U.S. Geological Survey Open-File Report 00-352

GEOLOGY OVERVIEW - A BRIEF SUMMARY OF THE GEOLOGICAL FINDS OF THIS STUDY AND WHERE THE RESULTING DATA CAN BE FOUND ON THIS CD-ROM

This CD-ROM releases data that were used in several publications describing processes associated with recent deposition on the Mississippi Fan.  Interest in the findings of this work makes it appropriate to release the data and not just the summary interpretive reports.  Petroleum geologists have used results from this research to modify models of deep-sea fan sedimentation, and in turn, hydrocarbon exploration techniques (Shanmugam, 1999).  Additionally, because of the detailed nature of this work, geologists studying ancient deep-sea fans are finding these results valuable because the scale of features resolved in this modern study is similar to that seen in the rock record and thus the link between the modern and ancient analogs can be made more readily (Bouma , 1997; Bouma and Rozman, 2000).  In this section we briefly summarize some of the key findings and cite the references for those interested in more complete discussions of these findings.  In all of the sidescan images shown on this CD-ROM, high-backscatter refers to strong acoustic returns from the seafloor and is represented by light tones while low-backscatter is shown in the darker tones.

GLORIA sidescan-sonar imagery was collected in the deep-water part of the Gulf of Mexico in 1982 and 1985, and these data provided an unprecedented view of an entire large deep-sea fan system (Fig. 1 McGregor and Twichell, 1985; EEZ-SCAN 85 Scientific Staff, 1987). Analysis of this regional data set identified the main meandering channel system of this fan (Garrison and others, 1982), and also showed that the surface of the fan could be broken into several smaller depositional units (Twichell and others, 1992).  The smaller depositional units suggested active channel shifting and redirection of fan sedimentation during the recent development of this system.  Additional studies have shown that backscatter strength increases with increased sand content in cores (Kenyon, 1992), and a quantitative analysis of the backscatter intensity of the surface of the fan (Wen and others, 1995) that revealed a similar pattern to that presented by Twichell and others (1992).  The different data types from this regional survey have either been archived at the NOAA National Geophysical Data Center in Boulder, CO (Lubinski and Twichell, 1989; Twichell and others, 1989) or were released in digital form on CD-ROM (Twichell and Polloni, 1993; Paskevich, 1996; Twichell and Paskevich, 1996; Paskevich, 2000).

Based on this regional understanding of the entire Mississippi Fan, a follow-up study was conducted on one of the depositional lobes identified in the GLORIA imagery (Fig. 2).  SeaMARC 1A sidescan-sonar imagery and sediment cores were collected during a 1-month field program in 1990. The SeaMARC 1A system is a deep-towed sidescan-sonar system that operates at 30-kHz (Kosalos and Chayes, 1983), and presents a higher resolution image of the sea floor than is provided by the GLORIA system.  The SeaMARC 1A imagery presented on this CD-ROM has a 25-m pixel size in comparison to the GLORIA imagery which has a 100-m pixel size.  The survey started on the main channel at a site where the levee was breached and traced the channel that originated at this breach continuously to its termination at the distal edge of the fan.

The main channel and the breached levee were studied to understand the processes by which sedimentation is redirected to a different part of the fan.  Twichell and others (1996) found that the main channel of the Mississippi Fan below the point where the levee was breached is choked with mass flow deposits, and that sediment also spilled through the breach in the levee onto the adjacent fan.  The smaller leveed channel that extends eastward from the breach formed after this event as it rests on top of mass flow deposits (Fig. 3).

The channel that originates at the breach in the levee of the main channel was traced to the distal edge of the Mississippi Fan where the deposits associated with the end of this channel system were mapped (Fig. 4).  The SeaMARC imagery also shows several small channels near the distal edge of the fan that each have high-backscatter deposits associated with them (Twichell and others, 1992).  These high-backscatter deposits have remarkably abrupt, fern-like edges.  The abrupt edges to the deposits and their close association with channels suggests that the flows are channelized and that they flow very short distances as unchannelized flows before “setting” and leaving the abrupt transition from high backscatter to low backscatter (Twichell and others, 1992). Cross-cutting channels indicate that only one channel system was active at a time.  At least ten separate channel systems have been mapped and were presented in large atlas form by Twichell and others (1995).  This interpretation has been digitized, and is presented on this CD-ROM as an ArcView shape file that can be openned using the appropriate ArcView "apr" file which can be found near the end of the "GIS Introduction" section.  In this interpretation, the oldest of the units is numbered 1 and the youngest is numbered 10.

Cores from the distal edge of the fan recovered a mix of turbidite sands and debris flow deposits from areas of high backscatter and hemipelagic clays from areas of low backscatter (Fig 5, core list table).  Detailed summaries of the coring component of the study are presented by Nelson and others (1992), Lee and others (1993), and Lee and others. (1996).  Scanned field descriptions of the cores and drafted visual summaries of most of the cores can be viewed from the "DATA" section of the CD-ROM (core list table).  The visual summaries have been incorporated in the GIS, and can be viewed while in ArcView by highlighting the "cores" shape file, clicking on the hot link (lightning bolt on top menu bar), and then clicking on a core located in the view.  X-rays were taken of parts of several cores.  These sections are highlighted on the visual summaries in yellow, and clicking on the yellow box that is labeled "click here to view the x-ray" will bring up an image of the x-ray (core 61 for example).

Grain-size measurements (grain-size table) were completed on many of the cores, and these results in concert with the imagery, age dates, and physical properties data have been used to study the mechanics of the flows needed to form these deposits (Lee et al., 1993; Locat et al., 1996; Schwab et al., 1996).  In light of the thickness of the deposits, size of the channels, and distance from the source area, the results suggest that these flows were slow moving and behaved as Bingham fluids that were in steady state (Schwab et al., 1996).  The physical properties measurements are not presented on this CD-ROM, but the grain-size analyses are summarized in tabular form.  The mean grain size and radiocarbon dates are also shown on the visual summaries of the cores to show the distribution of samples taken.
 
  [an error occurred while processing this directive]