Sea Floor Off Magdalena Delta and Santa Marta Area, Colombia

FRANCIS P. SHEPARD University of California, Scripps Institution of Oceanography, La Jolla, Cali- fornia 92037

Sea Floor off Magdalena Delta and Santa Marta Area, Colombia

ABSTRACT floor valleys off the Magdalena delta where in- stability had been indicated by the breaking of The slope bordering the Magdalena delta and cables after earthquakes (Heezen, 1956). Also, the adjacent outlier of the Andes has a complex the surveys of the late Eduardo Rico, of the morphology. This includes typical delta-front Colombian Public Works Department, had slope valleys directly off the Magdalena River shown that frequent depth changes occur. The and off its old mouths, bordered to the west by rough seas and 40-mph winds during the 1966 anticlinal hills and synclinal valleys, and to the work thwarted some of the program, but our north by fault blocks with a northeasterly inability to conduct adequate sampling led us trend. To the east, the slope has a concentration to devote more time to running seismic proof mud diapirs rising mostly from between 400 files across the area, and this resulted in our and 750 fm (730 and 1,370 m). Farther east, a discovery of an unknown field of diapirs partly filled synclinal valley extends seaward (Shepard and others, 1968). The ten-day from the Golfo de Santa Marta. Off the operation was too short to allow us to explore mountainous coast east of Santa Marta, a large these adequately. In fact, we found the only submarine canyon winds down the slope almost diapirs with 200 m of surface relief during a to Colombia Basin. This canyon cuts through a chance run the last night of our work. large fault scarp, which forms part of the south- The first opportunity to return for further east margin of the basin. The sea-floor relief investigations of the diapir field came in 1969, has been formed by a combination of complex when I was given use of the Argo for three faulting and folding, prograding sediment from weeks. The sea and wind conditions were almost the Magdalena with attendant formation of as bad as before, but the ship was more stable slump valleys, partial filling of structural and the seismic equipment (an air gun) was valleys with the sediment, diapiric intrusion of more efficient than the old arcer, so instead of plastic mud from underlying formations, and requiring a reduction of speed to four knots and marine erosion producing a large submarine running lines largely before the wind (as was canyon. The source of the sediment that has necessary with the old equipment), we could excavated the canyon is unknown but pre- make eight knots and could obtain the profiles sumably is supplied by streams entering the sea in any direction. from a north outlier of the Andes. During both operations we were dependent for positions on fixes by radar and, in the outer INTRODUCTION area, by even less accurate celestial navigation. The north coast of Colombia includes the Unfortunately, the newly installed satellite large Magdalena delta that has formed in the navigation system on the Argo had temporarily salient between the two branches of the north- ceased to function at the time of our 1969 ern Andes. The part of the area investigated in work. However, our sounding lines covered the 1966 (Shepard and others, 1968) was off the area widely (Fig. 1), and after a very large Magdalena delta, but in 1969 a three-week amount of adjusting of lines so that the sound- cruise of the Scripps Institution ship Argo ings agreed at crossings, it has been possible to covered also the slope off the northern tip of obtain a moderately accurate picture of the the Andes to the east. During the 1966 opera- topography. tion on the Scripps Institution ship Thomas Although the main purpose of the 1969 work Washington, the intention was to study the sea- was to study the diapirs, I was intrigued with a

Geological Society of America Bulletin, v. 84, p. 1955-1972, 12 figs., June 1973 1955

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submarine canyon discovered off the north end delta, this deeply incised canyon with its rocky of the Andes during an operation by Krause walls continues seaward across the slope virtu- (1971). As a result, a more complete survey of ally to Colombia Basin. It is clearly cut into the this canyon was included along with seismic great fault scarp that borders the basin on the profiles. Also, indications of unusual structural southeast. features seaward of the delta slope on the east side of the area led to more sounding and Mud Diapirs seismic lines to help interpret the tectonic The intrusive diapirs in this area, as else- history of the area. where, are recognizable in the seismic profiles by their dome shape and their acoustic trans- GENERAL TOPOGRAPHY parency. They generally lack the reflector One of the most striking features of the sea layers of bedded sediments characterizing other floor off northern Colombia is the virtual formations of the area. Figure 3, taken from absence of typical continental shelves. In the the 1966 operations, shows some of the diapirs area that we surveyed, the only appreciable with greatest surface relief, which we chanced shelf exists in Golfo de Santa Marta (name to encounter during the last night of our proposed for this indentation), the bight be- earlier work. In 1969, we ran a pattern of east- tween the Magdalena delta and the north-south west lines across the same area spaced about 0.8 coast between Ciénaga and Santa Marta (Fig. km apart. We located a large number of diapirs, 2). Apparently, the Magdalena sediment has as shown in the lowest profile of Figure 4. The virtually covered the continental shelf to the contours from the detailed survey of this area west. The slope directly northwest of the sub- are shown in Figure 5. It will be seen that, al- aerial delta is cut by valleys of small depth, though the hills rising above the surface are which resemble the valleys off the Mississippi very numerous, none are higher than those of delta where progradation has also eliminated our chance encounter in 1966. the shelf (Shepard, 1955). Somewhat the same We had hoped to make accurate surveys of type of feature is found off the Fraser delta one or more of the higher diapirs, but the area where prograding has also covered any pre- was too far from land for good radar fixes and existing shelf (Mathews and Shepard, 1962). our attempt to plant a buoy with a radar The interpretation of slump origin for these screen was unsuccessful. The anchor soon valleys appears to be well supported by avail- dragged because of the strong winds and heavy able information. seas. Two of the close-spaced lines apparently East of the delta, the slope along the outer indicate the same diapir, giving it a north- margin of a broad bulge is characterized by a easterly elongation (Fig. 5), although we have large number of hills, which constitute the no proof that this was not two separate diapirs. diapirs of the area. East of the bulge, a broad In several places, we had clear evidence of a indentation extends into the Golfo de Santa close juxtaposition of diapirs (Fig. 4, profiles Marta. Landward of this trough is the ex- S-S', W-W', and Hh-Hh'). The diapirs with the tensive lagoon, Ciénaga de Santa Marta, which largest surlace relief were usually isolated. is separated from the Caribbean by a long sand Basin depressions appear to exist mostly near barrier. Such depressions are a common feature the larger hills. These depressions are an inter- on the margins of the world's largest deltas. pretation of the contourer, and it is possible The north and northwestern part of the area that there is an outlet from some of them. studied is dominated by reliel, apparently of However, the basins seem the best interpreta- tectonic origin, where ridges and intervening tion because the squeezing upward of the troughs have a northeasterly trend. The major diapirs probably was accompanied by settling escarpment that flanks the southeast side of along their margins. Colombia Basin has this same structural Several attempts were made to dredge the alignment. In fact, there is some indication that sides of the highest diapirs. Only clayey the diapirs (dots in Fig. 2) extend in the same material was recovered in the dredge. Similarly, northeasterly direction, notable in the outlying a core taken by good luck on the very top of a examples. diapir yielded hard clay. An examination of the The east and northeastern portion of the area Foraminifera from this core by the micro- is dominated by a winding submarine canyon. paleontologists of ESSO Petroleum Corpora- Unlike the slope valleys off the Magdalena tion, Houston Laboratory, determined that the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1955/3428653/i0016-7606-84-6-1955.pdf by guest on 28 September 2021 Figure 1. Sounding lines used in construction of profiles indicated by heavier lines and by the letters Figure 2. The heavy lines refer to the seismic profiles A-A' and so on. Note that the letters run from A to Zz, used in Figures 4, 6, 7, 8, and 9. Location of seismic starting in the west, and proceeding east and northeast.

SHEPARD, FIGURE 1 Geological Society of America Bulletin, v. 84, no. 6

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74*00' 75*20 75?ICf 75*00 74*50" 74*40' 74*30' 74*20' 74* IO' Figure 2. Topography of the area off Magdalena Marta. The dashed lines show location of Figures 5 and See Figure 5 for larger scale of principal diapir area and delta and northern end of Sierra Nevada de Santa 10, and dots give location of diapirs with surface relief. Figure 10 for the head of Aguja Canyon.

SHEPARD, FIGURE 2 Geological Society of America Bulletin, v. 84, no. 6

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Figure 3. Seismic profiles of diapirs discovered in 170° from vicinity of A. Note that the beds adjacent to 1966 in the main diapir area. A shows three crossings of the diapirs appear to bend down, rather than up as is the same diapir, and B is from a line run on course of usual around salt domes.

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Figure 4. Examples of diapirs from the 1969 seismic west-east and east-west lines in the areas of Figure 5. shows a diapir on the east side of Aguja Canyon, far profiles. For locations see Figure 1. Note that beds Each break represents a gap between the east and west from the main field. The two profiles labeled Ee were commonly bend up along side of the diapirs that did not courses. Note the examples of two or more diapirs in obtained during maneuvering along various courses to reach the surface. Profile Hh-Hh is a combination of dose proximity in several of the profiles. Profile Uu-Uu' find good locations for dredging and coring.

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# 11°20' 11°20

74°50' 74°40* Figure 5. Detail of principal diapir area. Note the mately 0.8 km. Other contour interpretations of the numerous hills and the basin depression adjacent to soundings are possible, and locations of lines are only them. Based on sounding lines separated by approxi- approximate in this area, near the limit of radar ranges. fauna was older than Holocene and probably where the diapirs have appreciable surface mid-Pleistocene (C. C. Daetwyler, 1969, per- domes, sufficient material was displaced from sonal commun.). This information supports our an underlying clay formation to allow the previous conclusion that the diapirs are mud sinking of the beds along the diapir margin. instead of evaporites. Mud volcanoes are found This may also account for the basin depressions. along the west margin of the delta, and no In a few profiles, the sea floor seems to have salt domes have been discovered along the been little affectec. by the underlying diapir north coast of South America. (Fig. 4, sec. Jj-Jj')' perhaps due to deposition Along the margins of most diapirs, sediment after the intrusion. formations are bent upward. This is also true of some of the diapirs off Colombia, but, as in- Subsurface Folds £ind Their Surface Relation dicated previously (Shepard and others, 1968), In view of the numerous diapirs, one must there are more places where the beds bend down bear in mind that subsurface folds may be next to the contact (Figs. 3, 4). Upbending related to diapiric intrusions. However, some appears to be more common where the diapir of the best fold tracts are widely separated from did not break through to the surface. Possibly the diapir belts and contrast considerably in

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character from typical diapirs. On the slope profile of Figure 8 (lower right). All these northwest of the delta (Fig. 6, sees. A-A', sections have horst ridges and graben valleys, B-B', C-C'), the subsurface folds conform the latter have been partly filled with sedi- closely to the surface topography. The same ments several thousand meters thick with al- type of anticlinal ridges and synclinal valleys ternating good and poor reflector horizons. An were observed in a profile off the older portion unconformity in this fill is notable in profile of the Magdalena delta at Cartagena, 90 km to LI-LI', where a sloping layer covers the flat the southeast (Shepard, 1969, Fig. 9). Underlying formation. The fill no doubt has These structures appear to be similar to come from the Magdalena delta, carried along those off the east coast of Mexico (Bryant and the valleys by turbidity currents such as the others, 1968) and off southern Portugal ones that broke the cables off the delta on (Roberts and Stride, 1968). On land, it is several occasions (Elmendorf and Heezen, unusual to have such surface expression of 1957). Tectonic activity obviously has been the underlying folds, except on a rather small scale major cause for the relief in this outer belt, but where the folds are related to slumping. The the fault troughs have been partly filled with same explanation has been offered for sub- sediment. Faulting is also suggested in the dis- marine folds, and this appears as a likely cause continuous beds of the north-south profiles in of the folds off the Magdalena delta. In Figure Figure 8, notably near G' and R'. 6, another rough correspondence between folds and slope is seen on the left-hand side of Folding and Faulting in the Trough of section V-V'. A change in course from north to Golfo de Santa Marta east in the middle of this section shows a A series of seismic profiles extending west termination of this relation where the under- from the partially mountainous north-south lying formations appear to be displaced by a coast of the Santa Marta area reveals another fault. type of structure (Fig. 9). The deepest part of A partial agreement between folds and sur- the sections includes an underlying syncline, face relief is indicated in the north-south especially clear in profile Aa-Aa'. The syncline profiles Pp-Pp' and Oo-Oo' of Figure 6. Here, with its curving axis (Fig. 12) has been partly the surface slopes are not as steep as the inclines filled but still has appreciable surface relief. of the subsurface anticline. Section Nn-Nn' The most recent event has probably been the crosses both of these profiles and shows more diapiric intrusions that cut the center of the complex subsurface folding with only partial syncline in several places (Fig. 9, sees. Cc-Cc', relation to the topography. However, the con- Ff-Ff', Ll-Ll'). According to at least two of the tour map shows that the area under discussion profiles (Aa-Aa', Z-Z'), the east side has been has a hill in the center of this complex dome. downfaulted. This fault is presumably the Probably diapiric intrusion has been important north-trending Santa Marta fault that is said here. A slight break of the surface is seen at the to extend along the west side of the Sierra top of the dome. Section Qq-Qq' in the same Nevada de Santa Marta, an outlier of the Andes general area shows what appears to be an Mountains (Campbell, 1968). anticline under a valley, adding a complexity to the morphology. Aguja Submarine Canyon In profile Ee-Ee' (Fig. 4), subsurface folds The north coast of South America is unique with little or no surface relief indicate a rela- in having, as far as is known, very few sub- tion to the adjacent diapir, and are probably marine canyons. The only one that has been independent of tectonic activity. investigated is located off the mountainous In Figure 6, three profiles of major fault north end of Sierra Nevada de Santa Marta. scarps all lead down into Colombia Basin (Rr- The name Aguja Canyon is suggested for it Rr', Ss-Ss', Tt-Tt'). The last two show clear because the name is applied to a nearby cape evidence in the seismic profiles of high-angled and to its offlying island. The Krause (1971) normal faults. Fault scarps are also shown with map showing the canyon indicates that it somewhat less certainty among the troughs and trends directly into the mountainous north ridges in east-west profiles north and northwest coast, but we carried out soundings nearer of the Magdalena delta (Fig. 7, see especially to the land and, so far as we could tell, the L-L', M-M', and N-N'). These fault scarps canyon here extends along the shore (Figs. 2, also appear in the northwest-southeast E-E' 10). Presumably it heads into one of the land

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Figure 6. Folds and fault scarps indicated oil seismic Tt, and Rr) are at the margin of Colombia Basin. The profiles. The folds (upper left) may be due to slumping other profiles are related to a dome with some surface along the slope west of the delta. The fault scarps (Ss, relief on the southeast side of Aguja Canyon.

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Figure 7. The east-west seismic profiles extending more northerly sections indicate the effect of faulting in north from the Magdalena delta. Profiles H-H' and producing valleys. Also, slump folding is suggested in part of I-I' show the slope valleys near the delta, but profiles I-I' (left) and possibly iri some of the others.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1955/3428653/i0016-7606-84-6-1955.pdf by guest on 28 September 2021 Figure 7. (continued).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/1955/3428653/i0016-7606-84-6-1955.pdf by guest on 28 September 2021 Figure 8. North-south seismic profiles off Magdalena across the fault blocks of the northwestern part of the delta and Golfo de Santa Marta. The profiles show a area. Note the fill in the basin at the center of this combination of faulting, folding, and diapiric intru- profile. sions. The northwest-southeast profile (E-E') extends

Figure 8. (continued).

valleys east of the point, although there are no usual tributary valleys along this canyon. In- soundings to verify this supposition. stead, it appears to have several indentations The contours of the canyon indicate that it on the north and east sides, each coming to an winds seaward with alternating westerly and abrupt end. The seismic profiles in Figure 11 northerly courses. There is little sign of the give a strong indication that these indentations

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Figure 9. The east-west seismic profiles from the what is thought to be the Sani:a Marta fault. Note also area north of Golfo de Santa Marta. The profiles show a the diapiric intrusions in the center of the syncline in broad syncline partly filled and cut on the east side by profiles Cc-Cc' and Ff-Ff'.

Figure 9. (continued). are related to slumping along the right-hand not clearly verified but seem likely because they wall of the canyon. Apparently, this has are suggested by most of the profiles. produced isolated blocks, recesses, and ter- The extension of the canyon head along the races along that wall. The seismic profiles cross coast is perhaps related to what has been called the canyon in both an east-west and north- the Oca fault (Case and others, 1971). This, in south direction and contain the same slump turn, is believed to be the result of the eastward features in both trends. More accurately lo- movement of the Caribbean block passing cated and closer-spaced seismic lines would be northern South America. Presumably, the zone helpful. The hills shown in the contours are of weakness along the Oca fault allowed sub-

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11° 30'

Figure 10. The head of Aguja Canyon. For location the north and east sides of the canyon, indicative of see Figure 2. The dashed lines show the location of the slumping. The steeper slopes occur on the other side seismic profiles of Figure 11. Note the broken slope on and suggest valley migration toward that side. marine erosion to concentrate in this east-west scarp that, in turn, borders the Colombia direction. Basin. The outer portion of Aguja Canyon cuts These various features indicate that the sea- deeply into the fault scarp bordering Colombia floor relief has been caused by several processes. Basin. The cutting may have accompanied the Although the forward building of the Mag- development of the fault scarp, but it certainly dalena delta has coverec most of the con- could not have occurred prior to the faulting or tinental shelf off Barranquilla, the influence of we would have found a hanging valley ter- this prograding on the slope appears to be far minating the canyon at the fault scarp. The less than on other great deltas, like the Missis- outermost lines show that the valley does not sippi. Off the Magdalena delta, the immediate continue onto the floor of the basin. foreset slope is cut by slurr.p valleys, and 10 mi farther seaward, northeasterly trending horst CONCLUSIONS and graben features have taken over. Also, on the western slope, the folded formations ap- The principal features discovered during the pear to be the result of slumping, like the folds investigation of the area off the Magdalena off Vera Cruz, eastern Mexico. The concen- delta and the Santa Marta area are indicated in tration of the mud diapirs somewhat east of the Figure 12. These include, from west to east, the delta suggests that the weight of the delta has slump folds on the western slope; the horst forced up some of the underlying plastic mud ridges and intervening graben valleys partly formations into these sea-floor mounds, like the filled with sediment; the short slump valleys in Mississippi delta mud lumps. The large fault the foreset slope directly off the delta; the two scarp bordering Colombia Basin appears to be concentrations of diapirs with a general north- related to the same forces that produced the easterly trend; the synclinal valley in the Golfo horsts and grabens. The Santa Marta fault and de Santa Marta; the Santa Marta fault along its bordering syncline is perhaps part of the the west side of the peninsula; and the large plate movement that extended an outlier of Aguja submarine canyon, with its alternating the Andes into this area. What relation the east-west and north-south trends. Farther sea- Aguja Canyon has to this pattern is not clear, ward, the slope is terminated by a steep fault but it seems likely that its head follows a fault

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Figure 11. Seismic profiles across inner Aguja fathogram (not included), in contrast to the slight in- Canyon. Note the slump topography producing terraces dication of a terrace on the left side of the seismic pro- and displaced blocks along the right side of the valley. file. For locations of the profiles see Figure 10. In profile 1 the lower terrace is much clearer in the

that cuts off the north side of this mountainous and who shared with me the daily planning coast. Further study of the canyon will no of the programs. Others active in the ship doubt prove fruitful. operation included Walter Sproll, Fred Dixon, Baron Thomas, Jim Coatsworth, Elizabeth ACKNOWLEDGMENTS Shepard, and our three guests from Colombia, This work was supported in part by the Padre J. E. Ramirez, the late Eduardo Rico, Office of Naval Research under contract USN and Pedro Gutierrez. In processing the data, NOOO 14-69-A-0200-6006 and by the Na- Neil Marshall, Gary Sullivan, and Patrick tional Science Foundation Grant GA-4593. McLoughlin were of great help. Appreciation is I am indebted to Calvin Daetwyler who also expressed to the crew and officers of the accompanied me during the cruise of the Argo Argo.

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REFERENCES CITED Geol. Soc. America Mem. 130, p. 35-54. Mathews, W. H., and Shepard, F. P., 1962, Sedi- Bryant, W. R., Antoine, John, Ewing, Maurice, mentation of Fraser River delta, British and Jones, Bill, 1968, Structure of continental Columbia: Am. Assoc. Petroleum Geologists shelf and slope, Gulf of Mexico: Am. Assoc. Bull., v. 46, p. 1416-1438. Petroleum Geologists Bull., v. 52, p. 1204- 1228. Roberts, D. G., and Stride, A. H., 1968, Late Tertiary slumping on the continental slope of Campbell, C. J., 1968, The Santa Marta wrench southern Portugal: Nature, v. 217, no. 5123, fault of Colombia and its regional setting: 4th p. 48-50. Caribbean Geol. Conf., Trinidad, p. 247-261. Shepard, F. P., 1955, Delta-front valleys bordering Case, J. E., Duran, L. G., Lopez, A. R., and the Mississippi distributaries: Geol. Soc. Moore, W. R., 1971, Tectonic investigations America Bull., v. 66, p. 1489-1498. in western Colombia and eastern Panama: Geol. Soc. America Bull., v. 82, p. 2685-2712. 1969, Orientamenti di ¿eologia marine, in Enciclopedia della Scienza e della Technica Elmendorf, C. H., and Heezen, B. C„ 1957, Mondadore: Milan, Italy, S&T/69, p. 189-204. Oceanographic information for engineering submarine cables: Bell Systems Tech. Jour., v. Shepard, F. P., Dill, R. F., and Heezen, B. C., 36, p. 1047-1093. 1968, Diapiric intrusions in foreset slope sediments off Magdalena della, Colombia: Am. Heezen, B. C., 1956, Corrientes de turbidez del Assoc. Petroleum Geologists Bull., v. 52, p. Río Magdalena: Bol. Soc. Geog. Colombia, 2197-2207. nos. 51, 52, p. 135-142. Krause, D. C., 1971, Bathymetry, geomagnetism, MANUSCRIPT RECEIVED BY THE. SOCIETY AUGUST and tectonics of the Caribbean Sea north of 22, 1972 Colombia, in Donnelly, T. W., ed., Caribbean REVISED MANUSCRIPT RECEIVI-D NOVEMBER 13, geophysical, tectonic, and petrologic studies: 1972

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