• No results found

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

N/A
N/A
Protected

Academic year: 2022

Share "Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary "

Copied!
24
0
0

Loading.... (view fulltext now)

Full text

(1)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Subject Geology

Paper No and Title Sedimentology and Petroleum Geology Module No and Title Delta and Estuary

Module Tag SED & PG IVe

Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad

Vice-Chancellor Jamia Millia Islamia Delhi

Prof. Devesh K Sinha Department of Geology University of Delhi Delhi

Prof. P.P. Chakraborty Department of Geology University of Delhi Delhi

Paper Coordinator Content Writer Reviewer Prof. P.P. Chakraborty

Department of Geology University of Delhi Delhi

Prof. P.P. Chakraborty Department of Geology University of Delhi Delhi

Prof. D. M. Banerjee INSA Honorary Scientist Department of Geology University of Delhi Delhi

(2)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Table of Content

1. Learning outcomes 2. Introduction

3. Classification of Delta

3.1 Classification of Delta on the basis of Geometry 3.2 Classification of Delta on the basis of Feeder System 3.3 Classification of Delta on the basis of receiving basins

3.4 Classification of Delta on dominant grain size and depositional agent

4. Recognition of deltaic sediments and character of delta succession

5. Estuary: Its classification

5.1 Wave-dominated Estuary 5.2 Tide-dominated Estuary

6. Vertical Succession of estuarine product

7. Summary

(3)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

1. Learning outcomes

After studying this module, you shall be able to:

 Know about Delta and estuary, two major shallow- marine environments in contrasting sea level history.

 Learn about depositional agents, sedimentation patterns and relative roles of continental and marine agents operative in these environments.

 Know about the nature of products of these environments, their characteristic criteria for recognition in geological record and types of successions they form in the rock record.

2. Introduction

A delta is defined as the shoreline protuberance away from land at a point where a river meets and debouches its sediment load into a standing body of water i.e. a lake or sea. In contrast, an estuary defines an environment where riverine sediments are reworked by marine agents viz. tide, wave etc. While delta is a product of regressive coastline, an estuary is formed when there is transgression and drowning of fluvial mouth meeting the coastline. Although the word 'delta' is coined in geomorphic sense keeping in mind geometry of Greek alphabet '∆', absence of shape in deposits of ancient counterparts pose a major challenge for sedimentologists and stratigraphers for identification and characterization of deltaic deposits. Importance of delta study can be understood from the fact that 25% of world population live in deltaic setting and ~30% of oil and gas reserves of the World comes from deltaic reservoirs. With construction of dams, river discharge decreased manifold in deltaic systems and caused decay of modern deltas at many parts of Asia. This has become a serious concern for coastal settlements. Understanding of processes involved in growth and decay of delta is therefore a matter of utmost importance. Similarly, estuaries also represent very important depositional setting in terms of human settlement, construction of harbors and incidence of hydrocarbon reserves in course of transgressive sedimentation on a coastline. Sediments are mostly derived in a

(4)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

delta system from rivers unlike an estuary system, which derives sediment from both reverine and marine source.

3. Classification of Delta

Delta represents the transition zone that separates non-marine environments from marine processes and environments. Morphology of a delta and stratigraphy of its deposits depend on several factors such as drainage area lithology, physiography and climate, volume and sediment load of feeder (river) system, as well as nature and intensity of marine processes those rework and distribute riverine sediments.

Delta can be classified on the basis of their geometry or shape. Beside geometry, classification of deltas is also done on the basis of

 Character of feeder system

 Bathymetry of receiving basin

 Dominant grain size and prevalent depositional agent.

3.1 Classification of Delta on basis of Geometry

Depending on relative roles of continental (riverine) and basinal agencies (wave, tide, storm etc.) deltas assume different forms, commonly designated as bird foot, lobate, cuspate etc.

Bird foot delta:

Where marine (wave and tide) energies are low, coastal erosion becomes low and distributary channels build out into the sea forming bird foot delta geometry e.g. Mississippi delta.

Lobate/ Arcuate / Cuspate delta:

When wave and tidal energies are strong, sediment delivered by the feeder system get redistributed as curved ridges in the delta front or along the shore as beaches and spits giving a delta lobate or arcuate shape e.g., Senegal delta, Krishna-Godavari delta etc. (Fig. 1a and 1b).

(5)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Fig. 1a Arcuate delta of Krishna-Godavari Rivers. A vast range of geologic setting viz. coastal basin, delta, shelf-slope apron, deep-sea channel and deep-water fan complex has made this delta a frontier area for hydrocarbon exploration.

Fig. 1b Google Earth image for K-G delta.

(6)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Elongated Delta:

In tide-dominated deltas, tidal channels cut deep into the shoreline and tidal sandbars elongated parallel to the tidal flow are formed e.g.

Ganga-Brahmaputra delta (Fig. 2), India, Mahakam delta, Indonesia.

Deltas occur at a wide variety of scales ranging from continental- scale viz. Mississippi delta (28,500 sq.km in aerial extent) to sub environment-scale in any other depositional system viz. bay head delta in an estuary or lagoon, flood/ebb tidal delta in a barrier bar- lagoon system etc.

Fig. 2 Tide-dominated Bengal delta (Panoramic view from Google Earth). Tidal sand bars oriented parallel to tidal channel flow at a high angle to shoreline.

3.2 Classification of Delta on the basis of Feeder System

Feeder system for delta can be both alluvial and non-alluvial. Fig.

3illustrates different types of feeder system.

 In cases when river, braid plain, alluvial fan and scree cone act as feeder system for a delta, deltas are classified as alluvial delta.

Non-alluvial deltas are pyroclastic or lava flow feed delta systems (Fig. 3).

(7)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Fig. 3 Deltas subdivided into alluvial and non-alluvial varieties (after Boggs, 1995).

3.3 Classification of Delta on the basis of Receiving basins

Receiving basins can be of shallow-water, deep water and accordingly deltas are referred as shallow-water or deep-water delta. Depth (accommodation) of a receiving basin at any time in a delta history is a function of eustacy and tectonics.

 A shallow-water delta tend to have a large delta plain, delta front deposits being formed in shallow water are likely to get more reworked by wave action and sandy/gravelly deltaic sediments expected to be more well sorted.

 In contrast, the delta plain in a deep-water delta is relatively small in extent and large proportion of sediments get deposited in lower part of delta slope; possibility of wave-reworked mouth bar facies is limited because of small area of shallow water where wave action is effective.

(8)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

3.4 Classification of Delta on dominant grain size and depositional agent Sediments deposited in Mississippi and Ganges delta are mainly mud and silt, whereas sandy sediment is deposited in deltas like Rhone delta (France) and gravelly sediment character is noticed in deltas fed by pebbly streams, debris flows or sheet flood dominated alluvial fans (referred to as fan delta).

Lithology, climate and tectonics (relief) in the drainage basin control nature of sediment discharge within any feeder system.

As discussed earlier, dominant depositional agent viz. fluvial, wave or tide further control geometry of a delta (Fig. 4). While fluvial deltas are categorized as high constructive, deltas dominated by marine processes viz.

wave, tide are categorized as high-destructive ones.

Fig. 4 Morphology of modern deltas (after Nicols, 2009) with dominance of fluvial (b), wave (c) and tide (d).

(9)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Outflow dispersion and mixing in river-dominated delta:

Dispersion of feeder outflow within any receiving basin takes place in three different motifs and control sedimentation patterns in delta plain and delta front environments. These are

a) Inertia-dominated: This happens in case of three-dimensional homopycnal mixing i.e. when density of river outflow match well with density of receiving basin water. As a result, there is three- dimensional mixing of water bodies, deceleration of outflow jet and hence, outflow jet travel far into the basin. Delta forms in such case is Gilbert type i.e. with topset, foreset and bottom set.

b) Friction-dominated: Also referred to as hyperpycnal mixing i.e.

where higher density outflow enters into lower- density shallow nearshore waters. Outflows move as underflows, get decelerated and spreads laterally rapidly forming bifurcating channels with middle ground bars.

c) Buoyancy-dominated: In case of hypopycnal mixing i.e. less density outflow enter into higher density receiving basin water e.g. fresh water enters into a marine basin, lighter outflow water floats like a buoyant plume above salt wedge. Deposition occurs because of mixing of two different waters and deposits extend far distance from the feeder channel. Lateral bars with subaerial and subaqueous levees are common.

Delta environment and processes:

A delta environment is subdivided into a number of sub-environments depending on bathymetry, operative depositional processes, slope gradient and facies types developed. Besides, floral and faunal characters also help in identifying subaerial and subaqueous sub environments of a deltaic setting.

On the basis of depositional processes and facies characters, four major

(10)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

anatomical parts are defined from a deltaic setting viz. delta plain, delta front, delta slope and prodelta (Fig. 5).

Fig. 5 A fluvio-deltaic depositional model (plan view and cross-section) with sand deposition continuous from fluvial to marine environment.

Feeder system (river or alluvial fan) forms the delta top sub environment, also referred to as delta plain. Channel (meandering, braided, single or branched), levee and overbank constitute the subaerial parts of delta plain.

While sediments get actively transported as bedload through channels,

(11)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

sediment settlement from suspension takes place in overbank areas. In wet tropical regions large, vegetated swamps may form on delta plain those act as repository of peat. In addition, lense-shaped sand deposits can be found in delta top, deposited from crevasse splays at high-flood stages. Inter- distributary bays are sheltered areas of low energy between elongated channels along the edge of delta plain, protected from marine agents.

With decrease in gradient, the feeder channel commonly branches into a number of distributary channels. On meeting the receiving basin, velocity at the mouth of these channels drop abruptly and sediments carried as bedload in these channels get deposited at the mouth of these channels as lobe, termed as ‘mouth bar’ (Fig. 6). Association of distributary channel and mouth bar defines the second sub environment of a delta i.e. delta front.

Though both channel and mouth bar results in lensoid sand bodies, distinction between their respective products can be made in rock record from erosive base and fining-upward character of a channel sandstone and gradational base, coarsening upward character of a ‘mouth bar’ sandstone.

Mouth bars are a fundamental depositional element in any modern delta, which coalesce to form complex, bar assemblage and, in turn, build regional scale depositional lobes. Sediments deposited in mouth bar may further get reworked by marine agents i.e. wave or tidal current in wave-/tide- dominated deltas. Waves smooth out and elongate mouth bars in shore parallel orientation whereas tide current dissect mouth bars and elongate them in shore normal direction. In wave-influenced delta mouth bars may get reworked into a shoreface, which is considered as a part of delta. Shape and extent of such shoreface depends on interaction between sediment supply and wave energy in the coastline.

(12)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Fig. 6 Block diagram illustrating distributary channel-mouth bar complex in a delta front setting.

Delta slope is a high-gradient part of delta, incline away from delta top and common in Gilbert-type delta. Gradient of delta slope varies from 1-20 in fine-grained deltas to as high as 300 in coarse-grained deltas. Slumping, mass failures and sediment gravity flows are common in delta slope environment. Delta slope represents an extensive sub environment in deep- water delta where coarse sediments get remobilized in form of debris flows, relatively fine sediments mix with water to form turbidity currents and lower part of delta front becomes main site of deposition.

Most distal part of a delta i.e. prodelta receives river-borne suspended load in the form of sediment plumes. River water with suspended load being lighter than the saline marine water float as buoyant plume in sea and get distributed over a large area, much away from the delta plain. Mixing of two waters allow suspension settlement of fresh water borne sediments in the distal prodelta. Besides, occasional gravity failure in the delta front can

(13)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

bring relatively coarse sediments (silt, fine sand) in the prodelta in form of turbidities.

4.

Recognition of deltaic sediments and character of delta succession

Close association with fully marine and continental products is a key recognition element for a deltaic succession. A single delta succession may show a complete vertical transition from fully marine products at the base to subaerial continental products at its top. Common products of a delta i.e. conglomerate, sandstone and mudstone are found compositionally immature to sub-mature on delta plain and relatively mature in wave reworked delta front. Bed geometry is mostly lensoidal whether it is a product of a channel or a mouth bar, except in the prodelta where thin, tabular beds are common.

A long-lived, constructive delta develops a progradational succession. The succession is typically coarsening upward from low-energy fine-grained deposits of prodelta at the base to the high-energy delta front with wave-reworked mouth bar deposits where coarser sediments accumulate.

A typical deltaic complex consist of superimposed clastic delta wedges, each normally coarsening upward. Components present within each such coarsening- upward package include deposits of delta plain (channel-levee-floodplain, bar finger i.e. clean non-marine coarsening upward cross-bedded sandstone), delta front (delta fringe, distributary channel, mouth bar) and prodelta clay (Fig. 7).

However, within the overall progradational stacking motif, products of individual depositional element viz. channel, mouth bar may vary. A channel may be represented by a fining-upward deposit with a erosional base and unidirectional current element, except for when it is tide influenced where bidirectional current elements may be common. A shallow water delta front mouth bar may be extensively reworked by wave or tidal action resulting in cross-stratified mouth bar deposit. The geometry and extent of mouth bar, however, depend on the dominance of prevailing process i.e. river, wave or tide.

(14)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Fig. 7 An overall coarsening-upward succession of regressive fluvio-deltaic system.

(15)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

5.

Estuary: Its classification

Unlike delta, which is a product of regressive coastline, estuary is a product of transgressive shoreline. Estuary is formed with relative rise in sea level when seaward portion of a river valley gets drowned and reworked by marine agents. In estuary, sediment is delivered from both riverine and marine sources and combination of riverine, wave and tidal energy controls sediment transport, geometry, operating process and facies distribution. Estuaries are common at present day river mouths as the river mouths are drowned with Holocene transgression. Dalrymple (1992) identified two end members in estuary system viz.

wave-dominated and tide dominated and provided facies distribution model for either of them. Many estuaries also have mixed wave and tidal signature. Estuarine setting commonly contain brackish water fauna those include Oysters, pelecypods, gastropods and mussels.

5.1 Wave-dominated estuary:

Fig. 8 illustrates plan view of a wave-dominated estuary depicting facies, grain size and energy distribution. A wave-dominated estuary displays bay- head delta in its proximal fluvial part, low-energy lagoon at the centre and beach- barriers elongated parallel to the shoreline in the outer marine part where wave action is extensive. The bay head delta forms as the river system enters the central low-energy lagoon and decelerates. Processes and facies pattern of a bay head delta is similar to a fluvial-dominated delta as both tidal and wave energy are minimum. A coarsening-upward succession constituted of channel and overbank fine resting over channel mouth sand commonly represents deposit of a bay head delta. Restriction of riverine energy in the proximal part and wave energy within distal barrier bars make the central part of a wave-dominated estuary low-energy domain, fine- grained in character and often rich in organic matter. Depending on tidal range, distal barriers may either totally close the estuary from open marine system and thereby transform the estuary into a blind estuary/coastal lake or

(16)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

allow the marine energy to enter the estuary intermittently through inlets and result in influx of wave-rippled sandstones with draping of mud.

Deposition of mud in central lagoon is caused by flocculation of clay particles as a result of mixing between fresh riverine water and saline marine water, which allow neutralization of negatively charged clay particles by positively charged marine water.

Fig. 8 Plan view, energy distribution and spatial facies variation in a wave- dominated estuary (modified after Dalrymple, R.W, Zaitlin, B.A and Boyd, R., 1992 Jour. Sed. Petrol. V. 62, Fig. 4, p.1134).

Extensive wave action and operation of longshore drift result in reworking of marine sediments and formation of barrier bars at the distal part of wave- dominated estuary. Occasionally, marine sediments are also transported

(17)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

landward beyond the limits of barrier bars in form of wash either over fan or as flood tidal delta sands.

5.2 Tide-dominated estuary:

These estuaries commonly form in mesotidal (tidal range 2-4m) to macrotidal (tidal range >4m) coastlines where tidal current energy exceeds wave energy at estuary mouth. In comparison with wave-dominated estuary, much higher energy condition prevail in a tide-dominated estuary and thereby sediments in a tide-dominated estuary are more sandy. Because of funnel shape of an estuary, energy of flood tidal current increases near the constricted part of estuary resulting in formation of Upper Flow Regime Sand Flat (UFR Sand Flat) and further decelerates towards the proximal part of estuary until it reaches the tidal limit i.e. most landward limit up to which tidal influence can be traced.

The distal (inner) part of tidal estuary is represented by tidal bars, formed by reworking of marine sediments by tidal current, elongated parallel to the length of the estuary and at a high angle with the shoreline. Occurrence of herringbone cross-stratification, mud drape, reactivation surface, flaser bedding etc. bear evidence for tidal origin of these bars. Tidal bedforms of different scale ranging from ripple to dune are found migrating in sandy sediments on the bars and within tidal channels. In macrotidal set up strong tidal current moves sand, gravel and bioclastic debris and mud drapes form when tidal current slows down and returns back. Constriction of tidal current between the tidal bars energize tidal current and create sand flat of upper flow regime condition.

Landward, the low-gradient tidal channel adopts meandering form with development of point bars on inner banks of meander bends. Inclined stratifications, constituted of alternation between sand and mud deposited from unsteady tidal current, represent point bar deposit of a tidal channel.

Further landward in the outer part of the estuary the tidal channel becomes

(18)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

straighter, interact with fluvial channel, and thereby, offer fluvio-tidal signature. Beyond tidal limit, one may get only unidirectional fluvial signature.

Fig. 9 Plan view, energy distribution and spatial facies variation in a tide- dominated estuary (modified after Dalrymple, R.W, Zaitlin, B.A and Boyd, R., 1992 Jour. Sed. Petrol. V. 62, Fig. 4, p.1136).

6.

Vertical Succession of estuarine product

Character of estuary succession depends on type of estuary (wave-, tide- or mixed wave-tide dominated) and location within the estuary. Cross-bedded, often bidirectional and with reactivation surfaces, and bioturbated sandstones are products of inner part of tide-dominated estuary with fluvio-tidal channel in the outer most

(19)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

part. Products of non-channelized middle or outer parts of wave-dominated estuary are homogeneous or stratified mud. As estuaries are associated with shoreline transgression, an estuarine succession is commonly constituted of estuary-mouth sand i.e. cross-stratified bar sandstone, UFR sand flat etc. which is successively underlain by middle estuary mud/ heterolithic sand-mud and fluvio-tidal sandstone.

In case of regression, an estuary gets filled by riverine sediment.

7. Summary

 Delta and estuary are two most important environmental set ups in regressive and transgressive coastlines. While basin ward shoreline protuberance formed with discharge from a river system on meeting a marine/lacustrine basin is referred to as a delta, drowning of a river mouth in course of marine transgression result in formation of estuary.

 Geometry, size and internal architecture of deltas are functions of multiple variables including character of feeder system and nature of receiving basin.

 A tripartite classification of delta viz. river-, wave- and tide-dominated is well established.

 A single delta cycle may show a coarsening-upward succession made up of continuous vertical transition from fully marine conditions at the base to subaerial fluvial setting at the top. Except for prodelta deposits, which as thin bedded and sheet like, bed geometries in most parts of a delta are lensoidal and variably elongate, whether it's of channel origin or mouth bar origin.

 In wave-dominated deltas extending wave reworking of delta front mouth bars in shallow water give rise to formation of extensive shore face.

Characterization of any modern or ancient delta needs to be done in terms of grain size, depth of the receiving basin and dominance of operating system i.e. river, wave or tide.

(20)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

 In contrast to deltas, which are progradational sediment body built out into the sea, estuaries are mostly aggradational and developed with flooding of river mouth. Estuaries are either wave- or tide-dominated. Whereas wave- dominated estuaries have low-energy central lagoon, depositional energy, in general, remain high in tide-dominated estuary, as funnel shape of estuary tends to increase flood tidal current strength giving rise to formation of Upper flat regime (UFR) plain bed.

Frequently Asked Questions-

Q1. What are favorable conditions for formation of delta and estuary on a coastline?

Ans: A delta is defined as the shoreline protuberance away from land at a point where a river meets and debouches its sediment load into a standing body of water i.e. a lake or sea. In contrast, an estuary defines an environment where riverine sediments are reworked by marine agents viz. tide, wave etc. While delta is a product of regressive coastline, an estuary is formed when there is transgression and drowning of any fluvial system meeting the coastline.

Q2. Define 'mouth bar'. What kind of deposit you would expect in a mouth bar?

Ans: Stream mouth bars, also called distributary mouth bars, are formed when confined flow in a stream becomes unconfined with loss of gradient on entering a standing body of water (lake or sea) and deposit its sediment load at the mouth. They may form in relatively shallow or in deep water in stable or subsiding basins. There is a continuous textural variation along the depositional slope of a mouth bar with coarser material near the stream mouth that gets finer outward and down. A vertical section reflects facies variations found along the depositional slope i.e. fine sand at bottom and coarser upwards.

(21)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Q3. What are meant by terms 'constructive delta' and 'destructive delta'?

Ans: Deltas form at the interface of continental and marine environments. Processes, which control geometry, facies and evolution of a delta, include both continental (riverine) and marine (wave, tide) in origin. A delta is considered at its constructive stage when it is river dominated and developed in coastline, which is microtidal in character and where waves undergo active dissipation of energy before reaching the coastline. Deltas form in such cases are of 'bird foot' geometry and involve continuation of unidirectional fluvial current as subaqueous flow within lake or sea maintaining the channel form and well defined subaqueous levees and overbank areas. These river-dominated deltas develop well-grown delta top channel-levee- flood plain facies with prolific growth of vegetation in flood plain that give rise to peat and coal deposit.

In contrast, deltas where marine processes (wave and tide) become dominant sediments carried by rivers get reworked and redistributed by marine agencies, thereby produce more lobate geometry for the delta. In case of wave dominance wave reworking of delta front distributary channel mouth and mouth bar sediments result in formation of shoreface environment. Mouth bar sediments in such case become better sorted in comparison with mouth bars in bird foot deltas. In case of tidal dominance, the delta top channels come under tidal influence and overbank areas act like tidal flats. Strata with reversal of paleocurrent and mud drape are common product in such setting. Subaqueous mouth bars will be parallel with the orientation of river channel.

Q4. What is a 'fan delta'? What kind of process dominate in a fan delta setting?

Ans: A fan delta refers to a coastal prism of sediment, delivered by an active debris flow and sheet flood-dominated alluvial fan system and deposited in a subaqueous setting of a standing body of water i.e. lake or sea. Although the term 'fan delta' is, in general, coined for coarse grained deltas fed by rivers, dominance of fluvial, wave or tidal process is reported in fan delta settings case to case basis. Subaqueous portion

(22)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

of a fan delta system is dominated by processes like slumping and turbidity current, which carry sediments down slope.

Q5. What kind of facies stacking pattern one would expect in case of progradation of a river dominated delta?

Ans: An active delta growth and progradation results formation of a coarsening- upward sedimentary succession. At a relatively smaller scale progradation of a mouth bar sand on prodeltaic silt and clay, also result in coarsening-upward succession, which remain ingrained within overall coarsening-upward succession of a delta. Progradation of subaerial delta plain on subaqueous delta plain sediment also result coarsening upward succession. Hence, a progradational delta deposit is an overall coarsening-upward sedimentary succession which includes many relatively smaller-scale progradational successions at different hierarchy. In this backdrop, channel (feeder or distributary) deposits, particularly abandoned channels, may develop fining-upward successions.

Q6. Discuss how outflow processes in a river mouth influence delta character of a river-dominated delta?

Ans: Character of out flows in a river dominated delta depend on relative dominance of either of any three processes viz. i) inertia ii) turbulent bed friction and iii) buoyancy. Whereas inertia dominated outflow result narrow river mouth bars of the Gilbert type, turbulent friction cause channel bifurcation with formation of middle ground bar and buoyancy domination result in formation of long distributaries with subaqueous levees and narrow mouth bars those grade downward to fine grained distal bar and prodelta deposit.

Multiple Choice Questions- 1. A mouth bar deposit documents

(a) Sharp, erosional base (b) Gradational base

(c) Sharp, planar non-erosional base Ans: b

(23)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

2. Delta plain facies is best developed in (a) Lobate delta

(b) Cuspate delta (c) Bird foot delta Ans: c

3. We get delta slope environment in (a) Lobate delta

(b) Bird foot delta (c) Gilbert type delta Ans: c

4. Middle ground bar and bifurcation of distributary channels are common in fluvial deltas with dominance of

(a) Inertia in out flow

(b) Turbulent friction in out flow (c) Buoyancy in out flow

Ans: b

5. Bay head delta is an integral part of (a) Bird foot delta

(b) Tide-dominated estuary (c) wave-dominated delta Ans: c

6. Non-marine, cross-stratified clean sandstones found at fringes of channel mouths in a delta plain environment are referred as

(a) Mouth bar sand (b) Bar finger sand (c) Flood plain sand Ans: b

7. A progradational fluvial delta will result (a) Fining upward succession (b) Coarsening upward succession (c) Aggradational succession Ans: b

(24)

GEOLOGY

Paper: Sedimentology and Petroleum Geology Module: Delta and Estuary

Suggested Readings:

1. Sam Boggs Jr. (2011). Principles of Sedimentology and Stratigraphy, 5th Edn. Pearson Education, Inc., New Jersey. ISBN: 9780321643186, 0321643186.

2. Gary Nicols (2009), Sedimentology and Stratigraphy, 2nd Edn., Wiley- Blackwell, UK. ISBN: 978-1-4051-3592-4.

3. Posamentier, Henry W. and Walker, Roger G. (2006), Facies Model revisited, SEPM Special Publication 84, U.S.A. ISBN: 978-1-56576-121-6.

4. Reading, Harold G. (1996), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd Edn., Wiley-Blackwell, UK. ISBN: 978-0-632-03627-1.

References

Related documents

sand in the form of coalescing bars, over a relatively short period of geological time. However, these sand sheets are not as continuous as those of braided stream

 The cyclic sedimentation and their broad characteristics in the global stratigraphic record can be related to the changes in continental scale to changes in

However, mere presence of reservoir quality (porosity and permeability) is not a guarantee of getting oil or gas. Source rock and cap rock is also necessary

Paper: Hydrogeology and Engineering Geology Module: Geotechnical Investigations for Dam and Reservoir Sites.. Subject

 Major sedimentation processes in the deep sea include slow but continuous suspension fall-out of fine particles (pelagic and hemipelagic sediments) and local authigenic

(a) Fuel Geology: Maps and exercises related to coal geology, Study of geological maps and sections of important oilfields of India, Exercises and maps related to petroleum

Isle, Lake Erie, Pennsylvania. An analysis of long- shore currents and assosiated sediment transport in the surf zone. Observations of beach cusps and beach ridge

By averaging K samples out of the quantized sigma-delta output, the decimation filter achieves a high output resolution and also the frequency of the output data is at