Abstract: Tidal-Flat Sedimentation on Tide-Dominated Shoreline, Bahia de Lomas, Strait of Magellan, Chile
Ian A. Fischer, Miles O. Hayes
The macrotidal shoreline of Bahia de Lomas (spring tidal range, 10 m) consists of sandy and mixed sand and mud tidal flats up to 8 km wide. The flats are flanked by relict Pleistocene gravel cliffs and an uplifted Salicornia ambigua salt-marsh system. Patches of gravel are on the flats off the cliffs and off the end of an 8-km long gravel cuspate spit (Punta Catalina) at the east entrance to the strait. Localized deposits of gravel are the result of ice rafting.
Unusual climatic factors influence sedimentary processes in the system. A constant, unidirectional west-northwest wind (storm winds up to 150 km/hour) has developed a wind-dominated sand flat in the area of maximum fetch (east side). This part of the tidal-flat surface is featureless except for local detrital clumps of algae. All fine sediments transported into this zone are deposited against the cuspate foreland by wind-blown water.
The mud flat part of the system (central zone) is intensively mud-cracked and burrowed across its entire width. The mud cracks, which are related directly to the drying effect of the wind, control the drainage pattern in the mud. Mud cracks in more frequently inundated areas are filled with a ferrous precipitate. Wind processes also peel off the characteristic laminar beds, contributing mud clasts to the deposits in channel-point bars.
The mud flat system is being transgressed by marine sands that are filling tidal troughs and eroding mud. Offshore sand flats are characterized by flood-oriented sandwaves with a spacing of 12 to 14 m.
A regressive stratigraphic sequence formed by these environments would consist of (from top to bottom): (a) salt marsh with rooted muds; (b) upper sand flat (eastern and western section) and lower flat-bed fine-sand deposits; (c) (same level as b) highly channelized mud flats with burrowed and channelized mud; and (d) lower sand flat (dominated by flood-oriented sand waves) with cross-bedded sand.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC