Tireo Group roeks of late Cretaceous age occupy a 290-km-long belt along the Cordillera Central in the Dominican Republic and the Massif du Nord in Haiti. Lithologies and stratigraphy have been studied in three main areas in the Dominican Republic: Restauracion, Juan de Herrera–Constanza–Las Canitas and El Recodo. This chapter attempts a correlation among the recognized units in these areas with studied areas in the Massif du Nord. Two main units are recognized in the Cordilleran Central:

1. Lower Tireo Group (Basic Rocks). The oldest and most extensive rocks (here termed Tireo Formation) are a sequence (>4,000 m) of massive, green, vitric-lithic tuffs with intercalated mudstones, siltstones, and limestones found in the Dominican Republic and in the Massif du Nord. These vitroclastic tuffs probably originated as pyroclastic flows produced from phreatomagmatic and phreatic eruptions and then were deposited as submarine debris flows at abyssal depths. Other units mapped in the Restauracion area are interbedded red and green tuffs, well-stratified crystal-lithic tuffs and agglomerate; and a unit of fine silicified tuffs, andesite flows, and pyroclastic basaltic rocks on the border with Haiti.

In the Las Canitas-Constanza and adjacent areas important lithologies include high-TiO₂ metabasalt (Pico Duarte and Juan de Herrera areas); intercalated limestones (Constanza and Valle Nuevo Members); and cherts (El Convento Member) and gray-wackes, shales, and mudstones (Río Blanco and El Recodo areas).

Microfossil age determinations indicate a Turonian (possibly late Cenomanian) to early Senonian age for the main volcanism, but sedimentation may have continued until the early Maastrichtian in the northeast. These rocks pass without apparent discordance into the Upper Tireo Group.

2. Upper Tireo Group (Acid Rocks). This group consists of lavas, pyroclastic rocks, and reworked tuffs of mainly dacite and rhyolite (keratophyre) composition. The acid volcanism is concentrated in centers along the southern margin of the belt. The acid volcanism probably did not begin before the late Santonian and ended in the early-middle Campanian.

Lower Tireo Group rocks correlate with the Terrier Rouge Series in the Massif du Nord, Haiti, consisting of andesites, andesitic tuffs, tuffites, agglomerates, mudstones, and basalts. Upper Tireo Group correlate with the La Mine Series in Haiti, consisting of dacite flows and stocks, crystal-lithic tuffs, unsorted pyroclastic rocks, and volcaniclastic rocks. The Tireo Group is overlain conformably in the south-central part of the belt by sediments of middle Campanian-Maastrichtian age and younger of the Trois Rivieres–Peralta Belt.