3. Physical structure(inorganic)
1) Bedding structure
① horizontal bedding
: lake, beach, deep marine - parallel, continuous rather uniform, monotonous
ⓐ rhythmites
thin, repetitious sequence(cyclic) - texture, composition 의 교호
* seasonal charge (varve) - glacial lake
* beach bed - uprush and backwash (grain size, composition)
ⓑ upper flow plane bed or flat bed (no movement of sediment)
fluvial - braided, alluvial fans, glacial outwash
→sand, gravel horizontal bedding for a few meter scale.
② Bedforms and their internal stratification
ⓐ current - generated structure (lower flow regime)
* foreset stratification : influenced by sediment texture, velocity, bed shear stress.
by depth ratio
→ angular contact (increasing depth, increasing suspension load) → tangential → concave
* plan view
――――――――――― increasing flow strength ――――――――→
linear → undulatory → cuspate → rhomboid(planed off) → flat bed
| Ripple | Megaripple(dune) | Sandwave |
Spacing | 〈 60cm | 60cm-6m | 〉 6m |
Height/Spacing | variable | large | small |
Flow velocity | low (25~50cm/s) | high (70~150cm/s) | moderate (40~80cm/s) |
Geometry | high variable | high variable | straight to sinous |
* trough cross-bedding ← lenticular & trough shaped sets의 측방 이동 누적에 의해 형성.
* tabular cross-bedding ← linear bedforms의 수평적 이동 누적에 의해 형성
◎ Reactivation surface : interruption in migration or erosional surface.
caused by flow rates, tidal stage, tidal current direction
* Climbing ripple cross-stratification : 다량의 부유 퇴적물 유입
(ripple in drift): 측방 이동 + 상향 누적
(ripple in phase: 주로 상향 누적 by rapid sediment input)
* Hummocky cross-stratification
low-angle and gently undulation cross-stratification
(dip : 3~6°)
wavelength : ~ a few meters
height : up to 30~40cm (보통 수 centimeter)
caused by storm waves on shelf
ⓑ current - generated structures (upper flow regime)
* upper plane bed
* antidune - stream, tidal inlet, gentle-sloping beach
(line-crested) → low preservation
(뒤따르는 waning flow에 의해 대개 상쇄되어 보존 가능성은 낮음)
ⓒ wave - generated structures
wave ripples : symmetrical (peak-crest, round-trough)
- wave ripple crest - straight/slightly sinuous with common bifurcations
- wave ripple - chevronlike cross-stratification
controlled by wave size, grain size, wave period, depth of water
* ripple index (R.I.) : ripple length/height
* ripple symmetry index (R.S.I.) : length of horizontal projection of
(R.S.I.) ( 2.5 〈 current ripple )
( 2.5 〉 wave ripple )
(R.I.) ( 0~4 wave ripple )
( 15〈 current ripple )
ⓓ other bedding type
① flaser and lenticular bedding : tidal, streams, intertidal beach
altermating of energy 또는 sand mud availability
┌ flaser : thin mud + sand - 비교적 high energy
└ lenticular : discontinuous sand lens - 비교적 low energy
② wavy bedding : continuous mud (undulating)
③ graded bedding (inverse drading) : a few centimeters
- distribution grading - no fine in lower part
- coarse-tail grading - all size but grading in only coarse part
* by turbidy currents, shallow water, intertidal - sudden decrease of competence
flooding (waning) 또는 hurricanes에 의해 형성 가능
④ imbrication and internal fabric
* preferred orientation by gravitational field with current flow
: gravel, fossils on bedding surface
*imbrication : overlapping arrangement → dip current 지시
: long axes parallel to flow
normal to flow : rolling of clasts in bed load
* parting lineation : thin sandstone → flow parallel to lineation
2) Bedding plane structure
(1) Bottom mark (sole mark) : 퇴적층 층리면 하부에 imprint로서 보존, 상하 퇴적층은 조직이
상이함
: by current and gravity
ⓐ flute or flute cast - by current (sediment accumulation : rapid high turbulent flow)
up
down stream
turbidite에서 많이 산출
indicator of current direction and paleoslope
ⓑ scour mark(transverse)
- ridgelike transverse to paleocurrent direction
- a few cm in scale
ⓒ flute rill mark : long, narrow scour(anastomosing shape)
parallel to current direction (tidal flat에서 형성)
ⓓ longitudinal furrows and ridges
: straight, long, spaced closed ridge and furrows
ⓔ tool mark : by downcurrent transport of pebbles, shells, twigs, branches, fishbone
* Groove cast (striation cast) : linear 긁힘(물체에 의해 파인 흔적)
* Brush marks
* Skip casts
* Chevron marks
* Prod marks
* Bounce marks
(2) Surface marks : all not related to current
a few millimeter ~ a centimeter in scale
① current generation features
* current crescents (obstacle mark) : 장애물 주위에 생기는 구조
(stream, foreshore beach, tidal flat 환경)
* sand shadows : aeolian setting 의 current crescents
( low preservation )
* rill marks : small, bifurcation dendritic erosional feature
( low preservation )
② climate - related features
* raindrop impressions : mud or muddy sand surface
(a few millimeter) - arid, semiarid.
(not always symmetric <by falling oblique rain>: ripple-like)
* mud cracks : polygonal, V-shaped-supratidal, floodplain, playalake, sabkhas.
③ 기타
* swash marks : up-backwash on foreshore area
(Figure 3-35) : extensive, concave seaward
- diagnostic indicator of shoreline(기울기에 따라 간격 달라짐)
* adhesion ripple : wind blowing sand over a wet substrate.
(eolian 환경)(3-36)
* pit and mound structure : volcano - shaped or pitlike by water/gas escaping
* crystal impression : ice, halite, gypsum, pyrite in tidal flats, sabkhas, lagoons
3) Penecontemporaneous deformation structure
(1) Load structure by mass of overlying sediment unit (coarse sand over mud)
* Load cast : flute caster 유사 but not distinct linear alignment, no regular display
* Flame structure : mud intruding the overlying coarse unit
(2) Ball and pillow structure : elliptical or pillow-shaped (pseudo-nodule)
by slumping or downslope sliding(?)
(liquefaction of the mud and mobilization of mud)
a few centimeter ~ 1 meter
(3) Dish structure : discountinuous, concave-upward laminations in sandstones
* dish and pillar - water escape structure
dewatering → columns or pillars of massive sands.
(upward flow of water)
(4) Convolute bedding : complex folding in mud or fine sand,
remarkable continuity of individual bed in convolute unit
(slump structure 와 혼동
- due to gravity only, soft sediment faulting 와 함께 발생)
→ (liquefaction, turbidite 층에서 많이 산출)
(5) Slump structure : rapid sedimentation + slope instability
complicated, toothpaste-type folding w/
overturned folds and faults
from glacial, point bar, dune, delta, tidal flats turbidite
(6) injection structure : a few cm ~ several m
sandstone sill - concordant ┐ by liquefaction + loading
sandstone dike - discordant ┘
(jnjection from mostly below, some above)