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Figure Captions 

	Figure 1. Schematic dip cross-section over the northern Gulf of Mexico, showing basinward movement along extensional normal faults within the sedimentary wedge and within the rifting basement, causing the entire continental margin to migrate basinward and the occurrence of structurally weak materials (salt, shale, and overpressured sediments) at the base of the sedimentary wedge.
	Figure 2. Schematic dip cross-section indicating the occurrence of brittle-fracture earthquakes (marked by stars with released energy radiating omni-directionally) and slow/silent earthquakes (S/S E) (marked by “shaded patch” as semi-plastic, non-brittle flow, changing volumes, ergo, stresses, within the sedimentary wedge; as represented here, the S/S Es are originating circa the paleo-shelf-breaks and their listric faults and along the mid to lower slope; an infinity of other origins are conceivable).  Shelf-break/upper slope originating slump blocks/debris flows also provide energy to the margin.
	Figure 3. Massive short-term glacial-lake outburst depositions during lowstands generating low-frequency stress fields.  Figure 3a. During high sea-level stands, river-borne sediments reach the coastline with deposition occurring across the continental shelf. “Normal” river flowage rates ~ 104 m3/sec.   Figure 3b. During low sea-level stands in times of glacial advance, river-borne sediments reach the present-day shelf-break and continental slope with deposition across the continental slope. With the unusual (once every half century to multiples centuries) glacial-lake outbursts, the up to pea-sized gravel on the continental slope. “Normal” river flow rates of ~ 102 – 103 m3/sec are sufficient to create braided streams in the paleo-Red and Mississippi River valleys.  CL = coastline.
	Figure 4a. Interpretative dip geologic cross-sections across the Alberta thrust belt and potentially the Colorado Front Range, indicating that as the main ranges rose, secondary front ranges also rose. These massive uplifts may have interacted with the basement, releasing magma and energy to the evolving front ranges (Enkin et al., 2000 and discussion with William B. Hansen, independent petroleum geologist, Great Falls, Montana, 14 September 2007).  CL = coastline.  Figure 4b. Advancing Sigsbee salt wedge and overlying salt-floored basin of coastal plain, continental shelf, and slope sediments interact with the basement uplift, Sigsbee ridge, releasing energy and causing/influencing the salt to “ricochet” off the basement.  Scale in km.
	Figure 5. Referenced paleo-fracture zones, originating as ancestral continental breaks following the model of Sykes (1978). The Gulf floor is presumably cut by numerous fractures generally trending NW-SE with a spacing of some 60 miles, similar to that of the North Atlantic (from Lowrie and Moffett, 1998).

 

Introduction 

The classic passive continental margin model of previous decades was of evolution over a perceived structurally strong feature. That notion of stability is eroding as the impact of interlocking geologic processes are better understood. The entire continental margin, instead of being stable, may itself be dynamic as it “slides” intermittently into the Gulf of Mexico. For this to be true, there must be processes that individually, collectively, and synergistically, are weakening the margin in such a way that it moves minutely, locally, and regionally--each unit possibly moving independently, yet forming a single tapestry of deformation.    

Listing known geologic processes, a task heretofore never done, and tabulating rates or estimates of ranges of rates at which they may operate, also a non-existent compilation, may be a worthy exercise. Such process listings do provide a unique and a previously unavailable overview of how geology operates and interrelates, at least on the conceptual level. Continental margins, with their dynamic natures, appear to lend themselves to this novel exploratory format. Given that the northern Gulf of Mexico is the most extensively explored margin in the world and the authors' familiarity with it, this initial listing of processes was conducted in the context of the Mississippi-Louisiana-Texas offshore. This overview leads to avenues of improved comprehension of known interrelationships and possibly indicating future exploration.   

The sophistication and sensitivity of datatypes collected become ever more impressive with time as does the petroleum exploration into increasing deep waters and greater subsurface depths. Interpretations also become more precise, using such techniques as geometry-based sequence stratigraphy and using the geophysical data itself to derive geoacoustic/geomechanical characteristics. Greater abundance of digital data can be handled more complexly and speedily, employing continually increasing computer capabilities.  

Given this burgeoning range of capabilities in all aspects of geologic exploration, there does seem to be a shortage of synthesizing what these exploratory capabilities are actually measuring: the geology. The measured geology is the resultant of the interlocking processes that created it. A process is “a phenomenon that shows a continuous change in time;” change is continuous; yet the rate of change itself may vary within the process monitored.   

 

Objectives of This Poster 

The objective here is to list within the overall geologic context known geologic processes that apparently operate during the evolution and maintenance of a “dynamic” passive continental margin, such as the northern Gulf of Mexico, the Gulf of Cadiz, the SE Brazilian margin, the Angolan and Nigerian margins. This should be regarded as a “progress report.”  

 

Overall Geologic Context for Passive Margins 

The overall context for the movement of passive continental margins, even those such as the northern Gulf of Mexico containing dynamic elements such as migrating salt and shale, is that they migrate periodically downslope and basinward (Lowrie and Jenkins, 2007). In the case of the Gulf, the continental margin extends from the fall line/fall-line hinge fault, classically portrayed at Little Rock, Arkansas, to the abyssal plains. This margin represents an overall dip-oriented lateral distance of some 10 degrees latitude, circa 1200 km. It descends from continental elevations of ca. 500 m to submarine depths of greater than 3 km.   

As the rifting basin rapidly evolved during the tectonic subsidence phase, various rifted basement blocks have subsided and rotated basinward, with seafloor spreading and plate tectonics determining subsidence rates (Figure 1). Tectonic motions within the overlying sediment cover are primarily extensional, as shown by listric faulting. Localized compression occurs at the foot of the listric faulting. Dynamic migration of shale or salt serves as a “tectonic escape” moving basinward and downslope. These motions range from margin spanning, to regional, to local, to microscopic; the motions occur on time scales varying from instantaneous to geologically slow.   

The sedimentary units, determined from sequence and seismic stratigraphy, record characteristics of deposition itself. The sediments at present occur at greater depths in the subsurface than their original site of deposition, as a result of subsidence and lateral migration (Figure 1). Margin tectonics may be viewed as driven by two sets of inputs/energies: one is derived from internal mechanics of the basinward migration; the other, from exter