The Alajuela Formation, in older literature also referred to as Alhajuela Formation, was first described as a
member of the
Caimito Formation by
USGS geologist W.P. Woodring in 1957. The member was named after
Lake Alajuela, where the
type section was described. In 1970, Woodring elevated the member to a formation. The Alajuela formation crops out in the
Panama Basin along the southwestern shores of and on islands in Lake Alajuela. Woodring (1970) divided the elevated Alhajuela Formation into two members; a lower member consisting of the former calcareous sandstone member of the
Caimito Formation and a lens of sandy limestone formerly designated the Chilibrillo limestone member of the Caimito Formation. The upper member was described as the former Alhajuela sandstone member of the Caimito Formation. The author estimated the thickness of the formation to be .
Stratigraphy of of the Alajuela Formation. Fossil localities are indicated. While Woodring originally described the formation in two members, MacFadden et al. (2017) define three distinct lithological intervals in the composite section, measured in proximity to the fossil localities on the southern extent of Lago Alajuela, of the formation. The succession starts with a more than thick basal package of interbedded, clast-supported
conglomerates and
litharenite sandstones, grading into an approximately thick package of
calcareous sandstones and calcarenites, representing a transition from tide-dominated, potentially
estuarine, coastal
environment to a wave-dominated, shallow water carbonate environment.
Interval I 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 (), with an average thickness of about . The amalgamated conglomerates are generally clast-supported, but locally matrix-supported, with the coarse grain fraction rarely exceeding 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. s of the Alajuela Formation:
Conichnus (C) and
Ophiomorpha (O) 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.
Interval III The base of Interval 3 is marked by a cm-scale gradational contact between an underlying shell lag in Interval 2 and an overlying fine-grained calcareous sandstone with lithics and occasional trough cross-bedding and ripple marks (Unit E). A calcarenite occasionally interbedded with sandy limestone (Unit F) occurs stratigraphically above, separated from the underlying calcareous sandstone by an irregular, erosional contact. Sedimentary structures vary within the calcarenite from trough cross-bedding to wavy bedding to low-angle planar cross-bedding, suggesting substantial changes in flow velocities at the time of deposition. The lithologies in Unit F were originally described by Woodring in 1957 as the Alhajuela Sandstone Member of the Caimito Formation. However, based on the age constraints presented below for the Alajuela Formation, this attribution to the late Oligocene—early Miocene Caimito Formation is no longer supported. The stratigraphic thickness of Interval 3 was not measured in the study by MacFadden et al. (2017), but was reported by Woodring (1957) as being approximately . The equivalent
South American land mammal age for the estimated age of the Arajuela Formation is
Chasicoan. == Paleontological significance ==