. The Earth beneath the sea : History . KILOMETERS 5772 VERTICAL EXAGGERATION 16 X Fig. 6. Cross-sections of the Middle America Trench traced from continuous-sounding records. Sounding scales corrected after Matthews (1939). (See Fig. 5 for location of the sections.) show the trench floor to be flat or gently basined; within such basins the bottom commonly rises 10-20 m toward the land and seaward flanks, with sharp breaks in slope at the basin edges. Seismic-refraction measurements (Shor and Fisher, 1961) and projection of flank slopes indicate | to 1| km of sediments in several of the basins
. The Earth beneath the sea : History . KILOMETERS 5772 VERTICAL EXAGGERATION 16 X Fig. 6. Cross-sections of the Middle America Trench traced from continuous-sounding records. Sounding scales corrected after Matthews (1939). (See Fig. 5 for location of the sections.) show the trench floor to be flat or gently basined; within such basins the bottom commonly rises 10-20 m toward the land and seaward flanks, with sharp breaks in slope at the basin edges. Seismic-refraction measurements (Shor and Fisher, 1961) and projection of flank slopes indicate | to 1| km of sediments in several of the basins between Islas Tres Marias and Acapulco. In this segment of the trench, lying close to a coast composed dominantly of metamorphic and intrusive rocks, trench sediments are gray to greenish brown clays and silts. The fine sand layers occasionally present here in trench-floor cores are attributed to frequent slumping or turbidity-current transport down the steep flanks. Seismic-refraction measurements along the trench axis be- tween Islas Tres Marias and Acapulco indicate that beneath the thick sediments and a probable basement (volcanic?) layer, the main crustal layer (Fp =
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectoceanbottom
