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FIELD GUIDE TO THE GEOLOGY OF ST. CROIX, U.S. VIRGIN ISLANDS1 by John T. Whetten2 INTRODUCTION A popular misconception in tourist lore is that St Croix was formerly a volcano. Although volcanoes are present on many nearby islands, there are none on St. Croix, and there probably have not been any for tens of millions of years, if ever. Yet paradoxi cally, most of the rocks are originally of volcanic origin. The rocks underlying the mountain ranges on St Croix (and probably those deep underneath the Central Valley) are sedimentary rocks formed of the debris from eroding volcanic rocks and from vol canic ash spewed out from an erupting volcano. The sediments were deposited on the deep ocean floor in the late ^Cretaceous (Campanian Maestrichtian), and are approximately 80 million years old. The limestone (or marl) exposed at the surface of the Cent ral Valley of St. Croix is considerably younger (lovwe Miocene, 20 million years), and is probably the remains of a coral reef that formed as the island was uplifted. St. Croix has had a long and complicated geologic history. The general pattern of evol ution of the island is now known,but many of the details remain a mystery. The purpose of this guide is to summarize briefly the important aspects of the geology of St. Croix, and to indicate some of the places where the evidenceand the geologic record m ay be seen. A generalized geologic map indicates the distribution of the main rock types, and their strategraphic relationships are diagrammatically illustrated. A simplified map legend precedes the map indicating the type of rock and approximate ages they were formed or deposited. 1Reprint of Field Trip Guide used at 5th Caribbean Geological Conference, 1968. Figures from Whetten (1966) 2Department of Geology and Department of Oceanography, University of Washington, Seattle, Washington, 98105. Contribution no. 459 from the Department of Oceanography, University of Washington. Research sponsored in part by the office of Naval Research
CALEDONIA FORMATION The oldest rocks exposed on St. Croix are the alternating dark and light banded sedimentary rocks of the Caledonia Formation, named for Caledonia Valley where an excellent section is exposed. The formation is present in several areas on St. Croix (see map): in the northwest, in the vicinity of Hams Bluff, Mt. Washington (Frederiksted), Frenchman Hill, Punch, and Maroon Ridge; in a narrow outcrop along the coast at La Valee; and on the eastern half of the island from Christiansted to East Point, includin g Buck Island and Green Cay. Perhaps the most obvious characteristic of the Caledonia Formation is the alternating light gray and bluishblack beds, each seldom more than a few inches thick. The darkcolored rocks are either mudstones (very fine grained rocks composed of silt and clay) or, less frequently, silicified mudstone or chert. About 6070% of the Caledonia Formation is composed of bluishgray to black mudstone. The lighter colored rocks are sandstones and con glomerates, which are commonly grade d, with the coarsest grains at the bottoms of beds to fine grains and ultimately to mudstones at the top. The mineral composition of the sand and conglomerate layers suggests that they are derived in large part from the erosion of volcanic rocks. Some shallow water fossils (including snails, clams, and corals) are mixed with the volcanic debris, particularly in conglomerate beds exposed on Buck Island. The source of the sediment is unknown, but it may have been transported for a considerable distance. The only volcanic rocks now exposed in the area that are similar to the volcanic material in the Caledonia Formation are on St. Thomas, 40 miles to the north. It is possible that long before St. Croix became an island, while it was still a flat area on th e deep sea floor, a volcanic island in the vicinity of St. Thomas was eroded and sediment largely of sand size was transported southward and deposited. Much of the sandsize sediment was probably transported by turbidity currents (dense slurries of sand and mud that flow along the ocean floor by gravity). As a current decreased in velocity first the coarser sediment was deposited, and then the finer, eventually producing a graded bed. Certain s tructures within the sandstone beds such as flute casts and scour marks, and shallow water fossils (which invariably are broken or chipped, indicating considerable transport) support this hypothesis. The black mudstones inter bedded wi th the coarser layers consist of silt and clay (altered to chlorite) and marin e m icrofossils. The mudstone beds were probably deposite d during the interval between turbidity currents, and are normal marine deep water sediments. The recrystallized shells of micro fossils are abundant in the Caledonia mudstones.
The Caledonia Formation is at least 9000 feet thick as measured on the eastern half of the island, but may well be much thicker, as the bottom of the formation is nowhere exposed on land. The oldest beds of the Caledonia Formation are probably on Buck Isl and. TUFFACEOUS FORMATIONS The Caledonia Formation grades laterally and vertically into a variety of other sedimenta ry rocks, including the Allandal e Formation, the Cane Valley Formation, the East End Member (of the Caledonia Formation), and the Judith Fancy Formation. The complex relationships among these formations are indicated in a generalized way in Figs. 3 and 4. Although these rocks are variable in lithology, all contain tuffaceous material or fragments o f altered glass and crystals deposited originally as volcanic ash. The rocks are probably all of marine origin, and the sediments show signs of reworking by currents. The source appears to be from the south or southeast. Thus, the Caledonia Formation, which is composed of particles eroded from volcanic rocks probably to the north, interfingers with the tuffaceous rocks, which are in large part accumulations of volcanic ash from a southerly source (Fig. 2). The two rock types may be distinguished in part because of the remarkable consistency of t he Caledonia Formation in forming relatively thin beds of alternating black mudstone and tight gray sandstone. The tuffaceous formations are commonly greenish rocks, but may be quite variable in character. Some of the best exposures of tuffaceous rocks are located at Judith Fancy, Grass Point, Watch Ho (Vagthus Point), and along the north coast near Baron Bluff (Fig. 1). Because the tuffaceous rocks tend to weather rapidly the best exposures are along the coast, in some stream valleys, and in relatively recent road cuts. Rocks with a high percentage of tuffaceous material are characteristically an applegreen color, probably because of chlorite and epidote minerals. Medium to coarse grained sandstones are very common, but coarser rocks (conglomerates and breccias) and siltstones are also present. Some beds are graded, perhaps owing to the differential settling of windblown volcanic ash through water. Fr agments of fossil corals and pel ecypods are abundant in some places. Other rock types in the tuffa ceous rock sequence include at least two lava flows of unknown thickness, very hard si licified siltstones, and limestone. The best exposure of limestone is along the coast at watch Ho (Vagthus Point). The bedding in some of the tuffaceous sandstones is highly contorted, suggesting that slumping occurred relatively soon after
deposition. Some of the slumps appear to ha ve come from the south, which is in agreement with the proposed southerly volcanic source. The total thickness of tuffaceous rocks is approximately 18,000 feet, although there is considerable variation in thickness of some of the units. Furthermore, the bottom of the sequence is not expose d, and the top has been removed by erosion, so it may have been much thicker. ROCKS OF T HE CENTRAL VALLEY The white to buff limy rocks on the surface of the Central Valley (extending from western St. Croix eastward to Beeston Hill and Canegarden) have been named the Kingshill Marl, and may be up to 600 feet thick. The rocks are flat lyin g to gently folded, and probably formed as a large coral reef before St. Croix became fully emerged. Fossil coral, Foraminifera, gastropods, and pelecypods are present in abundance, and indicate an early Miocene age for the formation. Some land derived san d and clay is mixed with the lime stone. The formation has been used extensively for building stone on St. Croix, and there are numerous abandoned small quarries. Rocks beneath the Kingshill Marl are exposed only in a few places along the north flank of the Northside Range and in well cuttings from the Central Valley. For the most part they are clays and conglomerat es. A gravity survey by Shurbet, et. a1. (1956) indicates that the Central Valley is approximately 7000 feet thick. As of 1961 we knew nothin g of the sediments which lie at depths greater than the deepest wells have penetrated, approximately 14 00 feet below the Kingshtll Marl. A cross section through the Northside Range and Central Valley is shown in Fig. 5. INTRUSIVE IGNEOUS ROCKS Two smal l bodies of igneous rocks intrude the Cretaceous sedimentary rocks. The igneous rocks weather easily, and fresh specimens are difficult to obtain except in recent road cuts. The prominent valleys at Fountain and Southgate have formed on the easily eroded i ntrusions. The Fountain intrusion, called the Fountain Gabbro, is shaped like a malformed letter "H". Fresh specimens are (were) obtainable only along the Scenic Road. The rocks consist lar gely of coarsegrained crystallsS of plagioclase and augite, wi th accessory biotite, hypersthene, magnetite, and apatite. The sedimentary rocks adjacent to the intrusion have been metamorphosed. The Southgate Diorite is very poorly exposed, except at Punnett Point and Pu11 Point. Hornblende and plagioclase are essential minerals, and occur in about equal abundance, Augite, magnetite, and apatite are accessory minerals.
Numerous small dikes are present in both the East End and Northside Ranges. "THE BUILDING OF AN ISLAND" (Quin, 1907) The record of only a small part of geologic time is preserved on St. Croix. Although the history of the area prior to the Late Cretaceous (when the oldest rocks were deposited) is unknown, it is probably safe to say that St. Croix and other island areas of the Caribbean were undefined portions on the deep ocean floor for a very long period of earth history. The oldest rocks exposed on St. Croix were deposited by turbidity currents probably flowing from the north. East thin sand bed was rapidly deposited by a single current. Interbedded with the sand layers are mudstones that resulted from the settling out of fine particles between turbidity currents. Each mudstone bed may have taken hundreds or thousands of years to accumulate. Long after the turbidity and mudstones began to accumulate, one or more volcanoes grew from the sea floor, perhaps to the south or southeast of St. Croix. Volcanic debris was shed northward, and in the area of St. Croix the debris interfingers with the turbidites and mudstones. Organic reefs may have grown about the volcanic island and broken fragments of the reef now constitute the limestone of Watch Ho. After the youngest Cretaceous rocks were deposited and before the l ate Oligocene, mountainbui lding f orces compressed the area St. Croix and caused numerous large folds and at least one thrust fault (Fig. 6). Molten igneous material was intruded into the centers of two of the folds, and the Central Valley of St. Croix was depressed by faulting. At the same time, the ocean trough separating St. Croix from St. Thomas was probably formed. During this period of mounta in building, St. Croix was emerging from the sea. As the Nor thside and East End Ranges rose above sea level they formed two islands joi ned by a large coral reef at Kingshill (Kingshill marl). Further uplift joined the two islands and completed the building of St. Croix. SOME LOCALITIES OF GEOLOGIC INTEREST ON ST. CROIX Durin g the summer of 1959 and the fal l and winter of 196061, I compiled a geologic map of St. Croix. Figure 1, a generalized version of the map, shows the location of a number of localities of unusual geologic significance. Since the time I completed the map there has been extensive development of the island and a la rge number of road cuts and excavations have been made. Unfortunately, this new evidence could not be included in this report.
The location, assessibility, and geologic significance of some of the more important geologic localities wi11 be given below. T he locality numbers are keyed to Fig. 1. 1. East Point Location : East Point, the eastern tip of St. Croix, is reached by driving eastward along the north shore of St. Croix to the turnaround at the end of the road. A trail descends east ward approximately 200 feet to the point. ( Public land.) Geologjc stgnifica nc e : The Caledonia Formation is well exposed. Thin graded beds of sandstone and mudstone can be traced laterally for a considerable distance. The Caledonia Formation is also we11 exposed on many of the headlands along the north shore, including Pow Point, Tague Point, and Cottongarden Point. 2. Buck Island Location : Approximately 2 miles north of the east end of St. Croix. Numerous boats make the round trip from Christiansted to Buck Island daily. It is possible to walk around the island, but certain short sections must be waded. The island and adjacent reef comprise the Buck Island National Monument, administered by the National Parks Service, Christiansted. Geologic signi ficance: Buck Island is underlain by the Caledonia Formation, but the structural r elationship of Buck Island to St. Croix is uncertain. Assuming there is no major structure, then the Caledonia Formation exposed on Buck Island is probably the oldest rock unit in the St. Croix area. Exposures of the formation are virtually continuous around the coast except for the west end. The lithology is little different from the formation on St. Croix, except that some conglomerates are present, consisting of rounded pebbles 12 inches in diameter of igneous and metamorphic rocks, and broken fossils, particularly coral, gastropods, and rudists. The alternating sandstone and siltstone beds are characteristically white and black, respectively, on a fresh unweathered surface. The beds on Buck Island are steeply inclined. Some are vertical, and some are overturned by 5 10 degrees. A number of small drag folds are present on the eas t end of the island. Foliation i s well developed, especially in the finer grained rocks. Exposures vary considerably in quality around the island. On the south, wave action is gentle and exposures are somewhat weathered. On the north, storms periodically remove the weathered material and the exposures are relatively fresh.
3. Grass Point Location : South Shore Road, approximately 2 miles southwest of Grapetree Bay Hotel. Grass Point is a conspicuous headland located about 200 yard south of the road. From Christiansted it is most easily reached by taking the North Shore Road to Knight Bay, then turning right on the Grapetree Bay cutoff. Turn west on the South Shore Road f or approximately 1.5 miles to Grass Point. (private land) Geologic significance: The rocks exposed at Grass Point (mapped as the East End Member of the Caledonia Formation) are typical of the tuffaceous sandstones. The rocks are generally green finegra ined tuffaceous sandstones, with some volcanic breccias interbedded with the sandstones on the west side. The beds are highly variable in thickness. 4. Watch Ho (Vagthus Point) Location: South coast of St. Croix between Manchenil Bay and Canegarden Bay, due south of Christiansted. Property owned by the Nelthrop family. Located 0.5 mile south of the South Shore Road at the end of a dirt track that takes off from the South Shore Road 0.4 mile east of 900 bend at Estate Cane Garden. Alternatively, access may be made along the beach from Estate Cane Garden with permission of owners. Geologic Significance: The watch Ho section (approximately 600 feet thick) is in the upper part of the Judith Fancy Formation. F oraminifera from is are dated to Campanian or Maestrichtian, probably Campanian. The lower 300 feet consists of tuffaceous standstone and shale with a small amount of interbedded clastic limestone. Many of the beds are graded. The upper 300 feet consis ts largely of clastic limestone, for the most part conglomeratic, in beds from 6 inches to 8 feet thick. The particle size varies from sand tobolders of up to 10 feet in diameter. Rudists are the most conspicuous fossils; broken fragments are ubiquitous. In some cases the whole shell is preserved. The uppermost part of the section is largely covered. One thin limestone bed occurs very near the top and has an abundance of rudist shells. 5. Judith Fancy Location : Along the coast at Estate Judith Fancy Exposures are continuous from Hotel St. Croix by the Sea northwestward for approximately 0.6 mile. (private land) Geologic significance: This is the type of locality for the Judith Fancy Formation, which forms the bulk of the tuffaceous Sedimentary roc ks. Approximately one fourth of this section
consists of thinbedded Caledonia type sandstones and mudstones which are interbedded with the tuffaceous rocks. The tuffaceous rocks are for the most part green, medium to coarse grained sandstones, althoug h in the upper part of the formation (near Hotel St. Croix by the Sea) breccias with volcanic boulders up to 3 feet in diameter are exposed. Siltstones are not uncommon throughout the section. The lowest beds in the section contain broken corals, rudists, and gastropods. A few limestone cobbles and pebbles containing fossils are found higher in the section. There are several stump zones in this section; the largest is approximately 100 feet thick and is located about half way between Hotel St. Croix by t he Sea and a small bay 0.5 mile northwest of the hotel. 6. Tertiary Section along, the Scenic Road Location : Eastern end of Scenic Road. from base of Northside Range at Salt River to an elevation of about 240 feet. Take Scenic Road cutoff from the mai n Christiansted Frederiksted Highway; section starts as one ascends the Northside Range after crossing Salt River. Geologic Significance : This section is probably the Jealousy Formation (Oligocene Age) wh ich underlies the Kingshill Marl. The deposit is poorly stratified and consists of pebbles, cobbles, and boulders of tuffaceous rocks and minor diorite; thin oyster beds, corals, and other shallow water fossil fragments; pockets of red and green clay up to 20 feet long and 5 feet deep; a nd a calcareous sandy matrix. The clay is about 90% montmorillionite and 10% angular fragments of quartz, plagioclase, hornblende, and hematite. The fault contact between the Jealousy and the Judith Fancy Formations is well exposed on the Scenic Road an d on the East side of Judith Fancy. It dips 600750 southwest. 7. Intrusive Igneous Rocks on the Scenic Road Location : The Fountain Gabbro is exposed in two places along the Scenic Road, about 0.6 mile west of the junction of the Scenic Road and Parasol Hi11 Road, and about 0.4 mile east of the junction. Both exposures are continuous for some distance. Geologic significance : The Fountain Gabbro is a discordant unfoli ated pyroxene gabbro. Plagioclase and augite are essential minerals; hypersthene occurs in variable amounts and may be essential or accessory. A small contact aureole of metamorphosed sedimentary rocks surrounds the intrusion. Fresh specimens are obtainable only along recent road cuts. The intrusive rock weathers readily and has formed the broad valleys at Fountain.
8. Creque Dam Road Location The best exposures are in the stream valley for about 0 4 mile u pstream and downstream from Cre que Dam Reservoir. The road parallels the stream. : Creque Dam Road connects Sprat Hall with Estate Annaly. Geologic Significance : Some of the best exposures of the Caledonia Formation are in the stream bed on the north side of C re que Dam Road. P articularly good exposures are just upstream from Cre que Dam Reservoir. Pebble conglomerates, sandstones, and mudstones are present; the coarser units are generally graded and current bedding is not uncommon. Flute casts and load casts on the b ottom of sandstone beds are usually observable only in two dimensions owing to the extreme hardness of the u nderlying mudstones, which are in part silicified. 9. Ham Bluff Location : Northwest corner of St. Croix. Take the road along the west coast of St. Croix north to the Coast Guard Station at Hams Bluff. Obtain permission from Coast Guard to take the trail to the coast leading from the northeast corner of the easternmost house at the station. Geologic significance : The Caledonia Formation exposed at Hams b1uff consists of fine grained sandstone and siltstone. The section is structurally complex. There are numerous f aults and probably some large sl umps. Many of the siltstones are foliated Numerous quartz veins are present. The elevated beach roc k west of Hams Bluff suggests a recent change in sea level. BIBLIOGRAPHY Cedarstrom, D. J., 1950, Geology and groundwater resources of St Croix, U. S. Virgin Islands: Am. Mineralogist, v. 50, p. 752755. Quin, John T., 1907, The Building of an Island, Chauncy Holt, New York, 107 pages. Shurbet, G. L., Worzel, J. L., and Ewing, M., 1956, Gravity Measure ments in the Virgin Islands: Geol, Soc. America Bull., v. 67, p. 15291536 Whetten, John T., 1966, Geology of St. Croix, U. S. Virgin Islands Geot Soc. America Memoir 98, p. 177239. Whetten, J. T., 1965, Wairakite from low grade metamorphic rocks on St. Croix, U. S. Virgin Islands: Am. Mineralogist, v. 50, p. 752755.