BIOTURBATION RATES IN BAHIA LA CHOYA, SONORA, MEXICO

Experimental determination of sediment reworking rates in a subtropica l intertidal flat environment yielded information about the amount, na ture and implications of sediment reworking in near-shore deposits. Ca llianassid shrimp in Bahia La Choya, Sonora, Mexico, overturn the sedi ment in the inner flats at an average rate of 0.56 m(3)/m(2)/year. Ela smobranch rays overturn the sediment in the midflats at an average rat e of 1.01 m(3)/m(2)/year. Resin castings indicate that the shrimp are capable of burrowing to a depth of at least 1.15 m and, where present, can completely rework this interval in Bahia La Choya in two years. T he rays reach a maximum observed burrowing depth of 20 cm and, where p resent, can completely rework this interval in Bahia La Choya in 72 da ys. Reworking rates are high enough to preclude the preservation of mo st physical sedimentary structures under normal conditions. Only large -scale sedimentary structures or those buried deeply and rapidly are l ikely to escape reworking. Rates of biogenic sedimentation by calliana ssid shrimp are high enough to generate subsurface shell beds. Short-t erm biogenic sedimentation rates are higher than long-term rates, indi cating that such intertidal sediments are not only thoroughly reworked , but are incomplete at time scales of weeks to months.