Abstract During the last fourth-order glacial-interglacial cycle (e.g. post oxygen isotope stage 5) the Lagniappe Delta system located in the northeast Gulf of Mexico, prograded several tens of miles seaward along two main NE–SW and N–S trending fairways. The delta complex is underlain by a well-developed calcareous shale-rich condensed section that was deposited during isotope stage 5. The delta complex comprises many progradational lobes that were deposited during fifth-order sea-level falls and during the fourth-order maximum lowstand and early rise of sea level associated with isotope stages 4 to 2. Each significant fifth-order sea level fall developed a sequence boundary with an unconformity updip and a correlative conformity downdip on which a delta lobe was deposited. Autocyclic processes also lead to deposition of numerous lobes through lobe switching. During the maximum sea-level lowstand, deep erosion related to the development of an extensive incised valley system occurred across the top of the prograding wedges that were deposited during relative sea-level fall (the falling stage systems tract) and modified all of the previous updip unconformities. The base of the incised valley system, and its correlative downdip conformity, form the main fourth-order sequence boundary. It is on this surface that the last part of the delta complex was deposited during the maximum lowstand and early rise of sea-level. Thus, during the last fourth-order cycle, several fifth-order ‘initial’ sequence boundaries and one fourth-order ‘final’ sequence boundary were formed. Infilling of the incised valley system occurred mainly during the early and late rise of sea-level (isotope stages 2 and younger), prior to a major landward shift of deltaic sedimentation in response to the rapid eustatic rise in sea level during isotope stage 1.

ABSTRACT From a multidisciplinary study that utilized high-resolution seismic and a variety of analytical data from four continuously-cored boreholes, we have identified six major (and many minor) sequence boundaries and maximum flooding surfaces associated with the oxygen isotope stages 14 to 1 (since 500 KY BP) within the Late Pleistocene shelf margin Lagniappe delta complex in the NE Gulf of Mexico. We discuss here in detail a portion of the delta complex that prograded seaward for several tens of miles during the last, fourth-order, glacial-interglacial cycle since 125 KY BP. A well-developed calcareous-rich interval deposited during the isotope stage 5 (125-70 KY BP) underlies this portion of the delta complex. The delta complex consists of many lobes that were deposited during the several, fifth-order, sealevel drops within the falling stage and during the maximum lowstand-early sealevel rise of the cycle. We have recognized one main fourth-order sequence boundary that resulted from the erosion of an incised valley during the maximum lowstand of sealevel, and several, fifth-order sequence boundaries that were created during the minor sealevel drops within the falling stage. The incised valley-fill deposits consist of distributary channel deposits, bay-head deltas and estuarine deposits that were subsequently transgressed during the Holocene sealevel rise. This transgressive surface forms the base of isotope stage 1. On the whole, the isotope 5 to 1 stage interval of the Lagniappe Delta can be divided into four systems-tracts—highstand, falling stage, lowstand-early rise and transgressive.

Abstract: Although lowstands of sea level greatly favor the development of deep-sea fan systems, the timing and type of turbidite events in these systems may depend not only on the sea-level changes but also on the nature of available sediments, tectonic setting, size, and gradients of the basins. Thus, in basins with steep gradients, located on continental or transitional crust or along active margins close to sedimentary sources that could supply coarse-grained sediments, unchannelled turbidite sand lobes detached from updip channels or valleys might have been deposited during lowstands of sea level. Channel-attached sand lobes and channel levee complexes might have been deposited during sea-level rises in these basins. In contrast, large fan systems, such as the Indus Fan, located off passive margins in oceanic basins with flat gradients, distant sedimentary sources, and a predominantly fine-grained sediment supply have channel-attached lobes and channel levee, overbank complexes that were probably deposited during lowstands and to some extent during highstands of sea level.