The island of St. Croix is underlain by strongly folded Upper Cretaceous rocks which have undergone low-grade regional metamorphism, gently folded unmetamorphosed Tertiary sedimentary rocks, and igneous intrusions with contact-metamorphic aureoles of Late Cretaceous or Early Tertiary age.
The Mount Eagle Group includes all the Upper Cretaceous sedimentary rocks and is composed of two interfingering facies, epiclastic volcanic sedimentary rocks and tuffaceous sedimentary rocks, both deposited below wave base in a marine environment. A turbidite sequence more than 8000 and perhaps 18,000 feet thick of epiclastic volcanic sedimentary rocks called the Caledonia Formation (Campanian or older) consists of alternating thin beds of sandstone and mudstone with minor chert and conglomerate. The clastic grains are minerals and lithic fragments derived from keratophyre and spilite flows and tuffs of the Water Island Formation (Cenomanian or older) exposed on St. Thomas, St. John, and the British Virgin Islands. Sedimentary structures include graded, current, and convolute bedding; scour markings; load casts; and slump structures. Structures indicating current and slope directions confirm a northerly source.
About 3000 feet below the top of the Caledonia Formation a 900-foot unit of tuffaceous sandstone and volcanic breccia named the East End Member is exposed in the eastern part of the island. In western St. Croix the Caledonia Formation grades laterally to the south into tuffaceous sandstone and mudstone of the Allandale Formation (2000 feet exposed), and these formations are overlain by the Cane Valley Formation composed of three members: Hope Member (mudstone, 700 feet thick), Robe Hill Member (tuffaceous sandstone, 400 feet thick), and Springfield Member (mudstone, 600 feet thick). The Cane Valley Formation (in the west) and the Caledonia Formation (in the east) are overlain by the Judith Fancy Formation (Campanian, possibly Early Maestrichtian to Santonian), a unit of tuffaceous sedimentary rocks with an exposed thickness of 15,000 feet, including the Clairmont Member (limestone and volcanic-pebble conglomerate, 50 feet thick), Blue Mountain Member (siliceous siltstone, 3700 feet thick), and the Recovery Hill Member (mudstone, 1000 feet thick). Tuffaceous sedimentary rocks in these formations are composed primarily of andesitic and dacitic pyroclastic material intermixed with minor epiclastic keratophyre and spilite detritus. Pyroclastic debris in the East End Member and Judith Fancy Formation coarsens eastward, and slump structures in the latter indicate a slope to the north. The volcanic source apparently lay about 10–15 miles southeast of St. Croix in line with the trend of volcanic centers and belt of thickest accumulation of pyroclastic sediments of Campanian-Maestrichtian Age in Puerto Rico.
After deposition of the Upper Cretaceous sedimentary rocks the region was folded and faulted and intruded by igneous rocks. Two generations of folds are present, the older with axial planes dipping steeply north-northeast and the younger with axial planes dipping east-southeast. Two small stocks (Fountain Gabbro and Southgate Diorite) were intruded along the axial planes of several younger folds, and the sedimentary rocks adjacent to the intrusions were contact metamorphosed to a pyroxene hornfels facies.
All the faults, except the Prosperity thrust fault, are normal faults with little if any horizontal movement. A graben in the center of the island is covered by an estimated 7500 feet of Tertiary sedimentary rocks. Post-early Miocene subsidence of the graben produced gentle folding of the Tertiary rocks.
Early Tertiary uplift is indicated by a pre-late Oligocene wave-cut surface over the entire area.
Only the upper part of the Tertiary section is exposed. The montmoril-lonitic mudstone of the Jealousy Formation (upper Oligocene, minimum thickness 1400 feet) is probably altered volcanic ash from an eastern source deposited under marine conditions. The Kingshill Marl (lower Miocene, 600 feet thick) is a fossil coral reef. Recent sediments include beach sand (locally consolidated to beach rock) and alluvium.
There is little evidence for a previously postulated left-handed strike-slip fault between the St. Croix Platform and the Puerto Rico-Virgin Islands Platform. The Virgin Islands Basin may be a graben formed on the site of a Late Cretaceous geanticline.