PROVENANCE OF THE COASTAL SAND, GUYANACUSH, Athar-Rahman1, KHANDAKER, Nazrul I.1, SCHLEIFER, Stanley1, NICASIO, Micheal1, RAMPERSAD, Jessica1, and CUSH, Akbar Rahim Wazir2, (1) Natural Sciences Department, Geology Discipline, York College of CUNY, 94-20 Guy R. Brewer Blvd, Jamaica, NY 11451, wellingtoncyprus@yahoo.com, (2)

Earth and Atmospheric Sciences, City College of CUNY, 160 Convent Avenue, Manhattan, NY 10031

Guyana is located on the Northeast coast of South America. It sits amid Suriname (east), Venezuela (west), and Brazil (south) and occupies a land mass of approximately 83,000 square miles. There are three geographic regions, the coastal plane, the Hilly-sand and clay-belt, and the Highlands.

The Atlantic coastline of Guyana runs from southeast to northwest, is almost 270 miles in length, and as much as 30 miles wide in some areas making up less than 10% of the country. Its clayey, silt overlay are deposits of the eroding Highlands, brought by its many rivers. Shallow waters and sandbars are common to this region.

Swamps form a barrier between the hilly-sand and clay belt and the coastal plane. The hilly-sand and clay-belt, located south of the coastal plane, is between 100 to 150 miles wide with low sandy hills and an occasional rocky outcrop.

The Guyana Highlands region is where mountain ranges dominate.

Samples of the Guyana coastal sand were obtained from various points along the Guyana coast, beginning at the mouth of the Mahaica river in the North East, and concluding at the town of Bartica (Essequibo river) in the South West.

The samples were taken at an approximate depth of 18 to 30 inches on the beaches in groups of three per location – thirty feet apart and an approximate weight of three-hundred grams each. GPS readings were obtained at each location for future studies of those areas.

Each sample was divided in half and wet-sieved to restrict the range of study to sizes between -2.0, and +4.0 Phi. After drying, the samples were subjected to sorting by-hand and hopper at one-half Phi intervals. Each of the selected grain size was weighed, packaged, and labeled separately to facilitate other determinations later. The sorted samples were examined for mineral and opaque content.

Heavy Minerals Percentage

00.5

11.5

22.5

33.5

Barti

ca

Hope

Beac

h

Hope

Beac

h

Vigi

lanc

eBe

ach

Parik

a

Thom

asLa

nds

Barti

ca

Hope

Beac

h

Hope

Beac

h

016 07B 07A 028 023 004 018 017 008

Locations Name/Number

Heav

y M

iner

als

%

Heavy Minerals %

Selection and separation procedure: Nine samples were chosen from eight locations in the combined ranges of 2.5 3.0 Phi, and 3.5 to 4.0 Phi. The weight of each sample was between 12 and 37 grams. The samples were immersed in a 250 mL separatory funnel containing Sodium Polytungstate (Na6 (H2W12O40) H2O), density 2.85 gm/cc. This separates the heavy from the light minerals. Minerals with a density greater than 2.85 settled, and those of a lower density remained in suspension. A two-hour settling time was allowed before releasing the heavy minerals into a filter paper lined funnel and beaker collector. The heavy minerals were then washed in a 70% alcohol solution and allowed to dry on a heating plate with ultrasonic stirring. The light minerals were filtered into another funnel and beaker setup, washed with 70% alcohol solution, and rinsed with water and dried.

Representative sand samples were collected from the coastline located along the downstream section of the Mahaica River. Geographically, sample locations are in close proximity to the town of Bartica, eastern Guyana. Sand samples were analyzed for compositional studies with primary focus on the distribution of heavy minerals and utilization of heavy mineral data for provenance interpretation. Geologically, the sand samples are located within the Guyana Shield and the bedrocks belong to the Proterozoic Bartica Formation consisting mainly of gneiss and granulites. The Bartica Formation is bordered with the Proterozoic greenstone belts to the west and southwest and Trans-Amazonian felsic to intermediate igneous intrusive to the west and south. A prominent east-west trending outcrop of Proterozoic felsic volcanic rocks and granites marks the southern boundary of the Bartica Formation. Preliminary heavy mineral study indicates the presence of two distinctive heavy mineral suites. Samples located close to the downstream section of the Mahaica River are characterized by epidote, zircon, hornblende, augite, and minor hypersthene; whereas, samples located in the south of the Mahaica River are typified by garnet, reddish brown basaltic hornblende, augite, zircon, and hypersthene. The typical reddish brown hornblende has been well-documented by many researchers as a petrological signature tool for northern Brazil provenance. The overall composition and provenance of the sands may in part, have been controlled by the low sealevel stand associated with the last glacial episode when local streams incised deeply into the Proterozoic bedrocks and supplied detritus to the surficial coastal deposits.

Discussion: The samples collected can be divided into three groups.

1. High percentage of heavy minerals (2 to 3 percent).2. Moderate heavy mineral content (1 to 1.5 percent)3. Low heavy mineral content (0.14 to 0.5 percent).Group 1 comprised entirely of sample from Hope Beach.Group 2 was a mixture of Hope Beach and Bartica.Group 3 was made up from three locations Parika, Thomas Lands, Vigilance Beach–

each ten or more miles apart.A decreasing trend was observed in the samples when viewed from east to west. That is, the percentages of heavy minerals were highest in the north-east and lowest in the north-west. The middle group showed a diagonal pattern cutting through groups 1 and 2 from north-east to south west. These trends could possibly have been established during the glaciation of the Proterozoic, but cannot be substantiated without more in-depth studies.

Conclusions: Further detailed studies (Geochemical, Petrological, etc.) and analyses are necessary to get a better understanding of these observations

References: Http://www.guyanaguide.com; Walrond 1987; Gibbs and Barron 1993; Microsoft MapPoint; Expedia; Athar-Rahman Cush, Aug. 2006; Akbar Cush July, 2007.