LINGKUNGAN PENGENDAPAN
LINGKUNGAN PENGENDAPAN(LP)
(LP≈ Depositional Environment/ DE)adlh keadaan (a natural geo-graphic entity) yg komplek, yg disebabkan adanya
interaksi antara faktor2 kimia, fisika & biologi dimana sedimen di endapkan (terakumulasi) & keadaan tsb dpt
dibedakan dng keadaan yg lain.(Krumbein)
Krn adanya perbedaan, mk masing2LP dpt untuk mengidentifikasi media pengendapan, proses (energi fisik)
pergerakan material yang tersedimenkan.Sedimentary Environtment, is a part of the earth’s surface in which is
physically, chemically & biologically distinct from adjacent areas (Selley, 1970)
Interaksi antara unsur2 kimia, fisika & biologi dimana sedimen di endapkan, antara lain yi:
1. Material sedimen. Spt; apakah jenis & komposisi batuan sumbernya
2. Kondisi pembatasan (boundary condition). Spt; apakah diendapkan jauh dekatnya dari batuan sumber, apakah
proses fisik-mekanis yang berperan, atau unsur kimiawi?, atau interaksi kimiawi-fisik-energi
3. Enersi (Mekanis).Spt; apakah mempunyai kondisi tenang?, enersinya rendah/ tinggi
4. Kimia-fisika. Spt a). Apakah diendapkan pd lingkungan pH & Eh tertentu, atau b) Mempunyai kadar garam
(salinitas), & konsentrasi kelarutan karbonat tertentu, atau c) kondisi dengan temperatur tertentu.
5. Adanya Aktifitas biologi spt: a) struktur pertumbuhan, b) adanya cangkang, c) Material organik (C-H), atau d)
adanya struktur galian (burrow).
Upaya menentukan LP suatu bat sedimen mrpkn hal yg penSedimentology
ting, al: untuk mengetahui sejarah geologi, studi cek suatu
daerah, informasi untuk kegiatan eksplorasi dll Gambar.1.
Depositional
Illustrasi di atas bisa diartikan:
Environment
> Dng mengetahui sedimentologi (tekstur, kompss, strktr
Geometry
sed, proses) mk dpt untuk menentukan LP (de)
Setting Tectonic
Geography
> Dng mengetahui LP dpt untuk menafsirkan paleoGambar.1.Illustrasi keberadaan sedimentologi dng
geografinya (delta lobate, shore line dll),
ttnan tektonik, li-peng, & paleogeography.
> Dng mengetahui LP dpt menafsirkan ttnn tektonik ketika
bat tsb di endapkan,
DEPOSITIONAL MODEL(DM) adlh rangkaian depositional environment yg dikaitkan dng proses2 sediment.
 Depositional System bertanggung jawab thdp:
1) Besarnya ketebalan stratigrafi,
2) Perubahan2 lingkungan pd salah satu bag system (pd umumnya dpt dikenal dibagi lain dr system meskipun
jarak jauh)
 Depositional System& sedimen2 yg dihasilkannya merefleksikan integrasi autugenik &alogenik kontrol.
 Cek sed dng mekanisme pembentukanyg berbeda mengakibatkan kumpulan fasies & Depositional System yg
berbeda
 Proses yg berlaku pd LP tertentu menimbulkan sekumpulan karakteristik berskala relatip kecil pd endpn
sedimen yg dihasilkan
FASIES
 Fasies mrpkn suatu tubuh bat yg memiliki kombinasi yg khas dr litologi, strktr sed & strktr biologi serta
memperlihatkan aspek 2 yg berbeda dr tubuh bat di bawah, di atas &atau di sekelilingnya.(Boggs, 1987)
 Fasies umumnya dikelompokkan ke dlm facies association dimana fasies2 tsb berhub scr genetis, shg asosiasi
fasies ini memiliki arti lingkungan. Dlm skala lebih luas asosiasi fasies bisa dipandang sbg basic
architecturalelement dr suatu li-peng yg khas shg akan memberikan makna bentuk 3 dimensi tubuhnya (Walker,
1979).
 Fasies sedimen adlh suatu satuan bat yg dpt dikenali & dibedakan dng satuan bat yg lain atas dasar geometri,
litologi, struktur sedimen, fosil, & pola arus purbanya. (Selley ,1985)
 Jmlh total dr atribut2 berskala kecil ini membentuk satuan batuan tertentu yg berbeda, yg bila digabungkan
disebut fasies. Steno 1669, Gresely 1938, menyebutnya suatu satuan untuk menunjukan sekumpulan sifat
paleontologis & litologi dr suatu satuan batuan.
 Fasies sedimen mrpkn produk dr proses pengendapan bat sed didlm suatu li-peng nya. Diagnosa li-peng tsb dpt
dilakukan berdsrkan analisa fasies sed, yg merangkum hasil interpretasi dr berbagai data, al:
a. Geometri :
 Regional & lokal dr seismik (mis : progradasi, regresi, reef &channel).
 Intra-reservoir dr wireline log (ketebalan & distribusi reservoir).
Sedimentologi
1
LINGKUNGAN PENGENDAPAN
b. Litologi: dr cutting,&core dikombinasi dng log sumur (GR & SP).
c. Paleontologi: dr fosil yg diamati dr cutting, core, atau side wall core.
d. Struktur sedimen: dari batu inti.
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FASIES MODEL(FM) adlh suatu alat interpretasi yg digunakan untuk menerangkan asosiasi fasies. Proses
pemodelan fasies adlh fungsi penerangan dng mengkaitkan observasi pd proses2modern dan endapan2purba
menjadi suatu sintesa yang koheren
Keberadaan FM harus:
- Dpt digunakan dlm berbagai cara yg berbeda,
- Hrs menggabungkan banyak data menjadi suatu bentuk yg bersifat umum, mengenai proses2 sedimentasi,
- Hrs menjadi stimulan untuk penelitian selanjutnya & berlaku sbg peramal pd situasi geologis yg baru,
- Hrs membantu memberikan pandangan dlm interpretasi sat sed yg dinamis.
Genesa & perkembangan FM diekspresikan pd urutan (sikuen) vertikal.
Produk/ hasil pengendapan yg di endapkan scr unik di suatu LP disebut fasies sedimentasi (Sedimentary
Facies). Jadi pengertian fasies sedimen adlh tubuh bat sedimen yg dpt didefinisikan, serta dpt dibedakan dr
tubuh bat sedimen yg lain dr segi geometri, & ciri litologi (fisika, kimia, & biologi) yg sangat khas
Fasies umumnya dikelompokan kedalam asosiasi fasies, dimana fasies2 tsb berhub scr genetik shg asosiasi
fasies ini memiliki arti LP ((Walker, 1979)
Facies: overall aspect or set of characteristics of a rock, which reflectes its particular d-e & se it off from
adjacent facies within same rock unit.Fasies, term coined by Steno (1699), entire aspect of part of the earth
surface during a certain part of the earth history.1970s & 80s: facies linked to formations. Lithofacies: lithological
characteristic of a formation: sandstone f, shale f. Biofacies: biological characteristics of formation, shelly f,
crinoidal f, graptolitic f, etc.Also linked with d-egenetic interpretation: shallow marine f, fluvial f, deltaic f, reef f,
lagoonal f, etc (Walker & James)
Pola tumpukan perlapisan sedimen klastik (yg biasanya menunjukan suatu siklus - ritme), mrpkn indikasi adanya:
- Penambahan energi transport ke arah atas (pengkasaran & penebalan lapisan ke arah atas).
- Penurunan energi transport, mengekspresikan penghalusan & penipisan lap ke arah atas
- Siklus bisa dihasilkan dr proses2 alam di dlm LP (autocyclic) atau kontrol dr luar (allocyclic) LP.
- Tipe mekanisme autocyclic, spt: meandering, avulsion dr river channel. Tipe mekanisme allocyclic, tectonic
movement, climatic variation.
- Mekanisme Autocyclic & allocyclic, penting artinya dlm pembentukan paket stratigrafi: parasikuen.
Vertical sikuen dpt diinterpretasikan dr data out crop, geophysical, sumur. Contoh bentuk profil berdsrkan log:
 Upward fining cycles nampak sbg bellshaped log pattern (▲)
 Upward coarsening cycles nampak sbg funnel shaped log pattern (▼)
 Amalgamated fining & coarsening-up unit sbg symmetrical
 Unit dng tanpa vertical trend porositas atau kandungan lempung nampak sbg cylindrical log pattern.(▐)
Tiga mekanisme utama dlm sedimentasi, yi: proses erosi, transportasi&proses pengendapan. Ke3 proses utama
tsb di terjadi dlm ruang & waktu (space & time) Studi 3 demensi/ 2 demensi geometri sedimen menimbulkan kosep
arsitektur, cara dimana individu tubuh sedimen tertumpuk dlm ruang & waktu.
Ada bbrp cara untuk mengetahui LP, contoh: Phleger (1960), Bandy (1988) Tipsword (1980), menggunakan
aspek biologi sbg cara untuk menentukan li-peng, dimana dng menggunakan asosiasi, atau spesies tertentu dari
foraminifera bentonik. Bbrp ahli menggunakan fasies model sbg cara untuk menentukan LP spt: Walker (1984),
Wilson (1964), Reineck & Singh (1980), Miall (1978), Reading (1978) Allen, Horne (1978) dll. Ada juga yg
menggunakan pendekatan data sekunder spt wireline logging (Selley, 1978), seismic,
Berdsrkan aspek2 tsb (fisik, kimia & biologi) scr klasik, tdpt 3 kelompok li-peng utama yaitu:
1. Lingkungan Continental(lithoral), dimana proses erosi, transportasi & pengendapan terjadi di sungai, danau,
atau gurun, shg menghasilkan endpn fluvial atau lacustrin atau endpn eolian dng sub terrestrial(tdr dr endpn
gurun/ dessert, & endpn glacial/ salju) & ling aqueous (tdr dr endpn fluvial, paluda, lacustrin, & endpn gua /cave)
2. Lingkungan Transisi (Transisional) atau grs pantai (atau/ hingga endpn neritik),termasuk didalamnya al: Li-peng
delta dimuara sungai, dimana lingkungan tsb sgt dipengaruhi aktifitas aliran bawah permukaan (offshore flow)
serta panjang & bentuk dr grs pantai yg dipengaruhi oleh arus, gelombang, atau pasang-surut., dgn sub
lingkungannya adlh lingkungan deltaic, eustarine, lagoonal, litoral /intertidal
Sedimentologi
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LINGKUNGAN PENGENDAPAN
3. Lingkungan Laut (Marine), dng sub lingkungan terumbu karang (reef), neritic, bathyal, & abysal (Gambar -1)
Klasiffikasi li-peng menurut beberapa ahli, sbb:
TERRESTRIAL
CONTINENTAL
AQUEOUS
TRANSITIONAL
MARINE
Desert
Glacial
Fluvial
Paluda
Lacustrin
Cave
Deltaic
Eustarine
Lagoonal
Litoral (intertidal)
Reef
Neritic
Bathyal
Abysal
Classical type of classification
sedimentary Environtment
of
FANGLOMERAT
FLUVIAL
CONTINENTAL LACUSTRIN
Braided
Meandering
EOLIAN
SHORELINES
MARINE
LOBATE (DELTAIC)
LINIER (BARRIER)
Reef
SHELF
TURBIDIT
PELAGIC
Terrigenous
Mixed Carbonate - Terrigenous
Carbonate
Gambar.1. Classification of Depositional SedimentaryEnvirontments
(Selley, ‘78)
Gambar 2 Clastic Depositional Enviroment
(Walker, 1992)
Klasifikasian LP yg tdr dr 3 kelompok lingkungan pengendapan utama yaitu: (Selley, 1978)
1. Li-pengContinental, dg sub li-peng a) Flaglomerat, b) Fluvial (tdr dr endpn sungai teranyam/braided stream, &
endpn sungai berkelok/ Meandring), c) Lacustrin, & c) Eolian (endapan gurun)
2. Lin-penggrs pantai (shore lines), dg sub-li-peng a) Delta (lobate), b) grs pantai linier/ barrier (dng endpn
Terrigenious/ asal darat, endpn Mix carbonate-terrigeneous, & endpn Carbonat)
3. Li-pengLaut (marine), dg sub ling terumbu karang/ reef, paparan/ shelf, turbidite, & endapan pelagic
Klasifikasikan R.Walker & James (1992)
Lingkungan
Sub-lingkungan pengendapan
1. Continental
- Fluvial (alluvial fan, braided stream, meandering stream, anastomasing stream)
- Desert (subtropical desert, sand dunes, sand seas, ephemeral, salt lake, cold desert in
high latidue: deflected glacial outwash plains, Lacustrine, &Galcial
2. Marine –Marginal - Deltaic (Delta plain, Delta front, Prodelta)
- Beach/ Barrier bar, Estuarine (drowned river valleys, mud flat, swamp), &Tidal flat
(peritidal)
3. Marine
- Continental shelf, Continental slope, &Abyssal plain (deep oceanic floor)
CONTINENTAL SYSTEM & FACIES
- Fluvial Style: complex auto & allocyclic
- Allocyclic: iklim berhub dng aliran permukaan (run-off) & pelapukan bat induk/ asal. Tektonik mengontrol lereng
cek (basins slope) & relief hinterland dlm drainage basin. Dr interaksi tsb menghasilkan sistim sungai dng
karakteristik yg berbeda mis: sediment load, discharge, slope, vegetasi yg menentukan pola saluran.
- Tipe cek alluvial dpt diklasifikasikan menurut kriteria: (1) kehadiran sistim pengeringan longitudinal atau
transverse, (2) Sifat elemen proksimal, medial & distal
- Tectonically active basin margin, tdr dr 3 elemen yi: (1) Proximal: aluvial fan. (2) Medial:braidplain&high sinusity
alluvial system dr tipe longitudinal/ tranverse. (3) Distal: lake margin, terminal fan, sabkha, & eustuarin
- Pola saluran sungai adlh sngt tanggap thdp tipe load, discharge, slope, &vegetasi. Pola ini bisa dibagi menjadi
bbrp tipe berdsrkan: (1) suspended, mixed atau bed-load transport, (Schumam 1978), (2) channel sinusity
(Leopold & Wolman 1957, Rust 1978) (3) single atau multiple thalweg (cf. Rust1978), braiding parameter
Sedimentologi
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LINGKUNGAN PENGENDAPAN
- Pola saluran migrasi lateral & akresi vertical pd sungai & dataran limpah banjir semuanya akibat suatu keseimbangan yg rumit dr variabel2 scr luas.Tipe sungai: Meandering, Braided, Straight, dan Anastomoses.
- Lateral stability saluran sungai sgt mempengaruhi geometri tubuh sedimen (Friend 1983).
- Fixed channels menghasilkan endpn saluran dng pelamparan sgt terbatas pd penampang sungai ribbon.
- Mobile channel belt berasosiasi dng migrasi lateral, & pembentukan sedimen sheets
- Unchannelized menghasilkan endapan sheet flood juga sheets yg melampar luas.
I. Sistem FluvialFacies
 Ruang Lingkup Diskusi : (1)Geometry , (2) Proses & klassifikasi, (3) Kipas Aluvial (Alluvial Fan), (4) Sungai
teranyam (Braided river), (5) Sungai berkelok (MeanderRiver, (6) Sequence Stratigraphy
 Sisitim alluvial dibagi menjadi 3 area geografi (1) Upstream area – Piedmont, (2) Mid Stream area – Alluvial
Plain, (3) Down Stream area – Delta Plain
 Fluvial mrpkn aktivitas aliran sungai. Sistem fluvial dpt dibagi 3, yi (Reineck & Singh, 1973) :
Meander
menunjukkan gradasi normal yg didominasi oleh material
dng butiran halus & memperlihatkan distribusi butiran
menghalus ke atas (F-U) Bag dasar sortasi jelek &
ukuran butir kerakal - pasir kasar. Bergradasi ke atas
menjadi pasir sedang - halus dng sortasi baik. Batas bag
atas umumnya tdr pasir sgt halus, lanau &
lempung.Strktr sed yg berkembang merefleksikan
berkurangnya arus yg bekerja, yi through x-bedding pd
bag bawah & pararel laminasi pd bagchannel.
Penampang log elektrik merefleksikan arah umum F-U
yg terbagi atas 3 sub fasies utama yg menghasilkan
Gambar. 3. Sistem fluvial dng respon log GR
pengendapan pd 3 sub lingkungan yg berbeda, yaitu:
(Coleman,1981)
a. Sub fasies Flood PlainEndpn batupasir sgt halus, lanau & lempung diendapkan pd daerah overbank
floodplain sungai. Strktr sed yg berkembang adlh laminasi, ripple mark& kadang2 tdpt horison batupasir yg
mengisi struktur shrinkage, yg diasumsikan tdpt pd daerah subaerial.
b. Sub fasies ChannelPerpindahan lateral channel meander mengerosi bgn luar dr tepi sungai yg cekung,
menggerus dasar sungai & endpn sed pd point bar. Proses tsb menghasilkan karakteristik sikuen pd ukuran
butir & strktr sed. Pd dasar permukaan bid erosi disi oleh material sedimen berbtr ksr, mud pellet dr sisa2 kayu.
Endpn tsb disebut sbg lag deposit pd dasar channel& ditindih oleh sikuen btpasir dng distribusi F-U.
c. Sub fasies Abandoned ChannelEndpn btpasir hls berbentuk tapal kuda tdp pd sub fasies ini & disebut oxbow
lake yg terbentuk ketika sungai meander memotong bag lain dr permukaan disekitar sungai tsb. Endpn pd sub
fasies ini serupa dng endpn pd sub fasies floodplain, ttp dpt dibedakan dr geometrinya yi endpn yg menindih
abrasi konglomerat channel lag tdk tdpt selang dng sikuen batupasir point bar.
Braided,
sungai teranyam dihasilkan oleh channel dng intensitas kelokan yg kecl & kaya akan material pasir yg terbentuk oleh
tingkat intensitas aliran air yg kecil diantara channel bar2 (Boggs, 87). Strktr sed yg terbentuk & merefleksikan
pengendapan pd saat itu, salah satunya adlh x-bed tabular pd punggungan bar yg lurus memanjang. Pd log GR menunjukan bentuk bloky (Serra,89). Daerah ini, pengerosian terjadi dng cepat, dng proses pengisian sed yg cepat
dikrnkan sungai pd sistem ini mempunyai kelebihan material sed. Sikuen sedimentasi pd sistem braided ini umumnya didominasi material sedimen berbtr kasar dng sedikit material sed berbtr hls pd bag atasnya. Strktr sed yg
terbentuk merefleksikan pengendpn pd energi tinggi dgn arah aliran yg searah, tabular x-bedding& punggungan bar
yg lurus memanjang. Karakteristik log yg tampak memperlihatkan pola blockyatau cylindrical
Anastomasing:
Ling ini dipisahkan dr tubuh sungai utama oleh pulau alluvial, yg permanen, & ditutupi oleh vegetasi lebat yg distabilisasi oleh endpn sungai (Boggs,87). Braided juga naik dgn cepat, fluktuasi cepat pd pemberhentian sungai, kecepatan tinggi dr pasokan sedimen kasar & mudah tererosi (Miall,92)
Straight
Suatu channel dng bentuk lurus/ straight didominasi oleh lempung, dgn intensitas kelokan kecil, terbentuk krn
perpindahan arus pd pasir atau kelompok2bar. Segmen channel ini jarang terbentuk pd jarak yg panjang.
Lake Facies
Genetik dr li-peng ini dipengaruhi oleh: hanya ada gelombang (arus tdk ada), tdk ada pasang surut, ukuran dr danau
sgt penting (akan mempengaruhi model fasies pengendapannya), kedalamannya dangkal, danau di daerah gurun
Sedimentologi
4
LINGKUNGAN PENGENDAPAN
disebut dng danau playa (palya lakes) ini akan menghasilkan endpn evaporit, & danau yg besar ada sungai masuk
bermuara, mk akan membentuk delta
Sifat kimia
danau oligotrophic kaya oksigen (terbatas untuk bahan makan)
danau eutrophic sedikit oksigen dan banyak bahan makanan
danau distrophic, sedikit oksigen dan bahan makanan sedikit.
Danau air tawarsoft water lake
Danau air asin hard water lake
Struktur sedimen yg mungkin terbentuk al: a) Laminasi pd lepas pantai, b) Varve, c)Churned bedding, d) Struktur yg
teraduk (Policipoda air tawar yg menggali sedimen tertransport kmdn diendapkan kembali), e) Cross bedding, f)
Gradded bedding, g) Mud crack danau playa/ danau intermeten, h) Ripple mark
Pd lingkungan ini sering terbentuk endpn bijih besi, Fe hidroksida, Fe oksida, Fe Sulfida, Fe karbonat  danau
soperier, Endapan peat
Review (I)River Channel Patterns
1. Channel pattern refers to charateristies of a channel in plan view. The deposits of rivers vary depening on channel pattern, so different facies models have been developed for deposits of rivers with different channel patterns
2. Meandering - single, curved channels: tend to form on low to moderate slopes in fine-grained material
3. Braided - channel dividedes around islands, tend to form on higher slopes in coarser material
4. Straight - rare; form on very low slopes
Fig:Alluvian Fanis a sedimenttary deposit located
at a topographic break that
composed of fluvial &/or
debris flow sediments
&that has shape of fan
either fully or partially
extended
FluvialFaciesModelsRiver deposits consist of channel deposits & floodplain (overbank) deposits. The charac-teristics
of river deposits varies depending on channel pattern. First, let’s look at channel deposit that form in meandering
rivers
Deposits in Meadering Channels
Characteristics of these deposits is related to flow pattern that
develops in river bends, Flow is pushed toward outer bank of
meander bend, Helical flow (see diagram on next slide) developed
in meander, Flow near beds is directed toward inner bank
Point Bar(PB) Deposits
- PB slopes gently toward thalweg
Vertical variations in
(imaginary line connecting deepst
Point bar Deposits
Disrupted by burrows,
part of channel; in bends it’s near
Fig : Point bar on Brazos R.TX coastal plain. Note
Rooting, dessication cracks
sloping surface & cut-bank 0n ot side of bend
outer bank)
Small-scale
X-stratification
- In thalweg: coarse lag deposits (all
Secondary flow refers to small component of flow
velocity directed perpendicular to flow. Top diagram
but coarsest grains are croded away)
shows near surface & near beds velocity vectors
Large scale
A’
Most of PB is covered with dunes;
X-stratification
get large-scale X-stratified sand
- Top of PB commonly rippled: get
ow
small-scale X-stratified finer sand
A
Fl
A’
A
- Top PB deposits dry out during low
Mud
Sand
flows, become vegetated, & eventually becoming part of the floodplain
This diagram shows only the secondary components
of velocity vectors
as channel migrates laterally
- Top of PB deposits commonly show desiccation cracks & bioturbation by roots & burrowing organisms
Sedimentologi
5
LINGKUNGAN PENGENDAPAN
-
Lateral migration of bend produces a basal erosion surface
Deposition on sloping PB surface produces lateral accretion deposits: beds dipping gently toward outer bank
that consist of lg-se X-S sand that fine upward to finer-grained sm-se X-S sand
X-section perpendicular to flow
Bioturbated, dessicationt crackerd mud
Small-scale x-stratified sand
large -scale x-stratified sand
Channel fill
deposit
Note basal erosion surface & lag deposit also note bedding
geometry in bar deposit: lateral accretion beds dipping to right,
formed from lateral accretion on pint bar. Within lateral accretion
beds, paleocurrents are directed out of the out crop, if the outcrop is oriented perpendiculair to flow channel fill deposit consist of fining upward sediments in beds conformeable with channel, formed by vertical accretion
Channels Fill Deposits
When flow in bend is cut off due to chute or neek out off or largescale avulsion, channel will fill from deposition from suspension
(vertical accretion) & gravity flow deposition along outer bank..
Grainsize fines upward: bedding geometry is conformable with
channels form
This island formed by chute cut off when the chute channel formed
during a flood. Eventually, the older curved channel segment will fill
in with sediment & become part of the flood plain
CHUTE CUTOFF
ABANDONED
CHANNEL
This flood-plain area is an abandoned channel. Note the steeplysloping curved bank. it was the outside of meander bend. The
bend was abandoned
following a chute cut-off & filling of the ends of the older channel
segment. Note that the upstream end is filled more completely that
downstream end. Filling is accomplished by depositionduring floods
& by slumping of material from the old cut bank
Braided River (BR) Facies Models
- BR have a variety of bar types – PB, alternate bars (may be attached to one bank or mid-chanels bars)
- Alternate bars form in straight channels; they migrate downstream
- Depening on channel width depth ratio, alternate bar patterns may evolve into braid bars
Deposition on Braid BarsBar migrate downstream, but also grow laterally, Will produce lateral accretion surfaces
that can be seen in view perpendicular to flow, Downstream-accreting bars will produce large forsets; may be
internally large scale X-stratified
Distinguishing between braided&meandering river deposits (MrD)
- BRdeposits should show greater proportion of channel fills relative to lateral accretion deposits (because BRare
multi-channeled)
- May see convex-upward bedding geometry if outcrop provides extensive view perpendicular to paleo-flow
direction (see previous slide)
- MrDcommonly show grater contrast in grainsize between chanel fill & channel bars deposits
- Greater paleo-current variability in MrD
- Detailed studies of bedding may allow for recognition of braid bar migration (e.g.Willis, 1993)
B’
A’
A
A
A’ B
B
B’
Bedding geometry in fluvial
deposits in this Siwalik group,
Pakistan.
Note that some beds show convex-up beding geomerty; others
show lateral accretion surface
Bedding geometry in flow-perpendicular
X-section through braid bardeposits.
Sedimentologi
6
LINGKUNGAN PENGENDAPAN
Arrow in lower diagram show paleocurrent (After Willis,1993)
Overbank Deposits
- Overbank deposition occurs during floods
- Between floods, flood plain is vegetated; soil
develop; sedimens become bioturbated
- Over bank deposits are finer – grained than
channel deposits; the fine away from channel
The horizontal bedded red colored rock is mudstone (MS) deposited in over bank environment.
This area was once a river flood plain; later the
river sanged cousre & the channel flow through
this area, depositing the fan sandstone (SS)
SS
MS
Levee Deposits
- Over bank deposition may occur all along the banks of the channel during a flood
- Close to channel, coarser sediment is deposited; may see ripples or dunes
- Flow strength decreases through time ;levee deposit will show fining in grainsize vertically
- Farther from channel, sediments are fine grained, laminated
Crevasse Splay (CS)Deposits
- CS from break in a levee, water shoots through
crevasse, expands & decelerates on floodplain
- Forms deposit that is fan-shaped in plan view, &
wedge-shaped in x-cross section
- Erosive base
- Deposits fine upward & away from channel
Red rock is overbank mudstone; Tan rock is
crevasse splay sandstone
Review (II)
 Alluvial Fan Facies ( ►Common in modern & ancient deposits, ►Most common in semi-arid region where
pronounced gradient or abrupt relief exists (highland, hills, mountains,faulting scarps), ►Preservation potential
high when building into lakes, rivers,playas,flood plains,► Commonly divided into upper, mid & lower
fan►Commonly assosiated with intermittent current dunes & slt lake (playa) or salt flat (sabkha) deposits,
►Prograding alluvial fan: overall C-U succession )
 Braided Stream Facies (►Broad & shallow, ►Channel bars (longitudinal, transverse) are main sites of
sediment accumulation, ►Gravel sediment dominant vs sand-grade sediment dominant, ►2-D dunes most
common: planar (tabular) x-beds dominat, ►V succession: stacked dunes with planar x-stratification)
 Meandering Stream Facies(►Single channel with prominat meanders, oxbow lakes, floodplain, crevasse splays,
►Channel deposit : large dune-trough x-beds, ►Point bars: epsilon x-beds, upper plane beds, ripple x-beds,
► Channel-fill: abandoned channel deposits: mudrock, caliche, root traces, ►F-U sequences)
Approaches to Description
1) Descriptive approaches into 1960’s. Genetic approaches developed as integration of emperical modern data
with ancient systems
Sedimentologi
7
LINGKUNGAN PENGENDAPAN
2) Depositional Systems Approach : ►Concentrate on modern processes (phycical, biological, & chemical),
►Products of depositional processes (use rock properties & development of facies model)
3) Facies models are idealized scenarios, Natural systems differ
I Alluvial Fans
●Change in gradient & drop in hydraulic power ● Arid-humid environments ● Assosiated with rising fault scraps
(foreland basins, grabens, strike-slip basin) ● Transported by streams, debris, &mud flows●
Genetic Features
- Lobate mid-fan sieve-flood deposits (well sorted, well stratifiesd, x-bedded)
- Lobed (sieve) deposits progressivley coarser towarh front
- Lobate to tabular debris flow (poorly sorted, chaotic mixture w/o stratification
- Fanglomerat (stream channel conglomerat) & Fan deltas (deltaic CGL into oceans
Diagnostic Features
- Tectonic setting continental grabens, foreland basin, collisional overthrust belts, etc
- Geometry limited aerial extent, wedge-shaped
- SequencesC-U x-bedd ss, channel lag cgl, unsorted debris flow.
- Sedimentology Fining down fan, cgl & x-bedded sandstone common, unsorted debris flow
II Braided Fluvial Channels
●steep gradients with abundant coarse clastic ● morfological changes during high-discharge ● longitudinal &
transverse bar development ● cross channel bars●
Braidplain Bars Forms
1) Longitudinal bars-upper reaches (downstream aggradation of gravel, high velocity features/ planar beds,
imbrication, transverse (linguoid) bars-board & lobate, megariplles, modified by low-flow temperatur, ripplesdunes & trough x-beds
Diagnostic Features of braided fluvial channels
- Tectonic setting rapidly down dropping basins
- Geometry elongate, straight-lenticuler or sheelike sand bodies
- SequencesF-U of channel-lag gravels, trough x-bedded sand, tabular cross beds
- Sedimentology a) upstream gravel, downstream coarse sand, b) abudant tabular and trough x-stratification
III. Meandering Fluvial Systems
● Low gradient results in sinuosity ● Spiral currents results in erosional cut banks & depositional, laterally
accreting point bars. ● Channel cut-offs result in oxbow lakes ● Avulsion results from channel breach ●
Point Barsequences
 arcuate & swales known as scrool bars, chutes, chute bars
 capped by fine-mud & flow modified structures
 low-flow velocity sand ripples & climbing ripples
 planar lamination & large scale trough crossbeds of migating dunes
 channel lag of gravel - coarse sand
Diagnostic Features of Meandering Fluvial Systems
- Tectonic setting distal craton, passive-margin setting or down dropping basins
- Geometry long, ribbon-like sand bodies (shoe-string sands)
- SequencesF-U from channel lag trough point-bar deposits, high % fine-grained sediments
- Sedimentology gravel to muds, planar& trough X-beds, ripple x-lamination, floodpalin deposits, fossiliferous
IV. Lake Fasies
● Common where internal drainage dominates in close basins ● Grabens, calderas, glacial depressions, impoundments ● Low-energy silts & muds, occassional carbonate & evaporites ●
Unique Lacustrin Features
- graded turbidity currents in deep lakes
- Rhythmic fine laminations (circulations overturn) reflecting seasonality or large-scale climate ascillations
(Milankovic cyclicity)
- High concentration of organic matter (low-grade kerogen; e.g oil shales)
- Saline, evaporites deposits or low-magnesium carbonates
Diagnostic Features of Lacustrine Deposits
- Tectonic setting fault graben or downwarped basins, associated with other nonmarine settings
- Geometry circular or elongate, lenticular in cross sections
Sedimentologi
8
LINGKUNGAN PENGENDAPAN
Sequences C-U from laminated shale, marl to ripple & x-bedded sand. Cyclity possible
Sedimentology mudstone, shales, sandstone, carbonate, gypsum, halite, dessication features. Nonmarine
fossils
V. Eolian Deposits
Diagnostic Features of Eolian Deposits
Tectonic setting mountain rain shadows, associated with desert deposits
Geometry dune fields can over 100’ s Km2, tabular bodies up to 35 m thick
Sequences large-scale x-beds. Forset dip 25-30, deflation gravel or pebble lags
Sedimentology well-sorted, well rounded quartz-rich sand, large scale x-beds comprised of smaller scale
low-amplitude wind ripples.
-
BATTERY POINT MODEL SEQUENCE
MEANDERING FINING UPARD SEQUENCE
SS
G
C
B
C
A
SS
BAR
TOP
IN-CHNNEL
FLOOR
F
D
E
B
V.A
POINT BAR
TOP
POIN
BAR
CHANNEL
FLOOR
CHANNEL
FLOOR
Lingkungan Pengendapan Marine – Marginal (Transitional Depositional Environments)
Scr aspek fisik, kimia & biologi, lingkungan ini mempunyai 3 fasies sedimentasi; 1) Endpn sistim pasang-surut muka
air laut (Tidal flat), 2). Endpn sistim delta, dan 3). Endpn sistim garis pantai (shoreface line)
Inshore tidal flat deposit(Endapan sistim pasang-surut –muka air laut)
 Endpn yg terpengaruh pasang-surut, terlindungi oleh ombak laut, oleh sederetan pulau atau gosong, atau
posisinya yg masuk ke daratan (teluk). Hub dng laut terbuka melalui saluran pasang surut (tidal channels)
 Morfologi lingkungan ini yi: a). Tidal channel, b). Tidal flat (kering bila air pasang surut & terendam bila laut
pasang) & c). Salt marsh (terletak di atas air pasang ttp masih terendam bila ada taufan).
Lingkungan ini mempunyai ciri2 al: ●Sekuensi sering penghalusan ke atas ttp tdk teratur spt point bar, krn ada bag
yg tdk lengkap, sering penghalusan ini tampak, ● Tercampurnya strktr sed x- laminasi , berskala besar atau/dan
kecil, ● bimodalitas arah arus, ● endpn flaser, ● bioturbasi
Delta
Sistim Delta, mrpkn grs pantai yg menjorok ke laut, terbentuk oleh adanya sedimentasi sungai memasuki laut, danau
atau laguna & pasokan sedimen lebih besar dr pd kemampuan pendistribusian oleh proses yg ada pd cek pengendapan (Elliot,86). Morfologi delta scr umum tdr dr delta plain, delta front, &prodelta (Bhattacharya & Walker, 92)
Faktor2 yg mempengaruhi sistim pengendapan delta
1) Pembentukan delta, sngat dikontrol oleh hal2 tsb: ● Jmlh sedimen yg masuk (sediment input) (termasuk
didalamnya kecepatan, u-b butir, dll), ● Penambahan air masuk, dari air tanah, berat jenis air dibandingkan air
laut. ● Energi air laut/ danau, termasuk didalamnya gelombang, arus, & pasang surut, ● Kedalaman laut/
danau, ● Kompaksi & penurunan (subsidence), ● Struktur dari dasar(basement)
2) Hidrodinamik (Gambar.3, 4,& 5). Menurut Bates, 1953, ada 2 tipe dr free jet flow: ● Plane Jet, pada daerah
miring ada 2 arus ialah: horisontal & vertikal, ● Axial jet, pd daerah miring ada 3 arus, yi: mendatar, vertikal, &
searah kemiringan pantai (slope)
3) Berat jenis air sungai yg masuk di bandingkan air danau / air laut, mk ada 3 aliran yi: ● Aliran masuk > air
danau/laut, disebut hyperpycnal flow, akan menghasilkan arus turbidit, ● BJ air masuk = air danau/laut, disebut
homopycnal flow, mk di daerah miring terjadi 3 arah arus axial jet
4) Aliran masuk < air delta / laut (Hypopycnal flow). Material melayang layang, air sungai mengalir berada di atas
air laut dan aliran adalah plane jet
Ciri fasies delta konstruktif
Sedimentologi
9
LINGKUNGAN PENGENDAPAN
a. Pada chanel2 sungai dicirikan dng fasies sungai
b. Fasies delta plain ini termasuk marsh, cabang2 chanel subaerial/ levee&crevasse splay yg dicirikan : ● pasir
ukuran kasar – halus, &● clay, silt & lignit
c. Fasies delta front dibedakan
- Delta front platform (distributary mouth bar&delta front slope) yg dicirikan oleh: a) pasir hls, b). carbonaceous
silt & lignit
- Distal delta front, tdr dr pasir sdg- hls, sortingnya bagus & selang-seling shale
> Profil stratigrafi delta yg disebabkan proses: 1) Progading, 2) Aggrading), & 3). Transgressing), dimana 3
proses sedimentasi tsb berjalan scr simultaneously, menghslkan profil (delta set’s)
> Profil delta dpt dibagi menjadi 3 Set’s: (Gambar 6) (1) Top Sets: trdr dr coarse-grained sediments, & lapisan
hampir horisontal. Top set (delta plain), (2) Fore Set’s: trdr dr large-scale cross strata, u-b pasir (Delta front), (3)
Bottom Set’s : fine-grained (Pro delta sediments)
> Delta pd awalnya adlh istilah yg digunakan oleh seorang ahli filsafat Yunani, Herodotus (490 SM) untuk
menyebutkan sbh daratan aluvial di muara S. Nil yg membentuk triangular (Δ) & terbentuk akibat aktivitas
pengendapan Sungai.
> Pengertian delta adlh sbh lingkungan transisional yg dicirikan oleh adanya material sedimen yg terbawa lewat
aliran sungai, kmdn terendapkan pd kondisi dibawah air (sub-aqueous) pd tubuh air tenang yg diisi oleh aliran
sungai tsb, sbgan lagi berada di darat (sub-aerial) (Serra 1985).
Klasifikasi Sistem Delta
Distribusi, orientasi, & internal geologi dr endpn delta dikontrol oleh faktor yg bervariasi yi: iklim, morfologi, vegetasi,
pengisian air, banyaknya sedimen, proses mulut sungai, gelombang pasang surut, angin, arus, kemiringan shelf,
tektonik, & geometri cekungan. (Galloway ‘75, dalam Serra ’90)
Berdsrkan proses yg berpegaruh, mk dpt dibagi menjadi 3 jenis (Gambar 4.4.). yi: (1 ) Delta Dominasi Sungai, (2)
Delta Dominasi Pasang Surut, (3) Delta Dominasi Gelombang
1. Delta Dominasi Sungai
Jika gelombang, arus pasang surut, & arus sepanjang pantai lemah, volume sedimen yg dibawa dr sungai tinggi, mk
akan terjadi progradasi yg cepat ke arah laut & akan berkembang suatu variasi karakteristik dr li-peng yg didominasi
sungai dgn kontinuitas tubuh btpasir jelek (channel) - sedang (distributary mouth bar) serta umumnya memiliki
btpasir dgn struktur x- lam&// lam. Refleksi seismik oblique&sigmoid clinoform.
2. Delta Dominasi Pasang Surut
Mrpkn area dimana tingkat pasang surut tinggi, shg aliran balik (yg terjadi dlm distributary channel selama kondisi
banjir & surut). Memiliki kontinuitas btpasir jelek-sedang, arah sebaran tegak lurus pantai, dimana tidal
channel&ridgefacies sgt dominan.
3. Delta Dominasi Gelombang
Pd lingkungan dgn aktifitas gelombang kuat, endpan mouth bar scr menerus mengalami reworked. Tubuh pasir akan
cenderung paralel thdp grs pantai dgn kontinuitas tubuh btpasir baik mengikuti strike berbeda dgn delta dominasi
sungai yg mendekati tegak lurus thdp pantai.
Sub Lingkungan Pengendapan Delta
Delta dibagi dlm bbrp sub li-peng yg ditandai dng perbedaan geometri & fasies yg berkembang. yaitu : (1) Delta Plain,
(2) Delta Front, & (3) Pro Delta
Delta Plain. Ini mrpkn bag delta yg bersifat subaerial & umumnya td dr distributari chanels, & berbagai li-peng dr
lingk darat – payau, termasuk swamps, marshes, tidal flat, &interdistributary bays (Bhattacharya & Walker, 92)
Sublingkungan Delta Plain dapat dibedakan menjadi
a. Upper Delta plain: yi; bag yg terletak di atas daerah pasang surut, atau m.a.l, & endpnnya scr umum tdr:
- Endapan Distributary Channel (D.C): endpn ini td dr endpn braided, meandering channels, levee, &point bar.
Endpn D.C di tandai dng adanya bid erosi pd bag dasar urutan fasies & menunjukan F-U/ ▲(Bhattacharya &
Walker, 92)). Strktr sed yg umum dijumpai adlh: x-beds, wave x- beds, scour & fill, & lensa2 lempung. Endapan
point bar terbentuk apabila terputus dr channelnya. Sedangkan endp levee scr alami berasosiasi dng D.C, sbg
tanggul alam yg memisahkan dng inter-distributary channel.
Sedimentologi
10
LINGKUNGAN PENGENDAPAN
- Endpn Interdistributary Bay (I.B), daerah ini diisi oleh sed berbtr hls (lanau-lempung) saat terjadi banjir.
Umumnya endpn ini dipotong oleh endpn creveasse splay atau channel. Urutan ini bisa berubah menjadi
endpn berbutir hls karbonan/ batubara. Pd daerah ini hanya memiliki sedikit pasir & umumnya td dr urtan
fasies yg tipis, & saling berpotongan antara fasies mengkasar & menghalus ke atas
b. Lower Delta Plain, scr geometri terletak pd daerah dimana terjadi interaksi antara sungai dng laut, yi: dr low
tidemark sampai batas kehadiran gel yg dipengaruhi pasang surut.
Delta Front. Ini mrpkn tempat terjadinya sedimentasi scr aktif & tetap, khususnya pd mouth of distributaries.
Endpn delta front umumnya diwakili oleh distributary mouth bar yg menunjukan sikuen pengkasaran atasC-U/▼
Prodelta. Ini mrpkn sublingkungan transisi antara delta front& endpn normal marine shelf, yg berada di luar delta
front. Lingkungan ini didominasi oleh material hls yg mengendap scr suspensi, dng bioturbasi yg dominan. Daerah ini
mrpkn bag distal dr delta, dimana hanya trd dr akumulasi lanau & lempung.
Serra (1989), membagi dlm bbrp sub lingkungan sbb:
1 LingkunganDelta Plain
Mrpkn bag delta yg bersifat subaerial, terdr dr channel aktif &channel yg ditinggalkan (abandon edchannel). Sub
lingkungan delta plain ini dibagi menjadi :
a. Upper Delta Plain. Bag delta yg terletak di atas area tidal atau laut. Scr umum terdr dr endapan distributary
channelmigratory (yg berpindah), yi: endpn braided atau meandering, tanggul alam (natural levee), & endpn
point bar, lacustrine delta fill& endpn interdistributaryflood plain.
Strktr sed:: x-bedding, ripple x-stratification, scour & fill& lensa2 lempung. Ling Distributary channels membentuk
percabangan landas laut yg saling berhub, memotong delta plain& membawa sedimen fluvial ke pantai.
Kedalaman distributay channels dpt mencapai 10 – 20 meter & mengerosi lap bawahnya yg mrpkan endpn laut
atau lacustrinedelta front. Ling Distributary channels mengakumulasi endpn pasir & membentuk reservoar yg
baik. Geometri endpn distributary channels sgt bervariasi & tergantung pd tipe sistem fluvial & pembebanan
alami sedimen sungai.
b. Lower Delta Plain. Bag delta yg terletak dlm daerah dimana terjadi interaksi ant sungai & laut yi: low tide-mark
sampai batas pengaruh pasang surut (tidal). Endpnnya meliputi : Endpn pengisi teluk (bay fill deposit) meliputi
inter-distributarybay, tanggul alam, rawa, &crevasse splay. Endapan pengisi distributary yg ditinggalkan
2. LingkunganDelta Front
> Sublingkungan dng energi tinggi, dimana sedimen scr konstan dipengaruhi oleh arus pasang surut, arus laut
sepanjang pantai, & aksi gelombang (kedalaman 10 meter atau kurang).
> Endapannya meliputi: distributary mouth bar, endapan river mouth tidal range. Ditunjukkan oleh sikuen mengkasar ke atasC-U-▼ yg menunjukkan perubahan fasies scr vertikal ke atas (Gambar III.9).
> Sekuen ini hasil dr progradasi delta front yg mungkin diselingi oleh sikuen distributary channel dr sungai atau
tidal pd saat progradasi berlangsung, jenis fasies nya.
- Distal bar, urutan fasiesnya cenderung menghalus ke atas▲, umumnya tersusun atas pasir halus dng strktr
sedlaminasi tipe trough &burrow.
- Distributary mouth bar, Sedumumnya terssn atas pasir yg diendpkan melalui proses fluvial. Strktr sed
umumnya current ripple,x- bedding, &masive, graded bedding (Walker, 1992).
- Channel, Sedimen umumnya berukuran pasir dengan basal lag deposits. Struktur sedimen yg umum dijumpai adlh x-bed, ripple x-stratification, scour & fill.
- Pasokan sedimen fluvial yg menerus menyebabkan terjadinya pengendpn mouth bar yg maju ke arah laut
(progradasi).Endpn progadasi menghasilkan tumpukan sed C-U / ▼.
- Fluvial dominated-mouth bar:FD or RD artinya delta front yg paling dominan mempengaruhi pembentukan endpn mouth bar adlh proses fluvial (Gambar III.10). Mekanisme terendapkannya btpasir dng proses
fluvial masih memungkinkan terjadinya percampuran antara material pasir dng lanau atau lempung
(Gambar III.11).
- Tide dominated - mouth bar : TD Mouth bar pd delta front ini terbentuk di daerah pasang surut yg tinggi
arus, yg berasal dr proses pasang naik, dpt mengontrol sedimentasi delta front. Dari ciri pola log
menunjukkan bentuk funnel ▼ yg mengindikasikan pengkasaran keatas (Gambar III.12). Proses
pembentukan sedimen tidak banyak dipengaruhi oleh material dr daratan alluvial, melainkan dr fluktuasi
muka air laut. Pola geometri batupasir cenderung akan memanjang akibat proses pasang surut air laut
(Gambar III.13)
Sedimentologi
11
LINGKUNGAN PENGENDAPAN
-
Tubuh batupasir tdk setebal pd fluvial dominated mouth bar, namun blok-blok tersendiri yg dibatasi oleh
sedimen berbutir lanau-lempung yg terendapkan pd fase air laut naik .
Wave dominated - mouth barWDMouth bar pada delta front yg terbentuk dlm aktivitas angin & intensitas
gel laut yg tinggi. Delta ini pd umumnya berbentuk arcuate (Gb III.14). Proses tsb menghasilkan batupasir
yg melampar sepanjang grs pantai Krn adanya aktivitas gelombang air laut mangakibatkan terjadinya
pemilahan butiran yg lebih halus ketempat yg lebih dalam (fasies prodelta), shg menghasilkan batupasir yg
tebal & sera- gam. Oleh karena itu wave dominated mouth bar mrpkn reservoar baik krn pelamparan yg
luas & tebal.
3. LingkunganProdelta
Mrpkn sub lingkungan transisi antara delta front& endapan normal marine shelf, yg berada di bawah kedalaman
efektif erosi gelombang, terletak di luar delta front.
Sedimen yg ditemukan pd bag ini adlh material yg berukuran paling halus, lanau dan lempung, kadang-kadang
dijumpai lapisan pasir tipis. (Serra, 1990).
Struktur sedimen: masif, laminasi, &burrowing structure. Seringkali dijumpai cangkang organis- me bentonik yg
tersebar luas, mengindikasikan tidak adanya pengaruh fluvial (Davis, 1983). Endapan prodelta trdr dr marine
&lacustrine mud yg terakumulasi dilandas laut (seaward). Endapan ini berada di bawah efek gelombang, pasang
surut dan arus sungai.
Delta Environments
- A delta forms where a river carries in more sediments into the sea than sea can erode away
- Great geological significance: most basin- fill sediments are deltaic successions
- Great important economic significance: host rock of coal, oil & gas
- Complex sedimentary environment subdivided into:
 Delta Plain (delta top, topset, undaform): portions of the delta on the land side of the coast: Marshes (:wet or
periodically wet land), Swamps (:spongy land covered by very shallow standing water), Channels, Lake.
 Delta Front (foreset, clinoform): areas of delta progradation into the sea, where sediment carried by
distributary are deposited:
≈ Distributary mouth bars: with character as; important sites of sand deposit, dunes (x-beds),
ripples(: x-lamination & flaser beds), laminated mud & lentcular bedding in distal portion (distal bar),
indistributary bays: peats, coal, invertebrate fossils, bioturbation are all common
≈ Growth faults: with character as syndepositional: rapid sediments accumulation, abrupt change in
bed thickness across faulting plane, important feature for oil reservoir
 ProDelta (bottomset, fondoform), with character as: sediments becoming finer away from the delta,
finely laminated mud (silt, clay), bioturbation intense
- C-U succession of the delta front facies ( -prodelta – distal bar – mouth bar)
- F-U sequences of the delta plain facies : (distributary channel fill (x-beded sands) – swamps mud & coal
- Delta type
a. River dominated delta: (R/FD) with character as; low tidal range, minimal wave action, lobate: many
distributaries and interdistributary marshes & swamps elongate (bird-foot) few distributaries,
crevassingdeposits in bays along main channel; sediments carried in by one mayor channel (e.g. modern
Missisipi delta)
b. Wave dominated delta: WD) with character as; strong wave action, mouth bar sediments reworked into
coastal barriers and beach ridges paleocurrent flow: bimodal (onshore-offshore) or polymodal (plus
longshore drift) (Niger, Niles Delta)
c. Tide dominated delta: (TD) with character as; Tidal range high, Tidal current (flood-ebb) controlling
sediment dipersal & redistribution at the delta front Difficult to recognize in the geological record because
the deposits and stratigraphic successions are similar to tidal flat deposit (without presence of river influence)
(Ganges Delta –Bangladesh)
Review
Delta System
1) Influenced by fluvial & marine processes in coastal zone
2) Delta plain, delta front, & prodelta
3) River dominated, wave dominated, tidal dominated
Genetic features of RD (Gambar Pola tumpukan perlapisan endapan2 transisi/ delta)
Sedimentologi
12
LINGKUNGAN PENGENDAPAN
- Out-flow dominated by inertial forces generate turbulent bed friction and deceleration
- Bottomset beds (prodelta) are gently inclined, fine-grained sedimens
- Foreset beds (mouth bars) generally dip 100 in water depths of 10 m and are coarse grained
- Topset beds (delta plain)above aor within tidal influence and are overbank mud and silt, rooted
Genetic features of FD
- delta is elongate to lobate in geometry
- DMB and delta-front sheet sands; distributary channel fill
- Variable framework orientation, usually paralles depositional slope
- Channels are suspension-load dominated of fine-mixed clastic
Genetic features of TD
- shoreline is significantly modified by bidirectional tidal currents
- funnel-shape distributaries in lower delta plain
- reworked into linear tidal ridges that may extend out onto subaqueous delta front
- characterized by tidal flat, levee, floodplains
- tidal sand ridge and eustarine distributary channel fill sands
- paralels depositiona slope
Genetic features WD
- strong waves result in constricted or deflected river mouths
- distributed sediment along delta front by longshore currents
- beaches, barrier bar, and spits common along a smooth delta front
- arcuate geometry with sand composition
- paralles depositional strike, secondary dip trends.
Diagnostic Features
 Tectonic setting coastal plain passive margins in downwarped basins
 Geometry roughly triangular in plan view, wedge-shaped in cross-section
 Sequences C-U from prodelta mud to distributary bars sands, then floodplain rooted muds
 Sedimentology wide range dependt upon genetic regime
Pertidal
Environment
1.
2.
3.
4.
Coastal regimes with tidal flas, estuaries, lagoons
Microtidal (< 2m), mesotidal (2-4), macrotidal (>4)
Rhythmic tidal bundling, herringbone x-stratification
Shallow, wide fine-grained channel
1.
2.
3.
4.
Elongate sand bodies under micro & mesotidal conditions
shoreface deposits of sand & mud, low wedge-shaped planar x-beds
washover deposits breach the berm
lagoons & tidal flats behinds barrier
Diagnostic Features
 Tectonic setting stable, passive w/broad shallow coasts
 Geometry tabular, thin, parallel shoreline
 Sequences F-U sequences including herringbone x-stratification, flaser-and tidal-bedded sand, mud,
mudcracks, bioturbation, organic rich
 Sedimentology fine-grained sand/mud, lenticular bedding, interference ripples, pinstripe lamination
Barrier Island
Systems
Barrier Islands
Dyanmics
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Longshore lateral migration
onshore & offshore barrier migration
regressive sequence results in C-U profile
trangressive sequences erode barrier sands
Diagnostic Features
 Tectonic setting stable, sloping coastal plain settings
 Geometry long, ribbon-like sand bodies (shoe-string sands)
 Sequences C-U, offshore mud overlain bay shoreface silt & sand, subsequently by medium fine grained
beach & dune sand
 Sedimentology mature mineralgical texture with heavy mineral concentrations; lagoonal muds may be organic
rich, shallow, low angle tabular x-beds, uniq beach x-stratifications
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LINGKUNGAN PENGENDAPAN
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Delta Environments
A delta forms where a river carries in more sediments into the sea than sea can erode away
Great geological significance: most basin- fill sediments are deltaic successions
Great important economic significance: host rock of coal, oil & gas
Complex sedimentary environment subdivided into:
- Delta Plain (delta top, topset, undaform): portions of the delta on the land side of the coast: Marshes (:wet or
periodically wet land), Swamps (:spongy land covered by very shallow standing water), Channels, Lake
- Delta Front (foreset, clinoform): areas of delta progradation into the sea, where sediment carried by
distributary are deposited:
 Distributary mouth bars:
important sites of sand deposit,
dunes (x-beds),
ripples(: x-lamination & flaser beds),
laminated mud & lentcular bedding in distal portion (distal bar),
indistributary bays: peats, coal, invertebrate fossils, bioturbation are all common
 Growth faults:
syndepositional: rapid sediments accumulation,
abrupt change in bed thickness across faulting plane,
important feature for oil reservoir
- ProDelta (bottomset, fondoform)
sediments becoming finer away from the delta,
finely laminated mud (silt, clay),
bioturbation intense
C-U succession of the delta front facies ( -prodelta – distal bar – mouth bar)
F-U sequences of the delta plain facies : (distributary channel fill (x-beded sands) – swamps mud & coal
Delta type
 River dominated delta:
low tidal range, minimal wave action,
lobate: many distributaries and interdistributary marshes & swamps
elongate (bird-foot) few distributaries, crevassingdeposits in bays along main channel; sediments carried
in by one mayor channel (e.g. modern Missisipi delta)
 Wave dominated delta:
strong wave action,
mouth bar sediments reworked into coastal barriers & beach ridges
paleocurrent flow: bimodal (onshore-offshore) or polymodal (plus longshore drift) (Niger, Niles Delta)
 Tide dominated delta:
Tidal range high,
Tidal current (flood-ebb) controlling sediment dipersal and redistribution at the delta front
Difficult to recognize in the geological record because the deposits & stratigraphic successions are similar
to tidal flat deposit (without presence of river influence) (Ganges Delta –Bangladesh)
Marine Shoreline Environments
Comparison with deltaic environments: sediments supply not high enough to produce delta. Subdivision of shoreline
environments:
 Beaches-barrier islands: wave dominated shoreline, microtidal to mesotidal: (coast of Texas, Georgia, Virginia)
 Foreshore (swash zone): between low water & high water
Extensively reworked, supermature sands,
low-angle (2-100) planar x-beds dipping seaward
 Backshore: above high water
berm: sand ridge just above high water level, formed by high strom waves,
dunes: eolian in nature
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LINGKUNGAN PENGENDAPAN
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Shoreface: from low water level to fair-water wave base (FFWB)
lower shoaling zone: fine sands with mud interbeds, bioturbation,
middle breaker zone: fine to medium sand well sorted by waves, forming longshore sand bars, planar xbeds on seawards side of bars, and ripple trough x- beds on landward side
upper surf zone: strong wave action, multidirectional trough x-beds, interbedded with planar x-beds and flat
bedsl,
Vertical succession of beach-barrier complex: seaward progradation of beach-barrier complex: C-U succession:
offshore tempestite- HCS (above SWB) – SCS (just below FWWB) – shore face= foreshore- backshore/ eolians
dunes
 Tidal flats: tide-dominated shoreline, mesotidal to macrotidal (> 4 m tidal range); up to several kilometers wide;
occur in many settings: around lagoons, behind barries, in estuaries, on tide-dominates delta.
Tidal bedding (tidal bundle): daily cycles of onshore flow (cross bedded sand)-high tide (no flow, mud deposits)offshore flow (x-bedd sand) – low tide (no flow, mud)
Deep-water submarine fan Environments
sandstone & even conglomerate from in deep-water setting & continental margins, slope & basins
most commonly thruogh gravity flows: sliding, slumping, turbidity flow, debris flow, orginating from continental
shelf or even shoreline (when shelf deeply incised by canyons)
submarine fans & a spectrum of gravity flow deposits:
inner fan: debris flow, coarse turbidite
mid fan : proximal, medium – grained turbidite
outer fan: distal, fine –grained turbidite
vertical succession:
normal gradded bedding (Bouma Sequence) in fine scale
prograding submarine fan produces a large scale C-U succession, which differs from other C-U succession in
having turbidites when examined in detail
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