Structural Geology (Geo 381) Primary Structures In Sedimentary Rocks
Prof. Essam Abd El-Motaal Mohamed Geological structures
Nontectonic structures
Primary structures Secondary structures
During the formation of rocks After the formation of rocks
e.g. Folds, Faults, Joints….etc. In Sedimentary In Igneous rocks rocks Primary igneous structures: occur in intrusive and extrusive molten bodies. Primary sedimentary structures: occur in clastic sediments & generated by same processes (currents, etc.) that caused deposition. Importance of Primary Structures in Sedimentary Rocks
• Primary sedimentary structure = a syngenetic sedimentary structure determined by the conditions of deposition (primarily current velocity and sedimentation rate), developed before lithification of the rock in which it is found.
• Usually form during or shortly after deposition.
. Significance:
1- Depositional environment. 2- Stratigraphic facing (younging direction i.e. way-up structures) to identify right-side-up or overturned beds. 3- Current direction. Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Bedding
• Sedimentary rocks generally have bedding or stratification
– Individual layers less than 1 cm thick are laminations • common in mudrocks – Beds are thicker than 1 cm • common in rocks with coarser grains Bedding (Stratification)
• Bedding: The primary feature in sedimentary rocks is beds with different composition, texture, color, cement, differential erosion (make sure you recognize beds based on these criteria!).
• Different beds represent different sources, sedimentary processes, and environments of deposition.
Bedding (Stratification) Bedding (Stratification) Bedding (Stratification) Bedding planes Bedding plane: Nearly flat surface of deposition separating two layers of rock. Every layer has a lower bedding plane (lower surface ) at its bottom and an upper bedding plane (upper surface) at top.
Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Fissility in shale Fissility in shale
Fissility in shale - Closely-spaced parting in shale
• Reorientation of planar flakes (clay minerals) due to post-depositional compaction (squeezing of unlithified sediment due to the weight of the overlying rocks).
Clay Shale Clay minerals (Clay particles do not fit tightly) (Compacted clay) Fissility in shale Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Graded Bedding
• A layer with a vertical change in particle size (coarse to fine at the top). • Most likely to occur in a turbidity current.
Graded bedding
• Progressive fining of clast grain size, from the base to the top of a bed; form as a consequence of deposition by turbidity currents (e.g., in turbidite). • Can indicate which way is up. • Flash flood deposits and turbidites are typically graded beds. • Note that debris flows - deposit inverse graded beds.
Graded bedding
Younger
Y
Older Graded bedding
? Graded bedding
Older Y
Younger Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Cross-bedding
• Cross beds: Are surfaces within a thicker, master bed that are oblique to the bedding in the master bed. Eolian foresets with truncated tops (arrow),, concave-up morphology. Navajo Sandstone Cross bedding
Cross beds formed on the leeward side of a sand dune (top) and on the downstream side of river-deposited sediment (middle). Cross bedding Cross bedding Cross bedding in Dunes
Cross bedding from a Water Current Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Convolute structure
Convolute structures are intraformational folds occur within a restricted horizon of a sedimentary sequence (one or a few beds only). Due to syndepositional folding or slumping of wet sediments, which may be triggered by gravity or earthquake activity. Convolute structure Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Channel
Coarse channel deposit (conglomerate) Channel Channel Channel Channel Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Load-cast structures
• Load casts disrupt bedding due to density inversion (sand over mud) Load cast Generation
Load cast is similar to flame in terms of generation Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Injection (Flame) structures
Less dense material intrudes into material above
Caused by rapid loading of turbidite sands Injection (Flame) structures Injection Flame structure Generation
Flame is similar to load cast in terms of generation Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Mud cracks
Mud cracks in recently dried mud
• Polygonal cracks formed in very fine-grained sediment as it dries. • Only form in environments where sediment is exposed to water. • Good facing indicator (individual cracks taper downward). Mud cracks Mud cracks Mud cracks Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Ripple marks
Ripples forming on the shore.
Ancient ripple marks exposed Sand moved by wind, streams, and coastal waves is often rippled. on the base of a sandstone Useful in determine ancient current directions Good way up indicator Ripple marks Symmetric and asymmetric Ripples Wave-Caused Ripple Marks (Symmetrical) Ripple marks Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Rain drops
Rain Drops on Bedding Plane Rain drops Primary Sedimentary Structures
Primary structures in sedimentary rocks
On Upper Bedding Inside Bed On Lower Surface Surface
Fissility in Shale Mud cracks Channels
Ripple marks Gradded Load cast bedding Injection structures Rain drops
Cross bedding Biogenic structures
Convolute structure Dinosaur Trackways Burrows
• Vertical, dark-colored areas in this rock are sediment- filled burrows Biogenic structures (Trace fossils)
Leaf print
Worm burrows Worm burrows Biogenic structures (Trace fossils)
Foot print on ripple marks Biogenic structures (Trace fossils)
Cretaceous
Jurassic Biogenic structures
Tracks – Abu Dhabi
Snail Trails
Burrows
Stromatolites
Y
• Sharks Bay, Australia
• Cyanobacteria grow upward toward the surface Stromatolites
3 Billion Year Old Algal Mounds (Stomatolites) Outcrop of a stromatolite “reef” from 1.6- Australia billion year old
• These reefs were formed by colonies of photosynthetic “blue- green” algae, cyanobacteria and represent some of the first life forms on earth. Way-up Structures
Determining way up using:
• Inside bed features - cross-bedding, graded-bedding, convolute structure.
• Bottom surface features - channel, load casts, injection (flame) structures.
• Top surface features - ripples, mud cracks, raindrops, footprints, burrows, stromatolites. Which Way was Up?
Sedimentary Structures are governed by: • Gravity • Exposure to the Surface They all have a “right way up” Way-up Structures Way-up Structures
• Here, the mud crack “V” opens toward younger strata, and shape of current ripple marks • Indicate that the youngest layer is lower right. Way-up Structures