Benthic foraminiferal composition at the inner wall of the Middle America Trench, Costa Rica

The sediments of Deep Sea Drilling Project Site 565 and University of Texas Marine Science Institute Cores IG-24-7-38 to -42 taken on the landward slope of the Middle America Trench exhibit characteristics of material subject to reworking during downslope mass flow. These characteristics include a generally homogeneous texture, lack of sedimentary structures, pervasive presence of a penetrative scaly fabric, and presence of transported benthic foraminifers. Although these features occur throughout the sediments examined, trends in bulk density, porosity, and water content, and abrupt shifts in these index physical properties and in sediment magnetic properties at Site 565 indicate that downslope sediment creep is presently most active in the upper 45 to 50 m of sediment. It cannot be determined whether progressive dewatering of sediment has brought the material at this depth to a plastic limit at which sediment can no longer flow (thus resulting in its accretion to the underlying sediments) or whether this depth represents a surface along which slumping has occurred. We suspect both are true in part, that is, that mass movements and downslope reworking accumulate sediments in a mobile layer of material that is self-limiting in thickness.采自中美洲海沟陆侧斜坡的深海钻探计划（Deep Sea Drilling Project）565站位沉积物，以及德克萨斯大学海洋科学研究所（University of Texas Marine Science Institute）采集的岩芯IG-24-7-38至-42沉积物，均表现出斜坡下物质流改造作用下的物质特征。这些特征包括整体均匀的结构、缺失沉积构造、普遍发育的贯穿性鳞片状组构，以及搬运型底栖有孔虫的广泛产出。尽管这些特征在所有被研究的沉积物中均有出现，但565站位的体积密度、孔隙度和含水量的变化趋势，以及这些物性指标与沉积物磁学性质的突变界面表明，当前斜坡下沉积物蠕变活动在沉积物上部45至50米处最为活跃。目前尚无法确定，究竟是沉积物的逐步脱水作用使得该深度的物质达到塑性极限，使其无法再发生流动（进而加积于下伏沉积物之上），还是该深度代表了一处滑塌发生的界面。我们推测两种情况均有部分合理性：即块体运动与斜坡下改造作用在一个厚度自限的可移动物质层中发生沉积加积。