Bedforms and Sedimentary Structures

Bedforms and Sedimentary Structures

Why study sedimentary structures?

•Origin

•Physical

•Biological

•Chemical

•Indicate

•Way up

•Current strength (qualitative)

•Current direction

•Environmental conditions

Classes of Primary Sedimentary Structures(see Table 4.1)

Stratification and Bedforms

Bedding and lamination

Bedforms

Cross-Lamination

Irrigular Stratification

Bedding Plane-Markings

Tool marks

Flute marks

Load casts

Trace fossils

Mudcracks and synresis cracks

Scours, rills, and swash marks

Grading in sediment gravity flow deposits

Cryptic Bedding

Photograph and X-Ray prints of Berea Sandstone

Terms for describing beds(from Collinson and Thompson, 1988)

Bed

thickness (cm)

Very thick: > 100

Thick: 10-100

Medium: 10-30

Thin: 1-10

Very thin: <1

Parting or

splitting beds (cm)

Massive: >100

Blocky: 30-100

Slabby: 10-30

Flaggy: 1-10

Laminated: <1-3*

Bedforms can be used to determine flow direction

Orientation relative to flow

•Ripple crests - perpendicular

•Flute casts - parallel

•Tool marks - parallel

•Trees - generally parallel

•Cross-bedding - dip direction

•Imbricated clasts - dip direction

•Some fossils in living position - depends on organism

Bedforms vary with flow strength, flow depth, and grainsize

Bedforms in Catastrophic floods

•Missoula Floods

•Last deglaciation

•Ripples of boulders sized clasts!

Formation of Cross-bedding

Influences on ripple morphology...

•Flow direction

•Flow strength

•Water depth

•Grain size

•Sediment supply

Tidal influence: reactivation surfaces

Tidal influence:Herringbone cross-stratification

Tidal influence:Interference patterns

Influence of sediment supply

•Climbing (aggrading) ripples

•Climb in direction of flow

•Angle related to sediment supply

Influence of sediment supply: starved ripples

Fluvial vs. eolian ripples

Eolian:

•Transport by saltation, bed load

•Erosion on upstream (stoss) side

•Coarse crests; finer troughs

•Height not limited by flow depth

Fluvial:

•Transport by suspension, saltation, and bed load

•Erosion on downstream (lee) side

•Fine crests; coarse troughs

•Height limited by flow depth

Solemarks:

•Obstacle scours

•Flute casts

•Tool marks

Soft-sediment deformation structures

•Load casts

•“Ball and pillow” structures

•Flame structures

•Covolute bedding

Load casts

Develop when dense sediment sinks into less dense sediment on which it was rapidly deposited.

Pseudonodules(“Ball and Pillow” Structures)

Can form when balls of sand deform underlying soft sediments.

Convolute Bedding

•Intensive, soft deformation of bedding, not tectonic.

•Use care to distinguish from structural deformation.

(Look for large spatial distribution and deformation on multiple beds.)

Trace fossils can provide clues about the sedimentary environment

Sediment CT-Scanning

•Density

•Grainsize

•Bioturbation

Terrestrial bedding structures

•Polygonal Mudcracks (V- down)

•Raindrop prints/casts

•Rootlet casts

“Mud logging”

•Detailed description of sediment fabric

(e.g. cm resolution)

•Identify all visible sedimentary structures

•Develop qualitative rankings to classify sediment fabric

e.g. 1=Massive,

2= partially laminated;

3= laminated