Holocene evolution of the deltaic plain: a perspective − from Fisk to present

Abstract Before publication of Fisk's classic scientific papers dealing with the Mississippi River alluvial valley and deltaic plain, geological knowledge of the Holocene deltaic plain was the product of surficial geomorphological studies with a temporal framework provided by archaeology. Fisk and his co-workers provided the third and fourth dimensions, the three-dimensional characteristics of sediment bodies, by numerous deep borings and better chronostratigraphy through 14 C dating. This research, and the emphasis of his work on fluvial processes, was largely responsible for the form-process approach in sedimentary geology and the awareness that depositional environments are represented by unique sedimentary sequences and properties. Although Fisk made many contributions to deltaic geology, six major areas are noteworthy: delta response to base level changes, sedimentary loading, processes of delta switching, delta abandonment and formation of transgressive sands, river diversion (Atchafalaya distributary), and deltaic sedimentary architecture. In order to comprehend the complexly structured Holocene deltaic plain, the processes of channel migration, meander belt formation, avulsion, and delta switching must be understood. Preceding the work of Fisk, landforms resultant from these processes were recognized by geomorphologists, and this body of research formed the foundation for Fisk's major contributions. Using the concept of changing sea levels during the Quaternary, Fisk was one of the initial scientists to recognize the importance of a changing base level and its effect on valley cutting and filling episodes, particularly during the process of valley filling by fluvial and deltaic sediments during rising sea level. In documenting these cyclic processes, he first postulated the development of canyons or trenches carved across the continental shelf, the development of shelf-edge deltas, and the feeding of the deep-sea fans during periods of lowered sea level. Trowbridge (1930) and Russell and Russell (1939) described the dendritic shape of delta lobes and indicated that they were offset and overlapping, resulting in the Holocene deltaic plain configuration; delta chronology was provided by utilizing archeological methods (McIntire, 1954). The pioneering work of Fisk and McFarlan (1955) on the Mississippi delta set the stage for refinement of delta lobe chronology. Details regarding the timing of important depositional events have been added by McFarlan (1961), Frazier (1967), Morgan (1970), Penland and Suter (1989) and, most recently, Autin et al. (1991). In collecting data on offshore Mississippi River delta deposits, Fisk documented the regional subsidence associated with loading by continuous delta deposition as well as the localized subsidence associated with delta-front deposition and the formation of mud diapirs. He later applied this concept to the accumulation of peat deposits in interdistributary regions and the development of interdistributary bays. This research was the foundation for work by later scientists dealing with wetland loss in the deltaic plain. Fisk was the first major contributor to our current understanding of the stratigraphic-sedimentological architecture of the modern deltaic plain of the Mississippi River. The first cross-sections of the deltaic plain were published by Fisk (1947), prior to which there were little data for establishing the geometry of sediment bodies and spatial relationships of facies beneath the modern surface. Classic examples of this approach were the point bar and backswamp studies in the alluvial valley of the Lower Mississippi River (Fisk, 1944), his bar-finger sand paper (Fisk, 1961), and his more comprehensive sand facies paper (Fisk, 1955). This body of work on sedimentary facies relationships and specifically sand body geometries found wide acceptance among both academic and industry groups. At the time Fisk prepared his appraisal of sand facies in the Mississippi delta he was an employee of Humble Oil and Refining Company, and the paper was obviously directed toward understanding reservoir geometries and spatial relationships in a deltaic complex. In addition to the sedimentary architecture of the delta, Fisk also contributed to understanding of both the constructive and destructive cycles, the geotechnical properties of various delta facies, and subsidence, all of which helped establish an awareness of the critical need for geological knowledge in engineering applications. Since Fisk's contributions, the major advances in understanding the Holocene deltaic plain have been primarily in the realm of filling in the details of Fisk's sedimentary architecture. The mudlump studies of Morgan (1961) and work on crevasse splays and bay-fill sequences by Welder (1959), Coleman and Gagliano (1964), and Morgan (1970), as well as others, are good examples. Later, work on the new lobe of the deltaic plain, the Atchafalaya-Wax Lake delta complex (Shlemon, 1975; Roberts et al., 1980a, van Heerden and Roberts, 1980; van Heerden et al., 1983), added an element to the deltaic plain evolutionary story that was not available during Fisk's era. However, regardless of future research on the Mississippi River deltaic plain, the body of work produced by Fisk and his co-workers will always be recognized as the cornerstone of our understanding of this complex depositional system.