High energy systems in inner ramp settings are major targets for exploration and development in many of the worlds prolific petroleum provinces like the Middle East and U.S. Gulf Coast. Two new areas have been studied to expand our understanding of ramp systems beyond the classic Trucial Coast (Persian Gulf) model and, when compared to some siliciclastic barrier island systems, demonstrate some very interesting similarities.

The Northern Yucatan Ramp extends from the Holbox barrier island complex on the east for over 300 km to the Chicxulub impact crater site (K/T boundary impact site) in the west. The major longshore transport along this skeletal dominated ramp is east to west. This is provided by a NE prevailing wind direction which impinges on this microtidal system at an oblique angle and the Yucatan current which flows east to west as it passes from the Caribbean to the Gulf of Mexico.

The overall depositional style is that of barrier island/spit accretion but detail examination shows that the system has three distinct variations. The Holbox barrier island trend is an arcuate prograding series of beach-dune ridges approximately 30 km long. This harder island complex forms Yalahan lagoon which is open to the Gulf on the western end through the wide Boca de Conil and two smaller inlets. The lagoon is mud dominated, in some areas Thalassia covered mud mounds are almost awash at low tide. West of Holbox is the Rio Lagartos barrier island-lagoon trend. Here the barrier island system is 80 km long but very narrow, in some places only a single beach-dune ridge separates the lagoon from the open Gulf. This is in part due to Hurricane Gilbert which cut across the northern Yucatan in 1988. Some sections of the barrier island were completely eroded with major spillover lobes dumped back into the lagoon. The Rio Lagartos lagoon is very restricted and seasonally hypersaline with active evaporite precipitation. Some portions of the lagoon shoreline are sabkha-like with algal mats and salinas. This is in marked contrast to the more open, normal marine Yalahan lagoon. The remainder of the coastal system is dominated by shore-attached narrow beach-dune complexes. Sparse, very narrow barrier island-lagoon complexes do punctuate the coastal system in areas where the main coastline has relatively short, shallow reentrants. Immediately in front of the shoreline, along the entire coastal system is a band of subtidal migrating skeletal bars. The continuity of this subtidal longshore bar system testifies to the consistent wind-wave driven longshore transport system and a substantial offshore mid to outer ramp skeletal carbonate factory.

Wave dominated Holocene and modern ramp deposits from the southern Kuwait-northern Saudi Arabia coastal regions provide a complimentary model to the classic tide-dominated ramp system of the Trucial Coast, UAE. Orientation of the Kuwait coastline parallel to prevailing northwesterly Shamal wind direction, as opposed to perpendicular in the UAE, appears to be the main control on tide vs. wave dominated systems. Both systems are developed during the same Holocene eustatic sea level rise in a microtidal regime and implies that tide or wave dominated systems may not be linked to different portions of sea level cycles.

The Kuwait complex is a 100 km stretch of coast along a gentle ramp punctuated by three headlands and intervening broad cuspate reentrants. Three headlands, Ras Al-Qulay'ah, Ras Al-Zour and Ras Bard Halq, display Holocene prograding strand plain deposits consisting of ooid-skeletal-siliciclastic foreshore to beach/dune sequences. Reentrants consist of widely spaced, near continuous coastal ridges with inter-ridge sabkhas. Active and abandoned marine sabkha-tidal channel complexes are widely spaced and discontinuous, occurring landward of the coastal ridges and south (down wind) of each headland. The Kuwait coastal sabkhas are lean with respect to dolomites and evaporites in comparison to the Trucial Coast sabkhas. This reflects differences in wind driven flood recharge frequency that are controlled by coastline orientation (Fig. 1).

A comparison of these two newly described carbonate ramps with the classic ramp model produces a set of criteria that are useful in distinguishing between these two styles of inner ramp deposition summarized in Table I. These models could also be applied to some rimmed carbonate shelves. A reef or grainstone margin can significantly dissipate energy on a shelf. However, wide shelves have sufficient fetch to build wave and current energy to get high energy, low relief coastlines which are essentially ramps.

Ooid rich eolian dunes are a ubiquitous feature along the Kuwait Coast. Contemporaneous foreshore/beach and shoreface sequences usually contain a component of skeletal and siliciclastic sand in comparison to ooid dunes. In this polymineralic-polyparticle mixed system why are ooids concentrated in the coastal dunes? The answer may be as simple as aeolian sorting controlled by bulk density contrast due to ooid microstructure. Mineral density of quartz (2.65 g/cc) is less than carbonate (2.93 g/cc-aragonite, 2.71 g/cc-calcite) but bulk density must factor in porosity. If this system is driven by mineral density alone, then, for a given grain size fraction, quartz grains would be more mobile than carbonate grains of equivalent grain size and shape. This is counter to observations made in the Kuwait coastal system.

Accretion of tangential ooid cortices provides a significant amount of open pore space between aragonite needles so that the weight per unit volume (volume = grain size) is significantly reduced. Bulk density, and therefore wind mobility, will be controlled by the volume ratio of nucleus to cortices in a given grain size (Fig. 2). So, for an equal grain size fraction, grains with ooid cortices will have a lower bulk density than either grains composed of quartz or solid skeletal carbonate. This provides ooids with a significantly greater wind mobility resulting in preferential wind-sorting from the beach into coastal dunes. The microporosity normal in peloids and forams give these kinds of grains an inherent mobility. A continuous process of dune feeding by wind driven density sorting, coupled with coastal accretion, is autocyclic and precludes any need to lower sea level in order to expose ooid factories and develop significant accumulations of oolite aeolianite.

Fundamental differences between carbonates and siliciclastics include (1) source and sediment budget; extrinsic for siliciclastics and intrinsic for carbonates, and (2) early cementation potential; significant for carbonates but much less so for siliciclastics. In terms of certain aspects of depositional styles, I suggest that these differences may have little to do with what we may find in the geologic record in many cases. The well-understood coastal processes in siliciclastic systems can, in many cases, be directly applied to carbonate coastal systems along “ramps” and the coastlines of many rimmed shelves with significant width.

The work on Holocene deposits along the Kuwait-Saudi Arabian coast of the northern Arabian (Persian) Gulf and the Northern Yucatan Ramp compared to Abu Dhabi, UAE (the “classic” carbonate ramp model) led to a better understanding of the characteristics of tide versus wave dominated carbonate inner-ramp systems.

One of the best studied siliciclastic coasts in the world is the Georgia Bight which extends from Cape Hatteras, North Carolina, to Cape Canaveral in Florida. This coastal system includes wave dominated, tidally influenced, and tidally dominated sections of coastline and was summarized recently in an excellent review paper by Hayes (1994). The accompanying figure (Fig. 3) shows a comparison between the Holocene data from the Georgia Bight (modified from Hayes, 1994) and the three recently studied Holocene carbonate ramps. The absolute number of inlets (tidal channels) has been normalized because the lengths of sampled coastline range from 80 km to 270 km. Both the spacing and frequency of inlets, which define barrier island lengths, agree remarkably well between the two data sets.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah