MAZZULLO. S. J., WILLIAM D. BISCHOFF, and C. S. TEAL, Wichita State University, Wichita, KS

ABSTRACT: Evolution of a Holocene Carbonate Mudbank Complex, Cangrejo Shoals, Northern Belize

Cangrejo Shoals, a shallow water mudbank of Holocene age, is located at the south tip of Ambergris Caye along the transition between the outer-shelf (immediate back-reef) and inner-shelf (Chetumal Bay) - Unlike classic Florida Bay mud mounds, these shoals are areally extensive (18 sq km), thick (maximum 6.4 m), and transected by deep tidal channels. Dominant wind and water current directions in the area result in net bankward transport of sediments away from the platform-margin barrier reef. Depositional onlapping of the karst-eroded surface of underlying Pleistocene limestone occurred over the last 7000 years, during decelerating sea level rise. Sediment accumulation rate is about 0.9 m/1000 yrs. Core studies indicate that, except for some basal transgressive gravelly sands, the sect on is dominated by unconsolidated sandy, and locally gravelly, muds. Absence of mangrove peats suggests a catch-up and/or keep-up style of shallow subtidal deposition over time. Formation of mangrove islands (emergence phase), which assist in southward accretion of Ambergris Caye, likely occurred after sea level stillstand was reached 1000 yrs BP.

Comparison of particle types and sediment mineralogies in the shoal deposits to those in outer- and inner-shelf sediment standards suggests a dual sediment source. Through time, 60% of the mud on the shoals has been derived from the outer-shelf, whereas 80% of the particles composing the coarser size fractions has been derived from the inner-shelf. Data suggest that: (1) relatively high rates of mud produced on the outer-shelf, and continuously transported bankward to the shoals, overwhelm the supply of mud from the inner-shelf; and (2) some mud, and the bulk of the coarser sediments, are transported to the shoals from the inner-shelf on a seasonal basis only, likely during winter "northers" when current flow in the inner-shelf is to the SE.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.