Transcript Slide 1
Estuaries Tidal Flats and Other Sedimentary Structures Associated with Tidal Currents --Tavia Clark and JoDana Jones Structures Associated with Tidal Currents • A) Herringbone cross--stratification: the tide goes in the tide goes out causing a crossbedding in opposite directions • B) Reactivation surfaces--asymmetrical dune from the settling of suspended mud during slack tide • C) Tidal Bundles—an ancient record of a whole sequence of mud draped sand packets that show the clear thickness of variations of spring and neep tides. As you move further away from the tidal channels there is a proportions of sand increasing proportions of Mud thus you have some changes in structure: • • • • Wavy Bedding Flaser Bedding Lenticular Bedding Mud Cracks and Salt Casts Tidal Flats Wavy Bedding • Roughly 50% sand and 50% Mud. Found in areas more distant from channels than flaser bedding. Flaser Bedding • Sandy Beds in which mud-drapes often preserve part of the bed form. Often whole ripple shapes are preserved in cross-section by a bed of overlying mud, but more often the mud is eroded in the next current thus removing the ripple crests. The mud is thus more often preserved in the troughs and down current ripple sides. Lenticular Bedding • Bedding sequence in which a layer comprised of mostly mud/silt has isolated lenses and ripples of sand. Mud Cracks • Produced in a muddy environment when an area is periodically saturated. When the area dries out the mud cracks. The preservation of these cracks occurs when sediment is washed back over the area filling the cracks. • Since the sediments are often different sized the cracks are often visible. Tidal Flats • Definition- A broad flat, region of muddy or sandy sediment, covered and uncovered in each tidal cycle Where do we find Tidal Flats? • They generally occur in gently sloping, marshy, and muddy featureless plains dissected by sandy tidal channels and creeks exposed during low tide. • Found primarily along mesotidal- macrotidal coasts with LOW relief (to allow flooding; no cliffs) and low wave energy (may be protected by dry land); estuaries, bays, back-barriers, deltas, and open coasts. 3 main zones of tidal flats 1. Sub tidal: below mean low tide. a.) If flow is velocities are perfectly symmetrical, you may get cross-beds with herringbone stratification. b.) With Asymmetrical currents you may get xbeds are mostly in one direction. 2. Inter tidal zone: flooded and exposed once or twice a day. a.) Small distributary channels containing sand. b.) Traction Deposits may also be found c.) may see wavy, lenticular and flasier bedding d.) May have well sorted sediments 3.) Super tidal zone: flooded by spring tides or by seasonal storm surges a.) Common to see salt marsh vegetation or mangrove trees. b.) roots destroy bedding but peds often presence. Vertical Succession • Tidal are generally preserved only when the area progrades. ** you must bury sediments to preserver them** • Criteria For recognition: – 1. Biomodal X bedding – 2. Reactivation surfaces/tidal. Bundles; flaser/ wavy/ lenticular bedding. – 3. Alternations between tidal channels sands to sandy muddy tidal flat deposits – 4. Subaerial exposure evidence: Mud cracks, raindrop imprints, animal tracks, evaporite minerals casts