAIC EDS STAGE III (AggradaHonal-‐progradaHonal) -‐ Coarse sediment flux to the sea dramaHcally decreases and only mud can accumulate TS : transgressive surface or, beber, a fluvial retreat surface with no evidence for 9dal or wave ravinement and clearly related to base level rise
STAGE II
STAGE II (Mainly progradaHonal) -‐ Poorly-‐efficient stage. Smaller-‐ volume, flood-‐generated sediment-‐laden river oualows (as well as normal river oualows) are forced to deposit most of their sediment load at channel-‐exits (mouth bar). STAGE I (Mainly aggradaHonal) -‐ Highly-‐efficient stage. Large-‐ volume, flood-‐generated sediment –laden river oualows give way to hyperpycnal flows that can carry sand to shelfal regions and deposit tabular sandstone lobes. These lobes are in fact shallow-‐water turbidites.
Sequence boundary
Stage boundary
SOME CONCLUDING REMARKS
Sequence straHgraphy of marginal-‐marine fluvio-‐deltaic systems: Parasequences and Elementary DeposiHonal Sequences (EDS)
Meter to decameter scale
ELEMENTARY DEPOSITIONAL SEQUENCE (EDS) Regressive-‐transgressive succession bounded above and below by sequence boundaries (SB)
PARASEQUENCE Shoaling-‐upward succession bounded above and below by marine flooding surfaces
SL : delta-‐front sandstone lobes MB : mouth-‐bar (channel-‐exit) deposits TS : transgressive deposits associated with marine flooding surfaces Sequence boundary (unconformity and correla9ve conformity) Marine flooding surface (base level rise)
Sequence straHgraphy of marginal-‐marine fluvio-‐deltaic systems: Parasequences and Elementary DeposiHonal Sequences (EDS)
MAIN CONTROLLING FACTORS : A) Orbital forcing (Milankovitch cycles) governing high-‐frequency climate and base level varia9on B) Flood magnitude and frequency C) Sediment availability in the source area D)Tectonics (uplir and regional subsidence)
A + D control the amount of space available and cyclic climate and base level varia9ons B + C control processes and facies Meter to decameter scale
Shoreface, wave-‐dominated delta, or simply the most genuine expression of a fluvial-‐dominated delta front deposit? The Problem : Parallel-‐sided, sharp-‐based graded sandstone beds with HCS
HCS
Suggested InterpretaHon : Ver9cal and lateral stra9graphic rela9onships observed in tectonically-‐ac9ve basins definitely favour a “flood-‐ dominated delta-‐front deposit”, i.e. the genuine expression of fluvial processes
Normal deltas vs flood-‐dominated deltas
A
Conven9onal models of fluvial-‐ (A), wave-‐ (B) and 9de-‐ (C) dominated deltas. (D) – Most flood-‐dominated deltaic deposits are preserved away from the liboral zone and in rela9vely deeper waters. In this zone, these deposits have a much higher preserva9on poten9al than the more marginal ones.
D B
C
The basic ver9cal and lateral facies rela9onships observed in flood-‐ dominated fluvio-‐deltaic systems (9me involved is the dura9on of a Milankowitch cycle, i.e. approximately 100 000, 41 000 or 19-‐23 000) kyr)
(C) (B) (A)
The scheme shows the change in stream regime through 9me: (A) fluctua9ng discharge with episodic catastrophic floods ; (B) rela9vely constant discharge (flood events are the most commonly preserved); (C) stream ac9vity gradually comes to an end concomitantly with extensive pedogenesis
The basic flash-‐flood model: the meaning of flood-‐generated sigmoidal bars Sigmoidal bars
OUTCROP OBSERVATIONS
2
Fine-‐grained laminated sdst
Large mudstone clast
1 Unsegregated (very poorly-‐ sorted and matrix-‐ supported) parental-‐flow deposit
Unsegregated gravelly flows (parental flows) transform into beber-‐sorted and internally stra9fied conglomerates and pebbly sandstones. The se[ng suggests that the denser, coarser-‐grained and faster-‐moving head of the gravelly flow, moving under iner9a forces, is progressively bypassed by more dilute por9ons of the flow loaded with finer-‐grained sediment that deposit progressively finer-‐ INFERRED MODEL grained divisions in a downstream direc9on. Note both ver9cal and longitudinal grading within each flood-‐unit.
Downstream accre9on of sigmoidal flood units (Eocene lacustrine strata, south-‐central Pyrenees)
2 4 3
Each flood unit is characterized by ver9cal and longitudinal grading
1
DOWNSTREAM-‐ACCRETING FLOOD UNITS WITH MULTIPLE BASAL SCOURS Discorso
Eocene, South-‐Central Pyrenees
Oligocene, South-‐Central Pyrenees
Examples of sigmoidal cross bedding from the Cretaceous Troncoso Fm, Neuquen Basin)
Sigmoidal cross strata thinning in a downcurrent direc9on and associated both laterally and ver9cally with slightly finer-‐grained sandstone with HCS.
BEDDING PATTERN OF CHANNEL-‐EXIT DEPOSITS (Cretaceous, Sergipe-‐Alagoas Basin, Brazil)
Erosional surface
Bedding surface
Pebbles
CLOSE-‐UP OF CHANNEL-‐EXIT DEPOSITS – SIGMOIDAL CROSS BEDDING (SCB) PASSING INTO HCS (HCS) IN A DOWNCURRENT DIRECTION
HCS
SCB
SCB
Detail of the sediments recording the final deposi9onal zone (delta slope) of an Eocene fluvial system in the South-‐central Pyrenees (Cas9ssent Group) : sediments of this type contain the most complete record of the history of a fluvial system in terms of cyclically climate-‐driven varia9ons of the fluvial regime through 9me. The alterna9on of dark (silt) and light (mud) layers records seasonal events, mostly river plumes of varying size and sediment concentra9on.
ALTHOUGH OVERLOOKED OR MISINTERPRETED, SEDIMENTS OF THIS TYPE ARE OF PARAMOUNT IMPORTANCE FOR THE STUDY OF PALEOCLIMATES AT THE SCALE OF YEARS, CENTURIES AND MILLENIA -‐ a fascina9ng field of research for future years requiring close coopera9on among sedimentologists, stra9graphers, paleontologists, and paleclimatologists (note the similari9es with tree rings).
END
