page_title: Alajuela Formation
section_title: Interval I
hierarchical_section_title: Alajuela Formation / Description / Stratigraphy / Interval I
caption_attribution_description: English: Ichnofossils of the Alajuela Formation, Panama
context_page_description: The Alajuela Formation, originally Alhajuela Formation, is a Late Miocene geologic formation in the Panama Canal Zone of central Panama.
  The formation overlies the Caimito Formation and comprises sandstones, limestones and conglomerates deposited in a tidal-dominated estuarine to shallow marine environment.
  The formation crops out in a small area along the southern and western shores of Lake Alajuela, of which it derives its name, and preserves a rich fossil faunal assemblage of mammals, fish, invertebrates and flora. The fauna is of paleontological significance as an insight into the ecosystem of Central America preceding the Great American Biotic Interchange.
context_section_description: Within the basal-most Interval 1, laterally-extensive horizons of amalgamated conglomerate lenses fine upwards into fine-to-medium grained sandstones, truncated by
  erosional contacts with overlying units. The best exposed amalgamated conglomerate exhibits substantial variability in thickness (1 to 6 metres (3.3 to 19.7 ft)), with an average thickness of about 3 metres (9.8 ft). The amalgamated conglomerates are generally clast-supported, but locally matrix-supported, with the coarse grain fraction rarely exceeding 5 centimetres (2.0 in) in diameter. Weathered exposures of the conglomeratic horizons erroneously appear matrix-supported due to diagenetic dissolution of aragonitic shell material. Prior to such dissolution, the coarse fraction would likely have been dominated by mollusk shells, whereas well-rounded, pebble- to cobble-sized volcanic fragments (welded tuff and andesite) and silicified woods comprise the minor component of the coarse fraction. Fossil invertebrate remains primarily consist of internal and external molds of mollusk shells preserved in fine-grained sand matrix as well as some original calcitic shell material of scallops and oysters.
  The amalgamated conglomerates contain the most abundant vertebrate fossils, both well-preserved remains of marine vertebrates (e.g., sharks) and highly weathered remains of terrestrial vertebrates. The conglomerate exhibits a dm-scale gradational contact with a poorly sorted, fine-to-medium-grained sandstone with abundant lithic and feldspar grains. The sandstone is locally tuffaceous and exhibits, in some exposures, low-angle, dm-thick bedforms that are internally massive and dip perpendicularly to the overall attitude of the Alajuela Formation. Otherwise, the sandstone appears massive and highly bioturbated, containing a lower density of molluscan molds than the underlying conglomerate. Original molluscan shell material and marine vertebrate fossils are present but rare. No terrestrial vertebrate remains have been found within
  the sandstone horizons of Unit A.
  One exposure of Interval 1, within the 9–14 m levels of the composite section, includes a moderately sorted, fine-medium grained tuffaceous sandstone with distinctive mm- to cm thick horizontal bedding and wavy laminations (Unit B). Scour and fill structures with dm-scale widths and occasional mud drapes are present near the erosional contact with the underlying Unit A lithologies. No body fossils are present in this unit, but rare and well-preserved ichnofossils, primarily Conichnus and vertically-oriented Ophiomorpha, are preserved that cut across and deform bedding horizons. The infilling of these burrows consists of the same fine tuffaceous sand of overlying horizons.
caption: Ichnofossils of the Alajuela Formation: Conichnus (C) and Ophiomorpha (O)
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