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Introduction Unconsolidated clastic sediments Sedimentary rocks Diagenesis Sediment transport and deposition Sedimentary structures Facies and depositional environments Glacial/eolian/lacustrine environments Fluvial/deltaic/coastal environments Shallow/deep marine environments.
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Introduction Unconsolidated clastic sediments Sedimentary rocks Diagenesis Sediment transport and deposition Sedimentary structures Facies and depositional environments Glacial/eolian/lacustrine environments Fluvial/deltaic/coastal environments Shallow/deep marine environments Stratigraphic principles Sequence stratigraphy Sedimentary basins Models in sedimentary geology Applied sedimentary geology Reflection Contents EaES 350-6
Sedimentary structures • Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s–1000s of meters (large outcrops); most attention is traditionally focused on the bedform-scale • Microforms (e.g., ripples) • Mesoforms (e.g., dunes) • Macroforms (e.g., bars) EaES 350-6
Sedimentary structures • Laminae and beds are the basic sedimentary units that produce stratification; the transition between the two is arbitrarily set at 10 mm • Normal grading is an upward decreasing grain size within a single lamina or bed (associated with a decrease in flow velocity), as opposed to reverse grading • Fining-upward successions and coarsening-upward successions are the products of vertically stacked individual beds EaES 350-6
Animation EaES 350-6
Animation EaES 350-6
Sedimentary structures Cross stratification • Cross lamination (small-scale cross stratification) is produced by ripples • Cross bedding (large-scale cross stratification) is produced by dunes • Cross-stratified deposits can only be preserved when a bedform is not entirely eroded by the subsequent bedform (i.e., sediment input > sediment output) • Straight-crested bedforms lead to planar cross stratification; sinuous or linguoid bedforms produce trough cross stratification EaES 350-6
Sedimentary structures Cross stratification • The angle of climb of cross-stratified deposits increases with deposition rate, resulting in ‘climbing ripple cross lamination’ • Antidunes form cross strata that dip upstream, but these are not commonly preserved • A single unit of cross-stratified material is known as a set; a succession of sets forms a co-set EaES 350-6
Sedimentary structures Planar stratification • Planar lamination (or planar bedding) is formed under both lower-stage and upper-stage flow conditions • Planar stratification can easily be confused with planar cross stratification, depending on the orientation of a section (strike sections!) EaES 350-6
Sedimentary structures • Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry and by laminae dipping in two directions • Hummocky cross stratification (HCS) forms during storm events with combined wave and current activity in shallow seas (below the fair-weather wave base), and is the result of aggradation of mounds and swales • Heterolithic stratification is characterized by alternating sand and mud laminae or beds • Flaser bedding is dominated by sand with isolated, thin mud drapes • Lenticular bedding is mud-dominated with isolated ripples EaES 350-6
Sedimentary structures • Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry and by laminae dipping in two directions • Hummocky cross stratification (HCS) forms during storm events with combined wave and current activity in shallow seas (below the fair-weather wave base), and is the result of aggradation of mounds and swales • Heterolithic stratification is characterized by alternating sand and mud laminae or beds • Flaser bedding is dominated by sand with isolated, thin mud drapes • Lenticular bedding is mud-dominated with isolated ripples EaES 350-6
Sedimentary structures • Tide-influenced sedimentary structures can take different shapes: • Herringbone cross stratification indicates bipolar flow directions, but are rare • Mud-draped cross strata are much more common, and are the result of alternating bedform migration during high flow velocities and mud deposition during high or low tide (slackwater) • Tidal bundles are characterized by a sand-mud couplet with varying thickness; tidal bundle sequences consist of a series of bundles that can be related to neap-spring cycles EaES 350-6
Sedimentary structures Gravity-flow deposits • Debris-flow deposits are typically poorly sorted, matrix-supported sediments with random clast orientation and no sedimentary structures; thickness and grain size commonly remain unchanged in a proximal to distal direction • Turbidites, the deposits formed by turbidity currents, are typically normally graded, ideally composed of five units (Bouma-sequence with divisions ‘a’-‘e’), reflecting decreasing flow velocities and associated bedforms EaES 350-6
Sedimentary structures Gravity-flow deposits • Debris-flow deposits are typically poorly sorted, matrix-supported sediments with random clast orientation and no sedimentary structures; thickness and grain size commonly remain unchanged in a proximal to distal direction • Turbidites, the deposits formed by turbidity currents, are typically normally graded, ideally composed of five units (Bouma-sequence with divisions ‘a’-‘e’), reflecting decreasing flow velocities and associated bedforms EaES 350-6
Turbidite EaES 350-6
Sedimentary structures • Imbrication commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstream • Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial • Scour marks (caused by erosive turbulence) • Tool marks (caused by imprints of objects) • Paleocurrent measurements can be based on any sedimentary structure indicating a current direction (e.g., cross stratification, imbrication, flute casts) EaES 350-6
Sedimentary structures • Imbrication commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstream • Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial • Scour marks (caused by erosive turbulence) • Tool marks (caused by imprints of objects) • Paleocurrent measurements can be based on any sedimentary structure indicating a current direction (e.g., cross stratification, imbrication, flute casts) EaES 350-6
Sedimentary structures • Imbrication commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstream • Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial • Scour marks (caused by erosive turbulence) • Tool marks (caused by imprints of objects) • Paleocurrent measurements can be based on any sedimentary structure indicating a current direction (e.g., cross stratification, imbrication, flute casts) EaES 350-6
Sedimentary structures • Trace fossils (ichnofossils) are the tracks, trails or burrows left behind in sediments by organisms (e.g., feeding traces, locomotion traces, escape burrows) • Disturbance of sediments by organisms is known as bioturbation, which can lead to the total destruction of primary sedimentary structures • Since numerous trace fossils are connected to specific depositional environments, they can be very useful in sedimentologic interpretations EaES 350-6
Sedimentary structures • Soft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a sediment, and include: • Slump structures (on slopes) • Dewatering structures (upward escape of water, commonly due to loading) • Load structures (density contrasts between sand and underlying wet mud; can in extreme cases cause mud diapirs) EaES 350-6
