Late quaternary stratigraphy and environments of the west Mexican coastal plain

Abstract

The geomorphic events that formed the west Mexican coastal plain and the stratigraphy and chronology for those events based on soils, pollen zones, and radiocarbon ages are discussed. Three phases of geomorphic development are recognized. The Alluvial Phase represents deposition of the coastal bajada during the Neogene; oxisols and saprolites have formed on this surface. The Teacapan Peninsula Phase began during the Sangamonian Inter‐glacial with deposition of the spit; an ultisol and an alfisol have developed on the peninsula. In the Prograding Beach Phase, beginning about 13,000 B.P., four sequences of beach ridges were deposited during eustatic sea level rise and periodic uplift. On the resulting ridge and swale topography are four soil units, three inceptisols on the initial, early, and middle beaches and an entisol on the late beach.

The pollen stratigraphy and chronology are based on samples from eleven cores, twelve radiocarbon dates, and calculated deposition rates. The alternation in dominance of mangrove pollen taxa records relative changes in sea level due to tectonic uplift and sea level rise. The data record filling of the lagoon behind the Teacapan Peninsula beginning prior to 8,000 years ago during eustatic sea level rise. A mangrove pollen association that corresponds to this rise continues into the mid‐Holocene. A subsequent Avicennia rise indicates a relative lowering of sea level possibly due to uplift of the coastal plain. Between 7,000 and 6,000 B.P., increases in pine and oak indicate expansion of upland forests during a cooler climatic interval. A marine incursion around 5,650 B.P. documents a peak in sea level rise. In late Holocene time, marsh taxa become more important as beaches prograde westward and the lagoons receive more freshwater runoff. After 5,000 B.P., upland forest taxa again increase. The most recent, Avicennia‐dominated associations again show lower sea level or uplift. Late Holocene pollen zones include one of nonarboreal pollen dominance and lastly one enriched in pine, nonarboreal and lowland forest taxa, and cultivated plants.

The pollen data support age relationships inferred in the Prograding Beach Phase. Just as all of the beaches are represented in the Teacapan area of the coastal plain, the oldest pollen record occurs in the lagoon behind the Teacapan Peninsula. Higher sea levels are indicated by red mangrove prominence for the Initial Beach and by the Dictyocha zone for the Early Beach. The southern part of the coastal plain has only the younger beaches and only the younger Holocene pollen zones in the lagoonal records.