Abstract

Textural and compositional changes in fluvial sediments in the 3400 km 2 Rio Nautla drainage basin are discussed. Many different fluvial environments and rock types are present in this basin. The stream profiles are divided into three segments based on gradient. The headwaters segment is the steepest and contains high energy streams, while the intermediate segment has braided streams and the coastal segment consists of low-energy meandering streams. Sharp topographic breaks are present at the boundaries between the segments. Sediments found upstream from the coastal segment are bimodal mixtures of sand and gravel with metal grain sizes in the gravel range. They are poorly sorted, and are mostly positively skewed and leptokurtic. Skewhess and kurtosis indicate the proportions in which the sand and gravel are mixed in biomodal sediments and can be used to indicate the rate in which competence decreases after floodstage. Variation in grain size in the upper Rio Nautla drainage basin are determined by variations in the amount and size of gravels. In the coastal segment, unimodal, moderately well-sorted fine-to medium-grained sand is present. All of the gravel is deposited upstream from the coastal plain, and therefore never reaches the coastal plain. The composition of the Rio Nautla sediments reflects the dominance of volcanic rocks in the source area. Volcanic rock fragments, plagioclase, magnetite and pyroxenes are the most common constituents, but quartz, orthoclase and limestone are common and can even dominate the suite close to granitic and limestone outcrops. Sudden changes in composition are found in the headwaters segment, but not in the other segments which represent an integration of all lithologies found in the drainage basin. The area can best be compared to an alluvial fan, where gravel grades into non-marine sand and then into marine sand, silt and clay. The location of the inner margin of the coastal plain, where the last of the gravel settles out, may indicate the relative position of land and sea. If the land subsides or sealevel rises, the sand-gravel boundary moves inland. If sealevel is lowered or the land is uplifted, the boundary will move seaward.

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