Tllllllll:. in Two Modern Deltas, Veracruz, Mexico

Journal of Coastal Research 470-481 Royal Palm Beach, Florida Spring 2000 Selective Sorting, Storage and Progressive Dilution of Sediment in Two Tropical Deltas, Veracruz, Mexico Zhongyuan Chent, Daniel Jean Stanley], and Eric E. Wright§ tState Key Laboratory for :j:Deltas-GlobalChange §Marine Science Department Estuarine and Coastal Program Coastal Carolina University Research E-206 NMNH Paleobiology Conway, SC 29528, U.S.A. East China Normal Smithsonian Institution University Washington, D.C. 20560, Shanghai 200062, China P.R. U.S.A. ~ .. CHEN, Z.; STANLEY, D.J., and WRIGHT, E.E., 2000. Selective Sorting, Storage and Progressive Dilution of Sediment .tllllllll:. in Two Modern Deltas, Veracruz, Mexico. Journal of Coastal Research, 16(2), 470-481. Royal Palm Beach (Florida), ~ ISSN 0749-0208. ~ ~ Compositional and textural attributes of the surficial sediment in deltaic settings typically differ from the sediment susuTyJ -+; load carried by their source rivers. This phenomenon is evaluated here by semiquantitative study of the variability 1&r--- of petrologic characteristics in different delta subenvironments of the Nautla and Tecolutla deltas along Mexico's Veracruz coast. Sediment composition and texture in the different environments of both deltas are related to processes that involve selective sorting during seasonal flooding and temporary storage after deposition. Displacement of par ticles of different size, density and shape onto the two tropical delta plains is controlled by fluctuating fluvial discharge that responds to a highly variable seasonal rainfall pattern. These discharge variations induce changes in hydraulic equivalence and, consequently, the release of substantially different proportions of volcanic, mica, heavy mineral and grain-size fractions in the Veracruz delta settings. These phenomena, together with progressive dilution of sediment upon reaching the coast, result in significant modifications to the original petrologic attributes of fluvial loads as they are transported seaward. Comparable variations of composition and texture in subsurface Holocene sections would likely record former large scale climate, rainfall and fluvial discharge fluctuations. Petrologic analysis of core sections recovered at Prehispanic sites on the two deltas could be used to detect past long-term flooding pulses and droughts, natural events that archaeologists suggest caused deterioration of subsistence resources, stratigraphic hiatuses, cultural discontinuities and human migrations in this region. ADDITIONAL INDEX WORDS: Archaeological sites, delta sedimentation, fluvial discharge, GulfofMexico, Holocene, particle composition, rain pattern, sediment load, texture, transport processes. INTRODUCTION by the high Sierra Madre Oriental chain of eastern Mexico, provides an appropriate setting to gain further insight on the A river's unidirectional flow carries a sediment load that is nature of depositional variability in deltas. Preliminary sur generally representative of materials transported from upslo veys were made of two small tropical deltas, Nautla (SELF, pe source terrains. However, petrologic analyses of modern 1971, 1975) and Tecolutla (LENTELL, 1975), positioned -----35 deposits positioned in deltas and contiguous coasts indicate km apart on the tropical Veracruz margin of the Gulf of Mex that the proportions of compositional components accumulat ico (Figure 1). Building upon these earlier studies, the cur ed in such coastal settings usually differ substantially from rent investigation focuses specifically on the semiquantita those of their major fluvial channels in more landward and tive changes of composition and texture in these two deltas upslope sectors. Several petrologic investigations, such as to determine the relation that may exist among petrology, that of the Burdekin delta in Australia (ETHRIDGE et al., delta subenvironment and sedimentation process. Also con 1975), have detailed the compositional and textural variabil sidered are depositional processes that modify the fluvial ity in delta systems that are proximal to source terrains. loads of the Nautla and Tecolutla rivers as they reach deltaic Most such studies of modern deltaic deposits, however, have and contiguous coastal environments. Definition of petrologic been made in settings that are at a considerable distance variability in the two lower river systems in this investiga from terrigenous source areas. Examples include the Nile tion provides a means to interpret the past geological and (STANLEY and CHEN, 1991; STANLEY et al., 1992) and Yang human history of this region. tze (YAN and Xu, 1987) where composition and texture of the sediment released in various deltaic subenvironments differ DESCRIPTION OF STUDY AREA markedly from that of the sediment load carried by their main river channels. The Nautla and Tecolutla rivers are short (-----160 km and The southwestern Gulf of Mexico coastal margin, flanked -----220 km, respectively) torrential rivers, with headwaters in the proximal Sierra Madre Oriental mountain chain (Figures 99057 received and accepted in revised 20 July 1999. 1, 2). Tributaries of these two rivers reach elevations in ex- Sediments in Tropical Deltas 471 GULF OF MEXICO o 20km , , i o 20mi Legend IIAlluvial and L-..J delta plain . c..• Highland ... ':..' (200-3000 m) Highland (>3000 m) 19° -----....-------..... 00' Figure 1. Map (modified from several sources) showing the two delta study areas on the Veracruz coastal margin, southwestern Gulf of Mexico. cess of 3000 m, and channels flow down steep eastern moun vation headlands (TAMAYO, 1962; SELF, 1971). Rain falls pri tain flanks toward the northeast. Beyond the base of moun marily in summer and early fall, and is heaviest duringAugust tain slopes, the rivers traverse piedmont hills, alluvial fans and September. Differences in transport regime and sediment and terraces (Figures 3, 4A) and extend across the lower, nar supply to the two deltas are mainly a result of dissimilarities row ("-'15 km), near-horizontal coastal plain. Sediment source in fluvial basin area and consequent amounts of discharge (TA areas for the Nautla and Tecolutla deltas are primarily vol MAYO, 1962). The area of the Nautla drainage basin is ap canic terrains widely exposed in the Neovolcanic Cordillera proximately 2270 km-; the average river flow is 1,086,000 m'' of the Sierra. Peaks include the Cofre de Perote (4282 m) and per km 2 of basin area, accounting for a mean annual discharge Pico de Orizaba (5747 m), andesitic cones active during the of 2465 million m", In contrast, the Tecolutla drainage basin Cenozoic (GARFIAS and CHAPIN, 1949; TAMAYO, 1962; MEX area (8080 km-) is about 3.5 times larger, while the average ICO, DIRECCION GENERAL DE GEOGRAFIA, 1984; MORAN river flow per basin area (931,800 m" per km") is nearly the ZENTENO and Collaborators, 1994). same as the Nautla. This accounts for a mean total annual Sediment in both drainage basins is transported to the sea river discharge of the Tecolutla that is at least 3 times larger by rivers that are directly subject to seasonal rainfall and (7529 million m") than in the Nautla. During dry periods, flow flooding. Mean annual rainfall ranges regionally from "-'1500 in the lower Nautla is reduced to "-'20 mvsec, while in the mm on the coastal plain to as much as 3000 mm in high-ele- lower Tecolutla it is at least 30 m-/sec. Journal of Coastal Research, Vol. 16, No.2, 2000 472 Chen, Stanley and Wright TECOLUTLA RIVER o Gulf of Mexico o 20°30' o Sample Location o Dune and Beach Marsh • Mangrove Archeological o Site ~ Approximate *'\. Delta Boundary 20°25' I II 97°05' 97°00' ~lometers o NAUTLA RIVER " Gulf o of Mexico 20°15' o Nt o kilometers o 5 o 20°10' 96°50' 96°45' Figure 2. Maps of the Tecolutla (upper) and Nautla (lower) deltas, positioned on the Veracruz margin, showing surficial sediment sample sites in diverse environments. The Nautla and Tecolutla Holocene deltas are among sev are constrained by adjacent higher terrains formed by Ter eral wave-dominated and sharply truncated depocenters tiary and Quaternary deposits (Figure 6A). This coastal re along the NW-SE trending Veracruz margin (Figure 5D-F). gion, characterized by few, low magnitude, shallow earth The distance between apex and coast is '"'"'9 km in the Nautla quake epicenters, has remained relatively stable during the delta and '"'"'7 km in the Tecolutla delta, and both have a Holocene. Some faulting and neotectonic deformation (LEN coastal margin length of '"'"' 14 km. The trapezoidal Nautla del TELL, 1975), however, appear to control the configuration of ta area is '"'"' 115 km", and that of the triangular Tecolutla is the two depocenters, including angular paths of their fluvial '"'"'50 km". Delta margins and distributary channel patterns channels and distributaries (Figures 2, 3). The savannah veg- Journal of Coastal Research, Vol. 16, No.2, 2000 Sediments in Tropical Deltas 473 Figure 3. Space Shuttle image showing the base of the Sierra Madre Oriental (SMO) and adjacent piedmont, on the Tecolutla coastal plain and delta, including main river channel (T), marsh (M), mangrove (MA), and coastal barrier (B). In winter, wave-driven coastal currents (small arrows) drive sediment toward the SE. Near-vertical photograph was taken 118 nautical miles above earth surface on November 18, 1990. etation that now prevails over large parts of this coastal area (following the major Gulfcirculation pattern), with flow rang (Figure 4A-D ) results primarily from recent

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