ABSTRACT Two late Eocene impact spherule layers are known: the North America microtektite layer (from the Chesapeake Bay crater) and the slightly older clinopyroxene (cpx) spherule layer (from Popigai crater). Positive Ir anomalies occur at 5.61 m and 6.19 m above the base of a late Eocene section at Massignano, Italy. The age difference between the two anomalies is ~65 ± 20 k.y. The older Ir anomaly at 5.61 m appears to be associated with the cpx spherule layer. Although no impact spherules or shocked-mineral grains have been found associated with the upper Ir anomaly at 6.19 m, it has been proposed that it may be from the Chesapeake Bay impact. Comparison with other distal ejecta layers suggests that microtektites, but not shocked-mineral grains, from the Chesapeake Bay crater could have been thrown as far as Massignano. However, their absence neither supports nor disproves the hypothesis that the Ir anomaly at 6.19 m is from the Chesapeake Bay impact. On the other hand, the North American microtektite layer is not associated with an Ir anomaly. Furthermore, the average age difference between the cpx spherule layer and the North American microtektite layer appears to be ~18 ± 11 k.y., which is nearly one quarter the age difference between the two Ir anomalies at Massignano. This indicates that the Ir anomaly at 6.19 m is too young to be from the Chesapeake Bay impact, and thus is most likely not from the Chesapeake Bay impact.

We correlate seismic map units identified on industry seismic lines in the Gulf of Chiriquí, southwestern Panama, with onland igneous rocks and sedimentary formations described in this chapter. We propose six principal stages in the stratigraphic development of southwestern Panama based on our results and the results of previous workers in Costa Rica, westernmost Panama, and the western Colombian basin. The first stage in southwestern Panama is represented by basaltic basement rocks of Jurassic?-Late Cretaceous age interbedded with Upper Cretaceous pelagic sedimentary rocks. Following previous workers and data presented here, we suggest that these rocks formed in an intraoceanic, oceanic plateau setting. A second stage is represented by a major stratigraphic hiatus inferred to represent an erosional event that affected the basaltic basement of Panama in Paleocene time. A third stage is represented by a widespread basal transgressive section of coarse clastic rocks and reefal carbonate rocks of early to middle Eocene age. This section records initiation of clastic sedimentation over much of southern Central America. A fourth stage is represented by a thick section of mainly marine turbidites that appears to represent continued erosion of the land areas in southern Central America and upward deepening of adjacent marine environments. A fifth stage is represented by a stratigraphic hiatus in middle Miocene to late Miocene time that may represent the “breakup” unconformity associated with initiation of strike-slip faulting and rifting in the Gulf of Chiriquí. A sixth stage is represented by early Pliocene to Pleistocene rifting and syn-rift sedimentation in the Gulf of Chiriqu&iacute. Thick sedimentary fill of rift basins may reflect accelerated uplift of southern Central America and increased activity of the Middle America arc. The regional extent of the stratigraphic record of several of these stages across large areas of southern Central America and the western Colombian basin supports the previously proposed hypothesis that the crust of southern Central America represents the western upturned edge of a Late Cretaceous Caribbean oceanic plateau known from deep-sea drilling and seismic stratigraphic studies in the Colombian and Venezuelan basins of the Caribbean Sea.

A deep-sea core from the Caribbean contains a layer of sediment highly enriched in meteoritic iridium. This layer underlies a precisely deated (34.4 m.y.) layer of North American microtektites and coincides with the extinction of five major species of radiolaria. A causal relationship is strongly suggested: a massive (⩾3 km, ⩾50 billion ton), chemically undifferentiated meteorite collided with the earth, produced the tektites, and led to worldwide extinctions 34 m.y. ago.