Depositional Architecture of the Lagniappe Delta: Sediment Characteristics, Timing of Depositional Events, and Temporal Relationship with Adjacent Shelf-Edge Deltas
Harry H. Roberts, Richard H. Fillon, Barry Kohl, John M. Robalin, Johan C. Sydow, 2004. "Depositional Architecture of the Lagniappe Delta: Sediment Characteristics, Timing of Depositional Events, and Temporal Relationship with Adjacent Shelf-Edge Deltas", Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico Margin, John B. Anderson, Richard H. Fillon
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During the stepped fall of eustatic sea level from oxygen isotope stage 5 to the latest glacial maximum (oxygen isotope stage 2, approximately 18 ka), the Mobile River (possibly joined by the Pascagoula River) entrenched a delivery network into the continental shelf and deposited a complex delta that prograded to the shelf edge just east of the modern lobe of the Mississippi River delta. Over 4500 km of high-resolution 2D seismic profiles and four coreholes (MP303c1, MP288c1, MP242c1, and VK774c1) through these deposits have provided the data base for interpreting the depositional architecture, stratigraphic units, stratal surfaces, stacking patterns, and sediment characteristics of a beautifully preserved late Pleistocene shelf-edge delta (the Lagniappe delta).
The complex clinoform wedge that defines this delta built in two simultaneous progradational thrusts, defining western and eastern deltaic depocenters. Each depocenter contains constructional subcomponents composed of clinoform sets interpreted to be the products of both a stepped sea-level fall and autocyclic processes. The coreholes and associated seismic profiles define a coarsening-upward sedimentary sequence that thickens to over 100 m at the shelf edge and thins rapidly to less than 20 m on the upper slope. Clinoforms representing the stage 2 delta deposits downlap onto the toes of fifth-order late stage 3 clinoforms, which in turn downlap an outer-shelf shale underlain by a calcareous condensed section (oxygen isotope stage 5). Tops of the clinoforms are extensively eroded, resulting in a complex NE–SW oriented fluvial scour. Graveliferous sand fills this scour relief at the MP303c1 site. While clinoform development at the shelf edge is interpreted as a late-falling-stage depositional response, infilling of the complex fluvial scour has previously been assigned to the lowstand turnaround and early rise (oxygen isotope stages 1–2 transition). However, dating of the clinoforms in both the eastern and western depocenters by AMS 14C methods indicates an age of approximately 19 ka, i.e., just prior to the glacial maximum.
Age relationships between the Lagniappe clinoform wedge and a similar clinoform package that forms a protrusion of the shelf edge to the east has never been satisfactorily resolved. Correlation of reflectors of known age from VK774c1 along high-resolution seismic profiles acquired in 1997 to the first clinoform set to the east of the Lagniappe delta depocenters shows that these eastern clinoforms are much older than oxygen isotope stage 2 clinoforms of the Lagniappe system. The most probable correlation suggests that these clinoform sets were deposited during oxygen isotope stage 8 or stage 6.
Even though eustatic sea-level fall to the latest Pleistocene glacial maximum was impressive and rapid, researchers working on the Lagniappe delta have not previously reported evidence of fluvial incision of the shelf break and no large slope channels or canyons have been associated with the delta as sediment feeder systems to deep-water depositional sites. Furthermore, previous observations argued for a maximum fall of eustatic sea level during oxygen isotope stage 2 that was considerably less than the generally accepted -120 m level. Dates from both the western (MP303c1) and the eastern (MP288c1) parts of the delta suggest formation times of approximately 19 ka and at elevations of about –85 m to –89 m below present sea level. High-resolution seismic data collected by the authors in 1997, however, demonstrate that clinoform sets developed at approximately the –120 m level along the eastern progradational track of the Lagniappe delta. The deeper clinoform wedge is stratigraphically younger than the 19 ka lobes and formed at an elevation of approximately –119 m to –126 m below present mean sea level. With no recent faulting evident, these observations suggest a sea-level lowering of over 30 m in a 1–3 kyr period if the age of the glacial maximum is accepted as 16–18 ka. We believe that this sea-level fall is much too rapid to be accounted for solely by an increase in global ice volume; nevertheless, we are convinced that our age and level data are reliable.
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The northern Gulf of Mexico margin encompasses a variety of depositional settings characterized by different drainage basin size, physiography, fluvial morphology, climatic setting, and structural and diapiric activity. This, plus the abundance of long sediment cores and platform borings from oil industry activities, make it an unparalleled natural laboratory for sedimentological and stratigraphic studies and for testing sequence stratigraphic concepts. This volume contains twelve papers describing results from high-resolution stratigraphic studies of late Quaternary strata of the northern Gulf of Mexico, from the mouth of the Apalachicola River to the Rio Grande. These papers focus on fluvial response to climate and base-level change, variations in delta growth and evolution across the shelf, lowstand delta-fan evolution, the evolution of transgressive deposits on the shelf, the preservation of these deposits. The robust chronostratigraphic frameworks developed for the different study areas allows comparison of stratal geometries produced by contemporaneous depositional systems operating under identical eustatic conditions. This volume will appeal to sedimentologists and stratigraphers interested in source to sink issues, such as how various forcing mechanisms influence strata formation on continental margins.