Chapter 2 Nontectonic Structures

Nontectonic Structures ► Nontectonic structures help to determine the original orientation of strata  Primary structures such as mud cracks, ripples, sole marks, and vesicles help to determine original orientation  These features help us determine the facing direction of a sequence of rocks.

An overturned fold in Scotland

Nontectonic Structures ► Structures that formed in primary depositional environments may mimic tectonic structures  Examples ductile flow in water-saturated silt, glacial ice, or evaporites ► Primary structures are usually older than tectonic structures and cross-cutting relationships can help differentiate between them

Tectonic vs Nontectonic Structures Slumping Soft-sediment folds Tectonic folds

Primary Sedimentary Structures ► Bedding  Bedding Planes  Graded Beds  Cross Bedding

Cross Bedding

Cross bedding structures

Primary Sedimentary Structures ► Mudcracks  Polygonal blocks formed by desiccation and shrinkage of saturated sediments  Taper down from the surface until they terminate  The surface layer may also separate and curl upward

Mud cracks

Primary Sedimentary Structures ► Ripple Marks  Current (translational) ripple marks form under a prevailing direction of fluid flow ► Asymmetrical with the steep side toward downstream ► Not useful to determine facing because they have the same shape whether they are upright or overturned  Oscillatory ripple marks form with back- and-forth fluid flow ► Symmetrical with pointed peaks showing original facing direction

Ripple Marks

Primary Sedimentary Structures ► Rain Imprints  Can determine facing direction ► Tracks and Trails  May be used to determine facing direction ► Sole Marks, Scour Marks, and Flute Casts  Are later filled with sediment and can be used to determine facing direction

Rain Imprints

Sole Marks and Flute Casts

Primary Sedimentary Structures ► Load Casts  Form from dewatering of the underlying sediment from the weight of the newly deposited sediment  Can be used to determine the facing direction  The broadly convex bases of the load casts show the original bottom of the structure

Load Casts

Fossils ► Fossils  Provide relative ages  May be used to determine facing direction based on fossil sequences or growing conditions in life assemblages  May also be used as strain indicators

Sedimentary Environments ► Sedimentary Facies  Sedimentary rock units vary laterally and vertically as paleoenvironments change.  Facies are separated by composition, texture, sorting, physical and biogenic sedimentary structures.  Walther’s principle - Only those facies that once existed side-by-side can be observed vertically juxtaposed in outcrop.

Sedimentary Facies

Unconformities ► Unconformity – A break in the sedimentary record where part of a stratigraphic succession is missing.  Produced by erosion or nondeposition  Disconformities  Angular unconformities  Nonconformities

Unconformities

Cross section through the Grand Canyon

Primary Igneous Structures ► Igneous plutons and lava flows may form in shapes that resemble sedimentary features or may tectonically alter previously emplaced country rock

Geologic Map of the Sierra Nevada

Primary Igneous Structures ► Compositional banding  Occurs in igneous rocks and may result from crystal settling, differentiation, fractional crystallization, and multiple parallel intrusions or flows.  Study of these may reveal differentiation or fractional crystallization sequences

Primary Igneous Structures ► Vesicles  Cavities left by gas bubbles that form in magma due to pressure release  Vesicles generally accumulate at the top of the magma chamber providing the facing direction

Vesicle

Primary Igneous Structures ► Pillow Structures  Form from lava flows into or below water  Vesicular glassy curved tops and V-shaped nonvesicular bases indicate facing direction

Pillow Lavas

Pillow Lava

Primary Igneous Structures ► Contact Metamorphic Zones from lava flows  The metamorphic aureole forms beneath the lava flow and can be used to determine facing direction

Contact Metamorphism

Gravity-Related Structures ► Landslide and Submarine Flows  May resemble tectonic structures because of the flow patterns of these sediments ► Olistostromes are matrix supported bedded sediments ► Diamictites are matrix supported sediments with no bedding ► Turbidites are unsorted sedimentary deposits produced from rapid flow of sediment

Gravity-Related Structures ► Salt Structures  Occur in evaporite sequences of anhydrite, gypsum, or halite  These rocks flow more readily than any other common rock type and can form folded structures under surface temperature and pressure  These structures that move upward and gravitationally intrude overlying sediments are called diapirs.

Eroded salt dome in Iran and diapir evolution

Grand Saline Salt Dome in Texas

Experimentally produced diapirs

Impact Structures ► Structures with circular or elliptical outlines can be formed from meteorite impacts as apposed to tectonic forces  Shatter cones are produced from brittle deformation and can be good evidence of impacts

Geologic Map of Wells Creek Tennessee

Shatter Cone

Reconstruction of the impact at Wells Creek

Nontectonic Structures ► Nontectonic structures can appear to be tectonic features ► Nontectonic structures can also be used to determine facing direction, which can aid in the interpretation of tectonic features