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 external changes to the entire margin itself (Figure 2). In the former case of internal origins, the margin migrates downslope with movement often along extensional normal faults. These movements are characterized by faulting (i.e., the generation of earthquakes) and by slow/silent earthquakes as material moves by semi-plastic, not brittle, flow. Earthquakes provide “instantaneous” energy pulses that can activate more faulting as well as other deformation within the sedimentary wedge. Slow/silent earthquakes move sediments within the wedge and contribute to the next round of earthquakes. In both instances, more “instantaneous” energies are ultimately applied to the sedimentary wedge (Figure 2).   

External changes to the sedimentary wedge of the continental margin can come from sources foreign to the margin itself: “instantaneous” energy from truly large earthquakes, super-volcanic eruptions, meteor impacts, and lower-frequency energy inputs from large debris flows/slump blocks descending to new settling locations (Figure 2) and geologically rapid deposition of sediments from huge glacial-lake outbursts during episodes of glacial advance/sea level lowering (Lowrie and Meeks, 1999). Changes in sea level position at whatever frequency and their impact on erosion, transportation, and deposition of sediment dominate/influence the internal mechanics of the continental margin. Thus, the evolution of the continental margin provides energies for the basinward migration of the margin itself (Figure 2).    

 

Low Frequency Energies to Impact the Sedimentary Wedge 

Massive variations of up to 4-5 orders of magnitude in deposition rates during late Pleistocene lowstands/glacial advances must have impacted geologic processes operating within the underlying sedimentary wedge along continental margins. Evidence for such deposition variations are indicated by an alternation of high-energy coarse terrigenous deposits (sands and gravels) with low-energy, fine-grained calcareous deposits (Coleman and Roberts, 1988 a and b).  During times of glacial advance/lowstand with sealevel intersecting the shelf break and sequestration of water as ice on the continents, a suggested lowstand flow along the Mississippi River in the area of modern New Orleans could have been as low as 10³ to 10² m/sec, creating braided-streams (Fisk, 1944), (a “normal” or average river flow over the past few decades may approximate 10⁴ m/sec, from data and discussion with the Army Corps of Engineers, New Orleans, Spring, 1999) (Figure 3a).   

During the short-lived glacial-lake outbreaks, the flow could be as high as 10⁶ m/sec. The initial flow rate would have been reduced as the migration became elongated on its gulfward rush. If the initial flow were reduced to one-half to one-quarter, the flow/flood rate would still be 500 to 250 times greater than a “normal” historic flow rate. Such massive Ice Age floods would explain the up to pea-sized gravel drilled and sampled in the Deep Sea Drilling Project holes of Leg 96 and in the petroleum-bearing sand and gravel reservoirs up to 50-100 meters thick exploited along the northern Gulf continental margin (see Atlas of northern Gulf of Mexico Gas and Oil Reservoirs, Bureau of Economic Geology, University of Texas at Austin, Volumes 1 and 2, 1997, 277 pages).   

Given such sudden debouchings of sediments that are possibly deposited in a few months to less than a year means that these acts of deposition must act as generating low-frequency pressure waves. Variations in the grain-sizes found within these petroleum-bearing reservoirs indicate that there can be appreciable variations in the energy of the overall deposition process.  These low-frequency waves of huge wavelengths may apply yet another specific source of energy to the sedimentary wedge and its evolution.    

 

Basement Inputs, Provenance of Energy:

     Sigsbee Salt Basinward Migration Affecting Basement Interaction    

Arguing by analogy, a weak form of logic, it may be possible to apply some aspects from interpreted dip cross-sections prepared for the thrust belts of Alberta, Canada (Enkin et al., 2000, Figure 5) and those prepared for the Front Range of Colorado (discussion with William B. Hansen, leader, AAPG Thrusted Terrains Field School, 14 September 2007) to a regional interpretation of the Sigsbee salt wedge/thrust sheet across the northern Gulf of Mexico. The interpretation for the Gulf of Mexico has sediment being deposited onto the semi-plastic Sigsbee Salt as the proto-Gulf begins to subside (Lowrie, 1994; Lowrie et al., 1996). Subsidence continues and the salt continues its migration basinward until the Late Cretaceous - Early Tertiary, at which time the salt commences to be buoyant and continues to migrate basinward up to the present time. A standard interpretation is that the advancing salt impacts a basement high, the Sigsbee Ridge, causing the salt in essence to “ricochet” off the basement and become buoyant. Here the arguing from the North American thrust belts occurs: The advancing Sigsbee Salt interact with the basement such that the basement is “broken”/ruptured and applies heat to the salt wedge (Figure 4). Thus, a possible combination of the impact with the Sigsbee Ridge and basement heat cause the salt to rise. With this addendum, the basement plays an active role with the overall salt wedge migration and is not strictly passive (Figure 4). 

 

Ancestral Paleo-Fracture Zones Potentially Providing Heat to Gulf Basin 

Following the plate tectonic model of Pilger, 1981 or the back-arc model of Fillon (2007) for the opening of the Gulf of Mexico, there can be paleo-fracture zones across the Gulf Basin existent from the spreading event (Lowrie and Moffett, 1998, Sykes, 1978; see Figure 5). These paleo-fractures are characterized by some secondary magma, geo-thermal fluid, and/or heat intrusions. Such intrusions, not yet documented for the Gulf under great thicknesses of sediment yet known from the world ocean basement (e. g., Constantin et al., 1996, AAPG Hedberg Conference, “Hydrocarbon Habitat of Volcanic Rifted Passive Margins,” p. 8-11, September 2002, Stavanger, Norway) become another source of energy applied to the Gulf Basin.   

Basement inputs, therefore, may range from magma injections to geo-thermal fluids to heat with the total energy input being highly variable.  

 

Energy Waves Interacting with the sedimentary wedge 

In summary, this brief review describes various energy sources, both within and external to the sedimentary wedge, with a wide range of frequencies and amplitudes that impact the evolving passive continental margin. The frequencies apparently can range from some cycles/sec to cycle/seconds up to cycles/month(s). These differing frequencies must be interactive, from subtractive to additive, further complicating the full range of possible impacts.    

The later dispersion of energies/impacts/disturbances may appear to be an exponential decay. Such a decay may create far-reaching effects, territorially and temporal. The discussion of range of impacts is well beyond the scope of this presentation. 

 

Format for Presentation of a Process Equation for the Northern Gulf of Mexico   

The format here consists of two phases: first, a preliminary word equation is presented, incorporating the principal types of geologic processes that appear to operate along a continental margin (Appendix A); secondly, subsidiary processes and/or subsets of processes are listed beneath each of the main process headings (Appendix A). Note that at this time there is only a process listing: determining the interrelationships between the various processes and rates at which they may proceed, algebraically and/or synergistically, is not attempted at present.    

Geologic literature usually consists of a description of geologic histories for a given area. These classic descriptive efforts contain temporally-based recitations of geologic events; i.e., a discussion of the processes that occurred at any one time, with the various events (and their associated processes) listed chronologically. There is no attempt to list all the geologic processes that could indeed occur at any one time. Such a listing of all possible processes focuses attention on the actual mechanics of how geology actually “makes” a continental margin (Appendix A). Once a broad and simplified scheme of the operative processes for any one geologic “moment” is available and understood as to the interlocking of the processes, then it is appropriate to extend this one moment of understanding into a continuum of geologic time.

 

Principal Processes 

A provisional listing of the most salient geologic processes is given below (also see Appendix A). The desire here is to envision all that may be functioning in a continental margin from seafloor to basement. The listed main processes include:          

• Sedimentation            

• Compaction    

• Lateral Movements    

• Basement Tectonics   

• Vertical Movements   

• Fluid Movements       

• Tectonics        

• Critical Cohesive Coulomb Wedge   

• Discontinuities 

 

To emphasize, this list is provisional.  

 

Secondary Processes 

A listing of secondary processes is included beneath the tabulation of principal (or main) geological processes (Appendix A). These “secondary” processes may be thought of as either as minute or minimal aspects of the principal process or as processes, sovereign unto themselves, whose resultant effects, when summed with the results of other equally sovereign processes, comprise a single, easily recognizable geologic result.  

 

Process Interactions 

Algebraic and/or Synergistic   

Given that the various geologic processes listed herein do interact, mention is required concerning how the interactions may actually proceed. There appear to be two main modes: that the interactions are algebraically additive or synergistic. Detailed comprehension of each specified process will be needed before the interactions may be properly defined. Establishing such comprehension is well beyond the scope of this very preliminary compilation.

 

Conclusions 

Presented here is a preliminary process equation that has been developed to represent geologic processes operating at present along a “dynamic” passive continental margin, such as the northern Gulf of Mexico. Similarly dynamic margins include the Gulf of Cadiz, the SE Brazilian margin, the Angolan and Nigerian margins. Preparing a process equation for a continental margin is a novel effort designed to facilitate analyses of the geology thereof. The compilation of processes is at least a summary of what occurs, a cookbook, if one wishes such a designation. Review of the listed processes may facilitate the understanding of heretofore unrealized processes or process sequences, a series of processes that appear to function in common. Also, knowledge/data vacancies may be made more apparent. This is a progress report on an emerging inquiry tool, the final destination of which is not obvious.    

 

References 

Coleman, J. M., and H. H. Roberts, 1988a, Sedimentary development of the Louisiana continental shelf related to sea level cycles: Part I – Sedimentary  Sequences: Geo-Marine Letters, v. 8, p. 63-108.

Coleman, J. M., and H. H. Roberts, 1988b, Sedimentary development of the Louisiana continental shelf related to sea level cycles: Part II – Seismic Response: Geo-Marine Letters, v. 8, p. 109-119.

Constantin, M., R. Hekinian, D. Bideau, and R. Hebert, 1996, Construction of the oceanic lithosphere by magmatic intrusions; Petrologic evidence from plutonic rocks formed along the fast-spreading East Pacific Rise: Geology, v. 24, p. 731-734.

Enkin, R. J., K, G. Osadetz, J. Baker, and D. Kisilevsky, 2000, Orogenic remagnetizations in the Front Ranges and Inner Foothills of the Southern Canadian Cordillera: Chemical Harbinger and Thermal Handmaiden of cordilleran deformation (Extended Abstract): Montana/Alberta Thrust Belt and Adjacent Foreland, Volume II, Road Logs, Presentation Abstracts, and Foothills Analysis, R. A. Schalla and E. H. Johnson, eds., Montana Geol. Soc., 50th Anniversary Symposium, p. 13-15.

Fillon, R. H., 2007, Mesozoic Gulf of Mexico basin evolution from a planetary perspective and petroleum system implications: Petroleum Geoscience, v. 13, p. 105-126.  Fisk, H. 1944, Report for Army Corps of Engineers, Vicksburg, Mississippi.

Lowrie, A., and L. Jenkins, in press, Margin Tectonics: Passive continental margins migrating basin-ward: Gulf Coast Section SEPM.

Lowrie, A., and P. Meeks, 1999, Calcareous Upper Pleistocene deposits indicate shelf/slope tectonic instability, Energy Exploration and Exploitation, v. 17, nos. 3 and 4, p. 301-309.

Lowrie, A., and S. Moffett, 1998, Pressure compartments, existent and suggested, along the Louisiana continental margin, Energy Exploration, and Exploitation, v. 16, no. 4, p. 345-354.

Lowrie, A., R. Hamiter, M. A. Fogarty, T. Orsi, and I. Lerche, 1996, Thermal and time-temperature (TTI) patterns during geologic evolution of north and central Gulf of Mexico: Trans. Gulf Coast Assoc. Geol. Soc., v. 46, p. 249-260.

Pilger, R. H., 1981, The opening of the Gulf of Mexico: Implications for the tectonic evolution of the northern Gulf Coast: Trans. Gulf Coast Assoc. Geol. Soc., v. 31, p. 377-381.

Sykes, L. 1978, Intra-plate seismicity, reactivation of pre-existing zones of weakness, alkaline magmatism, and other tectonics post-dating continental separation, Rev. Geophys. Space Phys., v. 16, p. 621-688. 

 

Appendix A

“Dynamic” passive continental margin process equation: Prototype: Northern Gulf of Mexico (I + II + III + IV V VI) 

Factors in the equation     

I) Sedimentation   

1) Sedimentation

• a) first order depositional unit--duration: 250 million years +/- 75 million years.  Sea-level oscillation: 200-400 m.

• b) second order depositional unit--duration: 10 million years +/- 5 million years.  Sea-level oscillation: 100s of m.

• c) third order depositional unit--duration: 1.0 million years +/- 0.5 million years.  Sea-level oscillation: up to 125-135 m.

• d) fourth order depositional unit--duration: 20,000 to 40,000 to 100,000 years.  Milankovitch oscillations: Sea-level oscillation: up to 125-135 m.

• e) fifth order depositional unit—duration (upper Pleistocene highstands) - a thousand years +/- 500 years; (upper Pleistocene lowstands) - 1 to 5 thousand years +/- up to 2.5 thousand years.

• f) sixth order depositional unit--duration: 100 years +/- 50 years, 

 

2) Deposition rate -- varies from fluvial to abyssal plain; from meters/day to mm/1000 years.

 

II) Compaction and Subsidence    

1) Compaction-Subsidence = mechanical re-arrangement.    

2) Expulsion of free waters >> porosity: 90-75%,

• Depth range = 0 to 1000 m in slow depositional environment and 0 to 5000+ m in rapid depositional environment

  • “slow”—mm/1000 years

  • “rapid”—up to meters/day to m/decade     

3) Mechanical re-arrangement of particles

• Depth range = 0 to 1000 m

  • dewatering; elasticity in presence of water - porosity >> 75 to 35%

4) Expulsion of absorbed water – porosity >> 75 to 35%.

• Depth range = to 5 to 10-15 km  

5) Mechanical deformation (mechanical deformation of particles)

• Depth range—0.5 to 10-15 km

  • porosity—35 to 10 %           

6) Recrystallization - solution and recrystallization

• Depth range = 1-2 km to base of basin          

7) Water loss through chemical combination

  Depth range—2-3 km to base of basin    

 

III) Lateral Movements   

1) Lateral Movements—sediments—extrusives/salt, shale 

• Crustal blocks

  • Extrusives>> salt/shale/crustal blocks 

• Lateral motion rates>>>0 to 2-5 cm/yr.

  • Possibly most characteristic of lowstands—movements presumably intermittent

  • Prototypes: Gulf of Mexico/Gulf of Cadiz   

• Continental margins

  • Fractures >> lateral movements

    • initial fracturing

    • secondary fracturing

    • (gas accumulates, advancing gas wedge as fractures prograde)

  • Inversion tectonics

    • extensional faulting into compressional thrust faults, due to lateral movements

      • lateral movements>> local/regional

      • motion rates: up to 2-5 cm/yr

      • depth range: 1 km to base of basin    

 

IV) Basement interactions    

1) Basement interactions

• varying geothermal gradients

• paleo-fracture zones and reactivation

• varying stresses due to overburden thicknesses

• fluid extrusions

• magma emplacements                   

2) Geothermal gradients—heat conduction influenced by fluid inclusions:

marine water(1.03 density) - fresh water(1.0) - oils(0.9) - gases(0.01s)   

 

3) Paleo-fracture zones—ancestral continental breakage—possibly representing paleo-plate tectonics—possibly representing lithosphere-asthenosphere mis-adjustment (“inactive” marine fracture zones) 

4) Activities:

• magma and/or fluid intrusions

• loci of earthquake generation

• stress re-organization 

5) Varying stresses due to overburden thicknesses onto basement--basement serving as an isostatic horizon within a Pratt model--(What isostatic horizons may exist within overburden; i.e., as with salt wedges or extruded crustal blocks?) 

6) Rupture from seafloor spreading—fracture-zone spacing due to multiple processes 

7) Ruptures within transition zone of continental-oceanic crusts—aperiodic—spacing heterogeneous  

8) Magmatic emplacements—stress fields variable locally due to magma emplacement and/or magmatic-fracture zone interaction;

transitional zone elevation differences from continental-oceanic equilibrium levels     

 

V) Vertical Movements   

Vertical Movements—salt, shale, and magma diapirs        

 

1) Salt-shale vertical movements

• buoyant up-lift

• isostatically controlled by incoming sediment accumulation 

2) Magma vertical movements

• thermally driven uplift with magma chamber in regional equilibrium; geologic/sedimentary, atmospheric, and oceanic build-ups or devolutions may initiate magma diapir.

 

VI) Fluid Movements    

Fluid Movements—water: fresh-brackish-marine and hydrocarbons: Oil-gas, as gas, liquids, and solids/“hydrates” 

 

1) Fluid movements—transiting migration routes—from major regional faults, leakage through “shattered” sediments to buoyantly created migration paths

• flow rates = mm/(<10-100)cm/yr to (rarely) km/yr        

2) Buoyant forces penetrating through unconsolidated sediments opening and closing migration routes—           

• shales, predominantly clays, may smear along fault traces, closing faults.

• advancing salt/shale diapirs and /or salt wedges, opening and closing faults.

• natural gases rising generally vertically, creating pathways for buoyant fluids.

• predominantly gases, seasonally residing in near-surface 

3) Sediment blockage--hydrate formation between shelfbreak and continental rise(depth range, circa 0.5 and 0.2 to 4 to 5.5 km):

may serve as:              

• block to rising fluids

• hydrate formation intermittent—

• on multiple scales, from regional to local

hydrate formation—can be in multiple zones, often in a single zone