The stratigraphy and structural evolution of Nuclear Central America during the Cretaceous Period, analyzed with the aid of new time-stratigraphic data from Guatemala and Honduras, suggest the following sequence of events. Marine Cretaceous transgression commenced during late Aptian time with widespread deposition of platform carbonate sediments from El Peten on the north to Metapan and Lago de Yojoa on the south with no intervening land barriers. By late middle Albian time, downwarps had formed along a northeast-southwest trend from Guatemala City to Jocotan and along a northwest-southeast trend in the Tegucigalpa area. These downwarps were filled, during late Aptian-early Cenomanian time, with ammonite-bearing radiolarian limestones while evaporitic or carbonate-platfor conditions persisted on the north.

Widespread crustal rupturing is indicated by lava flows and deposition of coarse conglomerates during early Cenomanian time in southern Guatemala and Honduras. This diastrophism was followed, in later Cenomanian time, by structural quiescence and the return of carbonate-platform conditions over a wide area--at least as far south as Tegucigalpa.

A very thick sequence of metamorphosed graywacke, limestone, radiolarian chert and ultramafic rock, which crops out south of the Motagua Valley in Guatemala, is assigned to the early Late Cretaceous on the basis of radiolitid rudist fragments from metamorphosed limestone near Sanarate. This eugeosynclinal sequence was deposited in a mobile trough which extended northeastward from Guatemala City toward the Bay Islands in the Caribbean and probably covered the area now occupied by the Sierra de las Minas and Sierra Chuacus. The contents of this trough were deformed, intruded by granite, metamorphosed, and uplifted during a tectonic pulse which is referred to Turonian-Santonian time.

The uplift of this orogene caused the depositional axis to migrate northward so that, during Campanian-Maestrichtian time, the flysch-like sediments of the Sepur Formation were dumped into a trough which trended northeast-southwest across southern Alta Verapaz. This rapid sedimentation terminated with end-of-Cretaceous movements which induced the sliding of the massive Sierra de Santa Cruz ophiolite body, from its original location in the early Late Cretaceous orogene northward, into the Sepur depocenter.

South of the Sepur trough, Maestrichtian seas evidently were excluded from Nuclear Central America, which received continental deposits only. However, limited evidence suggests that there was, on the southwest, another trough, similar to that in Alta Verapaz but trending northwest-southeast from the Gulf of Tehuantepec in Mexico to Nicoya in Costa Rica.

Precise time-stratigraphic control for Cretaceous rocks in Guatemala and Honduras, although still very meager, allows a more accurate pinpointing of times of deformational events during the Cretaceous Period. Data strongly imply that short-lived periods of crustal disturbance in Nuclear Central America were synchronous with the well-established worldwide mid-Cretaceous (Austrian), Subhercynian, and Laramide orogenic pulses. Subsequently, Cretaceous structural grains and sedimentary patterns have been obscured greatly by major Tertiary block movements and volcanic extrusions. The breakup of Nuclear Central America and its greater surroundings, now hidden below Pacific and Caribbean waters, is attributed to crustal rupture along fundamental tension fractures aligned northeast-southwest nd northwest-southeast. Extensional rifting facilitated the injection of subcrustal material along block margins during abrupt orogenic pulses of worldwide synchroneity which are attributed to the process of global expansion.