Correction of Holocene sedimentation rates for mechanical compaction: the Tigris-Euphrates Delta, Lower Mesopotamia

Abstract

Holocene sedimentation rates have been calculated for the lacustrine/deltaic deposits of Lower Mesopotamia. The calculations depend on the radiocarbon dating of five organic-rich borehole samples and four near-surface shelly samples. Rates between 1 and 1.8 mm/year were dominant throughout the Holocene from 8400 years BP until about 3000 years BP . During the later stage of the Holocene, rates of 0.4 mm/year were not exceeded. The role of near-surface overburden compaction was excluded in these calculations. Near-surface overburden compaction is an effective mechanical process, particularly for muddy deposits such as those of the Tigris-Euphrates Delta. However, such compaction is usually ignored by most petroleum geologists down to >100 m depth. A geotechnical approach was developed to measure the relative amount of this compaction and to correct the above-mentioned sedimentation rates geotechnically. Correction curves were established, based on laboratory consolidation tests, for the three dominant sedimentary types sand, silt and clay fractions. In addition, a general correction curve was derived based on the general distribution of these three types of sediment in the Holocene succession of the Tigris-Euphrates Delta. Each curve shows the relation between the percentage of thickness reduction, due to mechanical compaction, and depth. Thickness reductions of up to >25% have been determined, particularly within the first 5 m of the sedimentary column. Below this depth interval, the slope of the curves reduces down to 15 m depth and more. When the previously mentioned sedimentation rates were recalculated using the established curves they increased to 1.3–2.2 mm/year. Based on these latter calculations, it is essential for petroleum geologists to reassess the concept of near-surface overburden compaction.