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Holocene Regressive Littoral Sand, Costa de Nayarit, Mexico*
HOLOCENE REGRESSIVE LITTORAL SAND, COSTA D E NAYARIT, MEXICO' JOSEPH R . CURRAY
and
D A V I D G. MOORE
Scripps Institiition of Oceanography, University of California, La Jolla, Calif.; U.S . Navy Electronics Laboratory, San Diego, CaliK ( U . S . A . )
INTRODUCTlON
The coastal plain and continental shelf of the west coast of mainland Mexico have been investigated as a part of the Scripps Institution of Oceanography project on the geology and oceanography of the Gulf of California. One of the areas of greatest geological interest is the area south of Mazatlan, Sinaloa, and north of San Blas, Nayarit, on the mainland side of the Gulf of California (Fig. 1). Both the coastal plain and the continental shelf in this region are dominated by the influence of the Rio Crande de Santiago, one of the major rivers of the west coast of Mexico, with lesser influence by the smaller adjacent rivers to the north and south. During Pleistocene periods of lower sea level, the Rio Crande built a complex delta system on the continental shelf, and in places prograded the edge of the shelf seaward into deeper water. The Holocene transgression moved the shoreline across the exposed upper surface of this delta complex to a position several kilometers inland of its present position. During the last few thousand years, the shoreline has regressed by deposition of a sheet-like body of littoral sand. This paper represents a progress report dealing with the general distribution of the sediments of the regressive sand body, its relationship to the underlying and overlying sediments, and the mechanism of deposition and regression at work along this coastline. The submerged Pleistocene delta complex of the adjacent continental shelf is the subject of another progress report (MOOREand CURRAY, 1963).
REGIONAL DESCRIPTION
The coastal plain (Fig. I ) consists of a low-lying marsh essentially at sea level, overlapping on the seaward-dipping flood plain surface of the Rio Grande de Santiago and the smaller adjacent rivers to the north and south. The coastal marsh exists mainly Contribution from the Scripps Institution of Oceanography, University of California, La Jolla, Calif.
I \
<
, I '
\ '/ /
2 \
7 /
\
/
/.
Fig.1. Bathymetric and physiographic chart of the Costa de Nayarit continental shelf - coastal plain complex. Coastal plain consists of system of abandoned Holocene beach-dune ridges, shown diagrammatically, overlying pre-Holocene-transgressive alluvium, and in turn overlain in part by younger 1963.) Holocene alluvium. Protuberance in continental shelf edge is Pleistocene delta system of Rio Crande de Santiago. (After MWREand CURRAY,
78
J. R. CURRAY A N D D. G . MOORE
Fig.2. Oblique air views of abandoned Holocene beach-dune ridges during flood season. Average width of ridges shown is about 80 m. (Photos courtesy of F. B Phleger.) a. Landward half of strand plain of abandoned beach ridges covered with mangroves. b. Closer to ocean than upper photograph, showing discontinuity between adjacent sets of ridges. Younger ridges closer to ocean are built higher and closer together than older ridges, and coastline was I eoriented before formation of younger ridees.
REGRESSIVE LITTORAL S A N D OF COSTA DE N A Y A R I T
79
in the depressions between scores of parallel abandoned beach-dune ridges (Fig.2). This strand plain of abandoned beach ridges averages about 5 km in width for the entire 225 km distance from Mazatlan to San Blas, and about 10 km in width for the better developed cer,tral 130 km length. Maximum width from the present beach to the oldest ridge is 17 km. The upper surface of the strand plain is uniformly furrowed by the parallel ridges, which are typically spaced 30-200 ni apart. About 250 ridges can be counted in air photographs from one of the better developed parts of the plain 12 km wide, giving an average width of about 50 ni per ridge. Relief varies from less than I n i to a maximum of about 5 m above the surrounding level. Detailed leveling shows that the entire plain lies at about sea level. The sand body is continuous between the ridges and all the way across the strand plain (Fig.3). Some depressions between the ridges contain elongate surface lenses of modern alluvium, but sand is always encountered at depth under the alluvium. Some of the smaller rivers of the area discharge directly into the ocean, but others discharge into the tidal marsh between the sand ridges. Seasonal flooding accompanied by influx of alluvial muds is gradually prograding the river flood plains over the landward edge of the strand plain. Only the depressions between the ridges have been filled with modern alluvium in most of the area, but locally some of the older ridges near the landward edge of the plain have been completely buried. The continuous sand sheet overlies the pre-transgressive or Pleistocene flood plain deposits of the coalescing river system (Fig.3). This alluvial surface has been traced by drilling under the coastal strand plain, and it has been delineated in detail on the shallow continental shelf by acoustic reflection niethods (MOORE and CURRAY. 1963). The surface is irregular as a result of subaerial erosion, and sand-filled river channels
mPRE-TRANSGRE5SlVE (PL EIST OC EN E) A L L U V I U M
Fig.3. Diagrammatic cross section through coastal plain and inner continental shelf. Formation of ridges is by successive accretion of submerged longshore bars. During period of low waves, bar is built to surface and becomes new beach, thereby isolating former beach and creating a narrow lagoon. Lagoon is partially filled by washover and later covered with modern alluvium.
80
J. R. CURRAY A N D D. G. MOORE
have been detected both under the strand plain and at sea. Average slope of the irregular surface under the strand plain is about 1.3 m/km (7 ft./mile). Sands composing the strand plain are of fine grain and well sorted, with a high proportion of feldspar and volcanic rock fragments. The older alluvial deposits consist of alternating sandy muds and muddy sands, while the younger alluvium consists mainly of fine mud.
MECHANISM OF FORMATION OF SAND RIDGES
Each ridge was formed individually as a shoreline deposit; the oldest lie farthest from the ocean, and the youngest lie closest to the ocean. The present shoreline is analogous in most respects to each of the ridges at its time of formation. The mechanism of formation proposed for these features is that each ridge started as a submerged longshore bar in front of the existing beach. Laboratory wave tank experiments (MCKEEand STERRETT, 1961) and beach and wave studies (KEULEGAN, 1948; SHEPARD, 1950; D.L. Inman, personal communication, 1961) have shown that longshore bars form initially at the plunge point of breakers. With sufficiently high rate of supply of sand, and with conditions of low wave action, a bar can build to the surface. If this occurs during a high tide and if the low wave conditions persist through the low tide and several tidal cycles thereafter, the longshore bar in effect becomes the new beach, and the former beach is isolated. This process has apparently repeated itself cyclically since the time sea level came approximately to its present position. Cause for the periodicity might be fluctuation in the amount of sand supplied to the beach shoreface, fluctuation in wave and tidal conditions, or the process may occur under conditions of more or less uniform rate of supply and oceanographic conditions. The writers favor the latter explanation: that rate of supply and wave and tidal conditions were approximately uniform throughout the period of formation of the entire strand plain. After formation of each successive new beach ridge, the process started over again after a sufficient supply of sand was brought into the area either by longshore transport from the rivers, or bv reworking of older sediments on the shallow continental shelf. The shoreface was then built seaward to provide a base for the formation of a new longshore bar, and the new bar was built to above sea level. This in effect creates a self-regulating cyclic process, independent of external periodic influence. Age of the oldest abandoned beaches and development of the strand plain is not yet known. The only radiocarbon date available suggests that sea level came approximately to its present position 3,000-5,000 years ago. Assuming a uniform and regular rate of formation, an average of 12-20 years were required for the formation of each ridge. It is known, however, that long term periods of change of conditions and interruption of the regular sequence did occur, and the entire coastline was reoriented to produce discontinuities. (Fig.2b). Another major reorientation of the coastline may be occurring at the present day, because the coastline is locally eroding or trans-
REGRESSIVE LITTORAL SAND OF COSTA DE NAYARIT
81
gressing landward with consequent better development of the present beach ridge. The problems of ages of the ridges, rate of formation, and position of relative Late Holocene sea level in this area cannot be given final answers until further information has been acquired by radiocarbon dating.
SIGNIFICANCE OF REGRESSIVE SANDS
Factors controlling the processes of transgression and regression (CURKAY, 1963) are change of relative sea level and net rate of deposition or erosion along the shoreline. Inasmuch as a geological record is commonly preserved only under conditions of rising relative sea level (usually subsidence of the basin of deposition), the common alternation between transgression and regression is due to periodic pulsation of the rate of subsidence or periodic fluctuation of the rate of sediment supply. Holocene sediments of the Costa de Nayarit display a transgressive-regressive record. The transgression across the continental shelf occurred as a result of the very rapid rise of sea level following the Late Wisconsin (Wiirm 11) glaciation, interrupted by periods of delta advance or regression. With final slowing or cessation of the rise of sea level in the last few thousand years, deposition along the shoreline became dominant, and regression started by the process described. Regressive sands are well known from all geological ages and from all continents. The mechanism of the process of regression should be understood in order to interpret the deposits properly. The process described here is one possible mechanism. but perhaps not the only one. With long continued stillstand of sea level and continued high rate of supply of sand, this deposit could develop to dimensions comparable to those of ancient regressive sands. Alluvial deposition and marsh growth over the top of the sand body will complete the regressive aspect with peat or thin coal measures and an overlying continental shale body.
SUMMARY
The coastal plain of the mainland coast of west central Mexico consists of a strand plain marsh of about 250 parallel abandoned beach-dune ridges. The sand of the individual ridges coalesces to form a sheet sand with a furrowed upper surface. The sand overlies the pre-Holocene-traiisgression flood plain surface of the Rio Grande de Santiago and other small adjacent rivers. The sand is in turn partially covered, especially in the depressions between the ridges, with younger or modern Holocene alluvium. The mechanism of formation of the sand body has been regression after approximate stabilization of Holocene sea level by successive accretion of longshore bars in front of the existing beach. Each successive bar was built to sea level by the rapid supply of sand during periods of low wave intensity, and thereby became the new
82
J. R. CURRAY AND D. G . MOORE
beach. The former beach was isolated, and the newly-created lagoon behind the new beach was partially filled by sand. Regressive littoral sand bodies are common in ancient sediments. This is one possible mechanism for their formation. With long continued stillstand of sea level and abundant supply of sand, this deposit could grow to dimensions comparable to those of ancient deposits.
REFERENCES
CURRAY, J. R., 1963. Transgressions and regressions. In: Papers in Marine Geology, Shepard Cotiimemorative Volume. Macmillan, New York, in press. G. H., 1948. An experimental study of submarine sand bars. U.S . Beach Erosion Board KEULEGAN, Tech. Rept. 3: 40 pp. MCKEE,E. D. and STERREIT,T. S., 1961. Laboratory experiments on form and structure of longshore and J. C. OSMOND(Editor), Geometry qf Sandstone Bodies. bars and beaches. In: J. A. PETERSON Am. Assoc. Petrol. Geologists,Tulsa, pp. 13-28. J. R., 1963. Sedimentary framework of the drowned Pleistocene delta of MOORE,D. G. and CURRAY, Rio Grande de Santiago, Nayarit, Mexico. In: L. M. J. U. VAN STRAATEN (Editor), Delfoic and Shallow Marine Deposits. Elsevier, Amsterdam, pp. 275-28 1. SHEPARD, F. P., 1950 Longshore-bars and longshdrktroughs. U. S . Beach Erosion Board Tech. Mem. 15: 32 pp.
and
D A V I D G. MOORE
Scripps Institiition of Oceanography, University of California, La Jolla, Calif.; U.S . Navy Electronics Laboratory, San Diego, CaliK ( U . S . A . )
INTRODUCTlON
The coastal plain and continental shelf of the west coast of mainland Mexico have been investigated as a part of the Scripps Institution of Oceanography project on the geology and oceanography of the Gulf of California. One of the areas of greatest geological interest is the area south of Mazatlan, Sinaloa, and north of San Blas, Nayarit, on the mainland side of the Gulf of California (Fig. 1). Both the coastal plain and the continental shelf in this region are dominated by the influence of the Rio Crande de Santiago, one of the major rivers of the west coast of Mexico, with lesser influence by the smaller adjacent rivers to the north and south. During Pleistocene periods of lower sea level, the Rio Crande built a complex delta system on the continental shelf, and in places prograded the edge of the shelf seaward into deeper water. The Holocene transgression moved the shoreline across the exposed upper surface of this delta complex to a position several kilometers inland of its present position. During the last few thousand years, the shoreline has regressed by deposition of a sheet-like body of littoral sand. This paper represents a progress report dealing with the general distribution of the sediments of the regressive sand body, its relationship to the underlying and overlying sediments, and the mechanism of deposition and regression at work along this coastline. The submerged Pleistocene delta complex of the adjacent continental shelf is the subject of another progress report (MOOREand CURRAY, 1963).
REGIONAL DESCRIPTION
The coastal plain (Fig. I ) consists of a low-lying marsh essentially at sea level, overlapping on the seaward-dipping flood plain surface of the Rio Grande de Santiago and the smaller adjacent rivers to the north and south. The coastal marsh exists mainly Contribution from the Scripps Institution of Oceanography, University of California, La Jolla, Calif.
I \
<
, I '
\ '/ /
2 \
7 /
\
/
/.
Fig.1. Bathymetric and physiographic chart of the Costa de Nayarit continental shelf - coastal plain complex. Coastal plain consists of system of abandoned Holocene beach-dune ridges, shown diagrammatically, overlying pre-Holocene-transgressive alluvium, and in turn overlain in part by younger 1963.) Holocene alluvium. Protuberance in continental shelf edge is Pleistocene delta system of Rio Crande de Santiago. (After MWREand CURRAY,
78
J. R. CURRAY A N D D. G . MOORE
Fig.2. Oblique air views of abandoned Holocene beach-dune ridges during flood season. Average width of ridges shown is about 80 m. (Photos courtesy of F. B Phleger.) a. Landward half of strand plain of abandoned beach ridges covered with mangroves. b. Closer to ocean than upper photograph, showing discontinuity between adjacent sets of ridges. Younger ridges closer to ocean are built higher and closer together than older ridges, and coastline was I eoriented before formation of younger ridees.
REGRESSIVE LITTORAL S A N D OF COSTA DE N A Y A R I T
79
in the depressions between scores of parallel abandoned beach-dune ridges (Fig.2). This strand plain of abandoned beach ridges averages about 5 km in width for the entire 225 km distance from Mazatlan to San Blas, and about 10 km in width for the better developed cer,tral 130 km length. Maximum width from the present beach to the oldest ridge is 17 km. The upper surface of the strand plain is uniformly furrowed by the parallel ridges, which are typically spaced 30-200 ni apart. About 250 ridges can be counted in air photographs from one of the better developed parts of the plain 12 km wide, giving an average width of about 50 ni per ridge. Relief varies from less than I n i to a maximum of about 5 m above the surrounding level. Detailed leveling shows that the entire plain lies at about sea level. The sand body is continuous between the ridges and all the way across the strand plain (Fig.3). Some depressions between the ridges contain elongate surface lenses of modern alluvium, but sand is always encountered at depth under the alluvium. Some of the smaller rivers of the area discharge directly into the ocean, but others discharge into the tidal marsh between the sand ridges. Seasonal flooding accompanied by influx of alluvial muds is gradually prograding the river flood plains over the landward edge of the strand plain. Only the depressions between the ridges have been filled with modern alluvium in most of the area, but locally some of the older ridges near the landward edge of the plain have been completely buried. The continuous sand sheet overlies the pre-transgressive or Pleistocene flood plain deposits of the coalescing river system (Fig.3). This alluvial surface has been traced by drilling under the coastal strand plain, and it has been delineated in detail on the shallow continental shelf by acoustic reflection niethods (MOORE and CURRAY. 1963). The surface is irregular as a result of subaerial erosion, and sand-filled river channels
mPRE-TRANSGRE5SlVE (PL EIST OC EN E) A L L U V I U M
Fig.3. Diagrammatic cross section through coastal plain and inner continental shelf. Formation of ridges is by successive accretion of submerged longshore bars. During period of low waves, bar is built to surface and becomes new beach, thereby isolating former beach and creating a narrow lagoon. Lagoon is partially filled by washover and later covered with modern alluvium.
80
J. R. CURRAY A N D D. G. MOORE
have been detected both under the strand plain and at sea. Average slope of the irregular surface under the strand plain is about 1.3 m/km (7 ft./mile). Sands composing the strand plain are of fine grain and well sorted, with a high proportion of feldspar and volcanic rock fragments. The older alluvial deposits consist of alternating sandy muds and muddy sands, while the younger alluvium consists mainly of fine mud.
MECHANISM OF FORMATION OF SAND RIDGES
Each ridge was formed individually as a shoreline deposit; the oldest lie farthest from the ocean, and the youngest lie closest to the ocean. The present shoreline is analogous in most respects to each of the ridges at its time of formation. The mechanism of formation proposed for these features is that each ridge started as a submerged longshore bar in front of the existing beach. Laboratory wave tank experiments (MCKEEand STERRETT, 1961) and beach and wave studies (KEULEGAN, 1948; SHEPARD, 1950; D.L. Inman, personal communication, 1961) have shown that longshore bars form initially at the plunge point of breakers. With sufficiently high rate of supply of sand, and with conditions of low wave action, a bar can build to the surface. If this occurs during a high tide and if the low wave conditions persist through the low tide and several tidal cycles thereafter, the longshore bar in effect becomes the new beach, and the former beach is isolated. This process has apparently repeated itself cyclically since the time sea level came approximately to its present position. Cause for the periodicity might be fluctuation in the amount of sand supplied to the beach shoreface, fluctuation in wave and tidal conditions, or the process may occur under conditions of more or less uniform rate of supply and oceanographic conditions. The writers favor the latter explanation: that rate of supply and wave and tidal conditions were approximately uniform throughout the period of formation of the entire strand plain. After formation of each successive new beach ridge, the process started over again after a sufficient supply of sand was brought into the area either by longshore transport from the rivers, or bv reworking of older sediments on the shallow continental shelf. The shoreface was then built seaward to provide a base for the formation of a new longshore bar, and the new bar was built to above sea level. This in effect creates a self-regulating cyclic process, independent of external periodic influence. Age of the oldest abandoned beaches and development of the strand plain is not yet known. The only radiocarbon date available suggests that sea level came approximately to its present position 3,000-5,000 years ago. Assuming a uniform and regular rate of formation, an average of 12-20 years were required for the formation of each ridge. It is known, however, that long term periods of change of conditions and interruption of the regular sequence did occur, and the entire coastline was reoriented to produce discontinuities. (Fig.2b). Another major reorientation of the coastline may be occurring at the present day, because the coastline is locally eroding or trans-
REGRESSIVE LITTORAL SAND OF COSTA DE NAYARIT
81
gressing landward with consequent better development of the present beach ridge. The problems of ages of the ridges, rate of formation, and position of relative Late Holocene sea level in this area cannot be given final answers until further information has been acquired by radiocarbon dating.
SIGNIFICANCE OF REGRESSIVE SANDS
Factors controlling the processes of transgression and regression (CURKAY, 1963) are change of relative sea level and net rate of deposition or erosion along the shoreline. Inasmuch as a geological record is commonly preserved only under conditions of rising relative sea level (usually subsidence of the basin of deposition), the common alternation between transgression and regression is due to periodic pulsation of the rate of subsidence or periodic fluctuation of the rate of sediment supply. Holocene sediments of the Costa de Nayarit display a transgressive-regressive record. The transgression across the continental shelf occurred as a result of the very rapid rise of sea level following the Late Wisconsin (Wiirm 11) glaciation, interrupted by periods of delta advance or regression. With final slowing or cessation of the rise of sea level in the last few thousand years, deposition along the shoreline became dominant, and regression started by the process described. Regressive sands are well known from all geological ages and from all continents. The mechanism of the process of regression should be understood in order to interpret the deposits properly. The process described here is one possible mechanism. but perhaps not the only one. With long continued stillstand of sea level and continued high rate of supply of sand, this deposit could develop to dimensions comparable to those of ancient regressive sands. Alluvial deposition and marsh growth over the top of the sand body will complete the regressive aspect with peat or thin coal measures and an overlying continental shale body.
SUMMARY
The coastal plain of the mainland coast of west central Mexico consists of a strand plain marsh of about 250 parallel abandoned beach-dune ridges. The sand of the individual ridges coalesces to form a sheet sand with a furrowed upper surface. The sand overlies the pre-Holocene-traiisgression flood plain surface of the Rio Grande de Santiago and other small adjacent rivers. The sand is in turn partially covered, especially in the depressions between the ridges, with younger or modern Holocene alluvium. The mechanism of formation of the sand body has been regression after approximate stabilization of Holocene sea level by successive accretion of longshore bars in front of the existing beach. Each successive bar was built to sea level by the rapid supply of sand during periods of low wave intensity, and thereby became the new
82
J. R. CURRAY AND D. G . MOORE
beach. The former beach was isolated, and the newly-created lagoon behind the new beach was partially filled by sand. Regressive littoral sand bodies are common in ancient sediments. This is one possible mechanism for their formation. With long continued stillstand of sea level and abundant supply of sand, this deposit could grow to dimensions comparable to those of ancient deposits.
REFERENCES
CURRAY, J. R., 1963. Transgressions and regressions. In: Papers in Marine Geology, Shepard Cotiimemorative Volume. Macmillan, New York, in press. G. H., 1948. An experimental study of submarine sand bars. U.S . Beach Erosion Board KEULEGAN, Tech. Rept. 3: 40 pp. MCKEE,E. D. and STERREIT,T. S., 1961. Laboratory experiments on form and structure of longshore and J. C. OSMOND(Editor), Geometry qf Sandstone Bodies. bars and beaches. In: J. A. PETERSON Am. Assoc. Petrol. Geologists,Tulsa, pp. 13-28. J. R., 1963. Sedimentary framework of the drowned Pleistocene delta of MOORE,D. G. and CURRAY, Rio Grande de Santiago, Nayarit, Mexico. In: L. M. J. U. VAN STRAATEN (Editor), Delfoic and Shallow Marine Deposits. Elsevier, Amsterdam, pp. 275-28 1. SHEPARD, F. P., 1950 Longshore-bars and longshdrktroughs. U. S . Beach Erosion Board Tech. Mem. 15: 32 pp.