The Geochemistry of Recent Mississippi River Delta Sediments: Gas Concentration and Sediment stability

Abstract

ABSTRACT River-mouth depositional patterns are modified by sediment-deforming processes of sufficient magnitude to severely endanger bottom supported structures. Several types of deformations are present, including (a) peripheral slumping, (b) differential weighting and diapirism, (c) radial tensional faulting, (d) mass wasting and flowage induced by wave motion and degassing, and (3) deep-seated clay flowage. The processes of bacterial methane production and the resulting effects on sediment-deformation have been investigated in four cores taken in the Recent deltaic sediments of the Mississippi River. Dissolved methane in the interstitial waters ranged in concentration from 2 × 10−3 to 1.7 m1/1. High concentrations of methane corresponded to zones of low shear strength and were observed where dissolved sulfate was depleted. Calculations of maximum in situ methane concentrations, based on chemical reduction of excess total C02, indicate that methane could be present above saturation levels (bubble phase). Classical anaerobic geochemical gradients were observed in sediment profiles where no movement had previously occurred. The pore water geochemistry of sediment profiles within peripheral mudflows suggest that the coexistence of methane and sulfate indicate convective mixing of sediments and bottom seawater. Calculations using the equilibrium slopes of wave-induced mudflows indicate that shear strengths during movement must be less than the values measured before or after the flow. An improved model of mass movement is presented which relates the physical and geochemical properties of unstable sediments. INTRODUCTION Analyses of sediment borings, high-resolution seismic profiles, side-scan sonar tracks, and repeated hydrographic surveys have disclosed that a wide variety of subaqueous mass movements are active in the delta region. The major types of deformational features that have been identified include (a) peripheral faulting and slumping; (b) differential weighting and diapirism; (c) radial graben and tensional faulting; (d) mass wasting and flowage induced by wave motion and degassing; (e) deep-seated clay flowage (mud noses); (f) shelf-edge rotational slumps and normal faults; and (g)'deep-seated normal faults. The generalized distribution of these features is shown off South Pass in Fig. 1. Over steepening of the bar front near the mouths of major distributaries results in the formation of peripheral slumps, which have longitudinaldimensions of 60 to 600 meters and lateral dimensions of 200 to > 800 meters. Differential loading by denser bar sands overlying low-density clays results in vertical and seaward flowage of clays contemporaneously with seaward bar growth. Diapiric folds and spines intrude into delta-front sediments on the seaward side of the deforming load, and vertical movement affects sediments to depths in excess of 200 meters. Seaward flowage of clays beneath the deforming delta load stresses the seaward periphery of the delta front, the stress being relieved by the formation of small radial tensional or graben faults 50 to 500 meters wide, several kilometers long, and ranging from 4 to 40 meters in vertical offset. Formation of these features has been observed within a 6-month period where repeated surveys have been conducted. The passage of extreme surface waves (i.e., during hurricanes or tropical storms) results in a dynamic change in bottom pressure.