Reading Question
1. What are sedimentary structures and how do they form?
Stratification
• Why do sedimentary rocks occur as strata or beds?
Stratification & Bedforms
• What is bedding?
• Having characteristics which distinguish it from layers above and below
• Lithology change, bedding planes
• Change in physical and/or bio conditions
• Depositional sedimentary structure
Sedimentary Structures
• The sedimentary fabric
• Form during or shortly-after deposition
• Primary versus secondary (bedding plane marks)
• Primary generating processes
– Deposition
– Erosion
– Deformation
– Biogenic
Stratification & Bedforms
• Beds > 1 cm
• Laminae < 1cm
• Subdivisions
Stratification & Bedforms
• Types of stratification
• Palnar stratification
• Cross-beds
• Bedsets
Stratification & Bedforms
• Graded Bedding
– Normal and inverse
• Massive Bedding
Origin of Bedforms
• Bedforms and Flow Regime
• Flume Studies
• Velocity and grain size
• Lower flow regimes
– Ripples
– Dunes
– Froude <-1 (inertia/gravity)
• Upper flow regimes
– Water waves in phase
– Plane beds
– Antidunes
– Pools and chutes
– Froude > -1
Ripples
• Ripples
• Moving fluids - water and wind
• Current ripples versus oscillation ripples
• Wide variety of shapes
• Not unique
– Cannot be used as the single criteria to interpret environments
Cross-Bedding
• Transport of sediments = dune forms (sand waves)
• Migration of dunes = cross-beds
• How do x-beds form?
• Forset and bottomset laminae
• Planar/tabular versus trough
Cross-Bedding
• Overall scale and geometry used to distinguishe environment
• Fluvial
• Beach
• Eolian - festoon
• Climbing ripples
• Can be used to determine ancient flow directions
Flaser and Lenticular Bedding
• Other types o ripple bedding
• Mixed hydraulic conditions and sed. Types
• Stingers of mud in cross-laminated sandy seds.
– Flaser bedding
• Pods of sandy seds in laminated muddy sediments
– Lenticular bedding
• Herring bone cross-stratification
• Tidal Flats
Hummocky Cross-stratification
• HCS
• Undulating concave-up (swales) and convex-up (hummocks) sets of cross-beds
• Cross-beds sets cut into each other with erosional surfaces
• Strong storm waves
Irregular Stratification
• Convolute bedding
• Interbedded laminated mud and sand
• Water-saturated mud = less dense
• Flame Structures
• Load casts
Erosion Structures
• No deposition -only erosion
• Channel
• Scour and fill
Other
• Dessication features
• Mud cracks
• Polygons - v-shaped cross sections
Bioturbation
• Trace fossils
• Can alter the sediment fabric
• Remains of animal behavior
• Ichnogenera - not unique
• Major types
• Locomotion
• Dwelling
• Resting
• Feeding
Bioturbation
• Trace fossils - environmental interpretation
• Ichnofacies
• Paleobathymetry
• Must use caution - preservation
Other Biogenic Structures
• Stromatolites
• Once the dominant form of reef
• Shape = environmental indicator?