ABSTRACT

The value of integrating structural influences with sedimentology is identifying the significant changes that occur along strike and downdip. Understanding where and how these changes occur can have a profound influence on exploration risk assessment along inner ramp and inner shelf trends, particularly in the search for stratigraphic traps. Dip-oriented structural elements along depositional strike of inner ramps of the discussed areas include major fault zones, folds, smaller faults, and impact craters. These tectonic influences control often discontinuous facies distribution patterns of barrier islands, lagoons, sabkhas, and inner ramp carbonate factories. In contrast, strike-oriented structural influences often control the location of facies trends which promotes trend continuity. Wave-dominated Holocene and modern inner ramp deposits from southern Kuwait-northern Saudi Arabia and Northern Yucatan, Mexico, provide complementary models to the classic channelized ramp system of Abu Dhabi. Structural influences relative to coastal orientations control regional and local depositional styles, trend continuity/discontinuity, the location of carbonate factories and landward lagoon and tidal flat depositional characteristics. As reservoir-updip seal pairs for stratigraphic trap exploration, these analogues also demonstrate that changes along depositional strike should be expected, and in many cases could be predictable. These relationships emphasize the need to carefully integrate structure with facies and paleo-environmental reconstructions when mapping trends and assessing exploration risk in the search for stratigraphic and combination traps.

INTRODUCTION

Inner ramp settings and models are important for interpreting and evaluating the evolution of ancient sequences, mapping trends, and defining play-types in hydrocarbon exploration. Detailed Holocene models have also continued to gain importance in the petroleum industry because of the geostatistical quantification of dimensions and attributes increasingly required for reservoir characterization and simulation programs (Lomando, 1998a). The southern Kuwait-northern Saudi Arabia coast of the Arabian Gulf and the Northern Yucatan, Mexico, offer the opportunities to examine the Holocene record and modern characteristics of wave-dominated inner ramp systems. Within these ramp systems, physiographic conditions, controlled and/or influenced by dip-oriented tectonic elements, have exerted a significant effect on the ultimate style of deposition along strike.

Read (1985) subdivided ramps into two geomorphic types: homoclinal and distally steepened which may be further subdivided by major facies types based on depositional energy. Burchetteetal. (1990) investigated the Mississippian stacked ramp sequence of southwest Britain. Grouping characteristics led them to develop distinguishing characteristics among storm-dominated detached shoals, wave-dominated shorelines and shoreface retreat sand sheets. Burchette and Wright (1992) suggested a classification scheme based on a process approach used in siliciclastic systems but went one step further to account for characteristics unique to carbonates such as boundstone buildups.

Burchette and Wright (1992) also clearly showed the range of large-scale basin settings on which ramps can build. These include extensional, foreland, and cratonic interior basins and trailing margins. There are a number of oil and gas field examples where structure has had documented influence on facies distribution patterns in ramp and inner shelf settings. Synsedimentary structural movements influenced a number of Carboniferous fields along the Nesson Anticline (Lindsey etal., 1988). Regionally, the Jurassic Jay Trend reservoir facies are influenced by underlying basement highs, faulting, and salt movement (Bliefnick and Mariotti, 1988; McGraw, 1988). The lower Cretaceous reservoirs in the East Texas Basin have been influenced by salt and salt-related turtle-structure anticlines (Lomando etal., 1984; Achauer, 1985). At Wafra field in the Kuwait-Saudi Arabia Partitioned Neutral Zone, both the Cretaceous and Eocene reservoirs facies distribution patterns were influenced by underlying structure (Longacre and Ginger, 1988; Danielli, 1988). What is still needed is a process-response model for ramps that link physiographic influence of structure with facies distribution patterns on exploration and development scales.

The objectives of this paper are to: (1)describe the Holocene and modern deposits along two inner ramp systems on an exploration scale; (2)link the Holocene and r ecent sedimentology with structurally controlled coastal morphology to develop an integrative understanding of the controls on the facies distribution patterns; (3)compare sparsely channelized systems with the classic highly channelized ramp system in the Abu Dhabi portion of the United Arab Emirates (UAE); and (4) compare the influences of strike-oriented versus dip-oriented structural elements on inner ramps systems.

THE KUWAIT-SAUDI ARABIAN RAMP

Setting

Regional depositional characteristics of the southern portion of the Arabian (Persian) Gulf have been the subject of a significant body of work (Kassler, 1973; Purser, 1973a & b; Purser and Seibold, 1973; and many others) and will not be reviewed here. Early workers in the coastal regions of Kuwait focused on small areas or on specific aspects of local depositional systems. Picha (1978) and Al-Sarawi etal. (1993) examined the ar ea south of RasAl-Zour at Al-Khiran (Figure1) wher e an ooid-rich beach ridge complex flanks a tidal channel system which separates the open Arabian Gulf from inland sabkhas. Aspects of the sedimentology and diagenesis in these sabkhas have been investigated by Gunatilaka (1986), Gunatilaka and Shearman (1988), and Robinson and Gunatilaka (1991). Recent sediments were studied by Al-Ghadban (1990) in the cuspate bay between Ras Al-Qulayah and Ras Al-Zour but focused on grain-size and bulk mineralogy distributions and did not differentiate particle types. Intertidal and subtidal cementation investigated by Khalaf (1988), was focused on the coastal and offshore areas of northern Kuwait.

Coastal Kuwait lies on the northwestern shore of the Arabian Gulf and is part of the variable shoreline along the shallow Mesopotamian Shelf (Figure1). The northern Gulf is an arid, micr o to mesotidal region with tides ranging from 1 to 4 meters (m) (Gunatilaka, 1986). Anticlockwise tidal currents in the northern Gulf and the prevailing northwesterly Shamal winds produce a shore-parallel prevailing energy orientation which contributes desert sand and silt to the Gulf, limits storm recharge of the coastal sabkhas, and produces a strong longshore wind-wave energy system.

From the coast to inland areas three broad depositional environments can be distinguished. Along the coast is a near continuous belt of Holocene to modern mixed carbonate-siliciclastic deposits with a wide range of depositional styles. To landward is a discontinuous trend of active and abandoned marine sabkhas. This passes into a broad desert system of continental sabkhas, desert flats, and linguoid dune fields (Figure1). The inland desert system is the flattest ar ea of Kuwait and is part of a zone of deflation, providing wind-blown sand which ultimately accumulates in the northern areas of the Rub Al Khali in Saudi Arabia (Al-Sulaimi and El-Rabaa, 1994; Frybergeretal., 1983, 1984).

From Kuwait City southward, the coast has a very steep shoreface with narrow siliciclastic-dominated beaches and actively eroding low cliff faces composed of Quaternary mixed lithologies (Khalaf, 1988). South of Mina Al-Ahmadi and continuing southward into Saudi Arabia steep shorefaces rapidly grade into a gentle ramp (Figure1). Paralleling the coast, the onshor e Ahmadi Ridge is the youngest fold structure in Kuwait (Carmen, 1996). The transition from a steep monoclinal shoreface to a gentle ramp slope coincides with the southern termination of the Ahmadi Ridge. Ramp morphology is punctuated, north to south, by three headlands RasAl-Qulayah, RasAl-Zour, and RasBar dHalq. These headland salients rest on possible major northeast-oriented structural features (Carmen, 1996) and form shallow-water noses extending into the Arabian Gulf (Figure1). Between these salients lie a series of br oad cuspate reentrants with gentle seaward slopes, often less than 1 meter per kilometer. Small-scale faults have been inferred (Picha, 1978) to explain the distribution of probable Late Pleistocene outcrops in the Al-Khiran area and small-elevated promontories, which occur along the coast to the south.

The broad, shallow-water ramp system in the northern Gulf and the overall relatively low siliciclastic input promotes high carbonate productivity, including patch reefs composed of variable amounts of mollusks, worm tubes, coral, and coralline algae. Nonskeletal particles, including ooids, hardened pellets, and lithoclasts, comprise a significant portion of the modern sediments. There are several sources providing siliciclastic sand, silt, and clay to the southern Kuwait coastal system. One minimal source is the Shatt Al-Arab (Tigris-Euphrates) delta system to the north (Larsen and Evans, 1978; Gunatilaka, 1986). Aeolian supply of silt and sand into the northern Gulf derived from the deserts of Saudi Arabia, Kuwait, Iraq, and Iran is significant but very episodic (Pilkey and Nobel, 1966).

Headlands

Two of the three prominent headlands, of which RasAl-Qulayah is the largest, together with RasAl-Zour (Figure 1), were examined in detail. All three headlands have important geomorphic similarities. North-facing shoreface and offshore bathymetric slopes are relatively gentle in contrast to much steeper slopes on their south and southeastern flanks. Offshore seismic data is not available in the nearshore areas to enable these structures to be more specifically characterized. The headlands and their geomorphically continuous bathymetric shapes demonstrate the size of these structures ranging from 15 kilometers (km) to 35 km long and 5 km to 15 km wide. This scale makes them unlikely candidates for inherited antecedent topography similar to the protruding paleo-dunes at Al Dabbiya (Kirkham, 1998a). A reasonable interpretation would be that these are either asymmetrical folds with steeper southern limbs or rollover anticlines against southerly bounding faults.

Each headland has major modern shoal complexes several kilometers in length extending from their tips to the northeast along the structural axis (Figure1). Lar ge bars within the shoal complexes are oriented roughly northwest-southeast, parallel to the longshore drift direction. At RasAl-Qulayah, a bar complex extends seaward from the point with an overall arcuate shape. Patch reefs occur along the windward northwestern flank of the bathymetric nose. Sediments from the nearshore to beachfaces are composed of fine- to medium-grained ooids with lesser amounts of quartz sand, intraclasts, and skeletal grains. At RasAl-Zour (Figure 2), the headland shoal and spit complex has the same overall geometry as RasAl-Qulayah, and has begun to build above sea-level to form a small island. The headland spit may be forming as a result of anticlockwise eddy currents.

The coastal geomorphology of the two main headlands reflect the offshore asymmetric bathymetry. At RasAl-Qulayah, the northern coastline has a well-developed coastal ridge which becomes discontinuous eastward towards the point, similar to the well-developed ridge along the north-facing coast at RasAl-Zour. Exposure of the ridge at RasAl-Zour contains all the elements of a ridge-type sequence from basal high energy, coarse-grained and cross-bedded upper shoreface and foreshore units to finer-grained, well-sorted, oolitic aeolianite at the top. This shoreface deposit, which is now 1 to 3 m above the present-day beach, may reflect either a slightly higher, earlier Holocene sea-level or recent uplift along these structurally controlled features.

The southern flanks of the northern two headlands are very similar, consisting of a 2 to 3 km wide, 4 to 6 km long trend of parallel to subparallel, closely spaced, low-relief mixed, olitic-siliciclastic coastal aeolianite dune ridges (Figure 3). These are the tops of a rapidly accreting beach ridge system which forms strand plains. The headland at RasBar dHalq is half the size of RasAl-Zour and RasAl-Qulayah but shows the same overall pattern.

Inland of the western landward limits of all three headlands is a rapid transition to elongate marine sabkhas which parallel the coast. At RasAl-Zour, the southern edge is marked by the Al-Khiran tidal channel-sabkha complex (Figure 2). At RasAl-Qulayah, the southeastern limit is a broad sabkha which separates the headland complex from the beginning of the coastal ridge trend. The eastward extension of the RasAl-Qulayah sabkha is separated fr om the coast only by the youngest coastal ridge. Northern ends of the two older coastal ridges that are part of the re-entrant system turn inland, terminating in a now abandoned channel. RasBar dHalq also has a similar abandoned tidal channel, connected inland to a broad sabkha and truncated at the coast by the youngest coastal ridge.

Observations at RasAl-Qulayah and RasBar dHalq, with their marked similarity in form, scale and facies distribution to the RasAl-Zour - Al-Khiran complex, leads to the conclusion that the abandoned tidal channel-sabkha complexes which were once connected to the Gulf by single or closely spaced multiple tidal inlets are terminated by the construction of the youngest Holocene ridge. Extensive marine sabkha complexes are discontinuous features on the southern flanks of the major coastal headlands. As discussed previously, the bathymetry offshore of the headlands shows a marked asymmetry with gentle slopes on their north to northwestern flanks and steep linearly deepening trends on their southeasterly flanks. In all three cases, the relationships of the headlands to the tidal channel-sabkha complexes, suggest that they control the regional Holocene (and possibly Pleistocene) depositional patterns, including the present-day carbonate shoal factories at the tips of the headlands and intervening coastal re-entrants.

Re-entrants

Al-Khiran

The Al-Khiran tidal delta-channel-sabkha complex consists of a series of poor- to well-cemented oolitic-siliciclastic ridges flanking two tidal channels which are backed by supratidal sabkhas (Figure4). The two tidal channels converge towards the coast where they are separated by less than 200 m at their mouths, and have a common, poorly developed tidal-delta. Sediments of the latter are composed of ooids, peloids, ooid intraclasts, and bivalve fragments from the channel/delta complex and echinoderm fragments derived from the shoreface system. Quartz-nucleated ooliths are common and similar to those forming in the Jebel Dhanna in the UAE (Kirkham, 1998b). Fine to coarse siliciclastic sand makes up 2% to 20% of the total sediment. Tidal channel sediments are rich in ooids in the lower reaches and become more peloidal and skeletal (small bivalves and gastropods) in their upper reaches as they merge into the sabkhas. Rapid aragonite cementation on the channel flanks has produced well-cemented levees in places.

Sabkhas that form broad flats behind the ridge complex have been studied in detail (Picha, 1978; Gunatilaka 1986, 1991; Gunatilaka and Shearman, 1988; Robinson and Gunatilaka, 1991). Gunatilaka (1991) investigated the distribution of dolomite and gypsum in the Al-Khiran sabkha complex and concluded that the overall scarcity of these minerals results from minimal flood-recharge to rejuvenate the sabkha brines, due to the effectiveness of the ridge system as a barrier and infrequent coincidence of spring tides and onshore winds.

Holocene beach ridge accretion is a complex record of deposition of barrier-spits, beach-ridges, and tidal-delta systems. The resulting facies mosaic is best observed west of KhorIskander wher e clear variations in patterns of beach ridge and spit coalescence and bifurcation during progradation are found (Figure 4).

Active ooid production in the tidal channels contributes to an ebb tidal delta that feeds the barrier island and spits at Khor Iskander. Robinson and Gunatilaka (1991) showed that the seawardmost oolitic deposits can be up to 7m thick and contain several har dgrounds. Picha (1978) assigned a Pleistocene age to most of the oolitic ridge complex at Al-Khiran, but did so without evidence from radiometric dating. He attributed faults to be the controlling factor for the position of the major tidal channels, the location of the oldest oolitic ridge and the termination of the coastal ridges at the coastline. Gunatilaka (1986) stated that the formation of the ridges extended from 8,450 to 7,650±70 years before the present (yBP) to 2,190±70 yBP. Al-Sarawi et al. (1993) attributed the complex and related ridge accretion to Holocene eustatic sea-level fluctuations which have occurred over the past 8,000 years. Two small (1 to 2 km long) modern barrier spits occur south of the mouths of the Al-Khiran tidal delta (Figure4). These ar e the only barrier spits observed along the coast. The northern and larger of the two is Khor Iskander. The smaller southern one is unnamed. Both have narrow beaches, tidal inlets and a 2 to 3 m high coastal dune-ridge. Dune, backshore, foreshore, and nearshore sediments are composed of unconsolidated oolitic sand with less than 10% skeletal debris and quartz grains.

Offshore, the barrier spits are fronted by well-developed intertidal to subtidal sandbars of the same general composition. This system is fed by ooids formed in the Al-Khiran tidal-channels and seaward bars. Southward longshore drift sweeps sediment from the tidal delta, limiting delta size, and promoting active southward accretion of the barrier-spits. Lagoons (khors) behind the barrier spits are well swept by tidal action, making them mud-lean. They are filling primarily by aeolian input from both sides in addition to sediment transported through the inlet itself. Reexamination and mapping of Al-Khiran during this study, with the aid of high-resolution satellite imagery, differs from previous mapping (Picha, 1978) only in minor details.

Coastal Ridges and Shore-Attached Shoals

Three near continuous parallel ridges exist between RasAl-Qulayah and RasAl-Zour (Figure 5). Their spacings range from 300 m in the north, gradually converging to less than 100 m southward, where they change to a series of offset ridges subparallel to the present-day shoreline. Heights of the innermost ridges decrease southward from a maximum of 5 m to 2 m but are somewhat variable due to erosion. At their northern limits, these coastal ridges abruptly turn landward in an area interpreted as an abandoned channel-marine sabkha complex. The most landward ridge rises in elevation to almost 6 m until it appears to pass offshore. This coastward ridge reappears some 200 m farther south where it is continuous for several kilometers and is being actively eroded along the north coast of RasAl-Zour.

Coastal ridges, which are almost continuous, are the most prominent features along re-entrants in the study area and may be described by a type vertical sequence consisting of three main components. The best exposures occur where the coastline truncates the ridges or where tidal channel erosion or excavations have exposed the section. Lowest in the sequence is an upper shoreface unit of moderate to poorly sorted and cross-bedded oolitic, skeletal and siliciclastic grainstone, sometimes with abundant Ophiomorpha-type burrows. The middle unit is a beach-foreshore sequence of low-angle planar laminae with alternating grain sizes. Finer laminae contain fine to medium quartz sand and ooids, while coarser laminae are rich in medium to very coarse sand with accessory skeletal fragments and ooids. The sequence is always capped by cross-bedded coastal dune deposits composed of well-sorted, fine ooids, and minor amounts of fine quartz sand.

South of Al-Khiran, discontinuous Holocene coastal ridges are parallel to the coast for short distances, then rise in elevation and curve seaward where they are erosionally truncated by the modern coastline (Figure6). One of these seawar d-deflected coastal ridges has been mapped as fault-controlled (Picha, 1978). Field observations during this study indicate that the upthrown north side of this fault creates a seacliff over 12 m high, with a different fault orientation than previously mapped. This fault is one of a series of closely spaced southerly downthrown faults with offshore extensions that control the locations of shore-attached shoal complexes.

There are two occurrences of shore-attached shoals coincident with the fault-terminated ends of Holocene coastal ridges between RasAl-Zour and RasBar dHalq. The termination of each discontinuous ridge-segment is complemented seaward by a shore-attached bar-shoal complex. Shoals are oriented roughly perpendicular to the shoreline, curving southward distally in response to longshore waves and currents (Figure 6).

It is important to note that similar fault-controlled complexes are absent between RasAl-Qulayah and RasAl-Zour. This provides an opportunity to compare structurally active with structurally passive portions of the coast. The structurally passive re-entrant between RasAl-Qulayah and RasAl-Zour has significant continuity of Holocene coastal ridges and runnels, both individually and collectively, but have lower overall relief today compared to the structurally active portion between RasAl-Zour and RasBar dHalq. The faults south of Al-Khiran cause higher relief Holocene coastal ridges, deflection and truncation of the ridges, and control the location and orientation of modern shoal complexes oriented perpendicular to the coastline. Faulting, therefore, produces significant heterogeneity and complexity to the coastal and nearshore sedimentary systems. South of the Kuwait-Saudi border, coastal ridges become scarce, discontinuous low relief features. Generally, only a single additional ridge occurs landward of the shoreline ridge.

NORTHERN YUCATAN RAMP

Setting

The Northern Yucatan Ramp (Figure 7) lies along the northern east-west trending coast of the Yucatan Peninsula, Mexico. Ahr (1973), following the work of Logan et al. (1969), classified the Campeche-Yucatan shelf as a distally steepened ramp. This broad, shallow shelf is underlain by an extensive carbonate platform which began to develop in the Cretaceous. The break in slope that creates the distally steepened portion of the ramp occurs in water depths greater than 150 m and 250 km offshore. Isphording (1975) subdivided the peninsula into three geomorphic regions. The western region is a relatively low relief area of juvenile karst. The central region has greater relief and more extensive karst development. The easternmost geomorphic district is characterized by a region of major block faulting that extends from the northeastern Yucatan southward along the entire Yucatan Peninsula and into Belize and Guatemala.

Previous work on the depositional systems of Northern Yucatan Ramp focused on two small areas. Logan et al. (1969) studied the middle and outer ramp portion of the western region and focused on sediment distribution and reef development in the mid and outer portions of this shelf. Later, Ward and Brady (1975) worked on the inner portion of the eastern end of the shelf, north of Cancun, where the coastline strikes north-south (Figure7). This ar ea is transitional from the barrier fringing-reef margin trend along the eastern Yucatan to the gentler north Yucatan Shelf. Sediment distribution in this transitional area are influenced by the strong, northward flowing, Yucatan current and the near perpendicular prevailing east-northeast wind in addition to significant pre-Holocene topography. Brady (1972) studied the lagoonal sediments of Yucatan and the eastern portion of Lagartos Lagoon.

Along the Northern Yucatan Ramp, the prevailing wind direction and major storm tracts are from the east and northeast. The coastal trends are storm-fed by inner and mid-ramp carbonate factories. The Yucatan current flows up the east side of the peninsula and curves around to parallel the northern coast (Logan etal., 1969). The combination of pr evailing wind which generates wave-formed littoral drift and the Yucatan current produces a strong westward transport of sediment.

The coastal or inner ramp portions of the Northern Yucatan Ramp may be subdivided, from east to west, into three trends, each with distinctive sets of characteristics: Holbox, RioLagartos, and Chicxulub. The major differences are width of barrier islands and the size and facies of lagoons (Figure 8).

Holbox Trend

Isla Holbox (Figure9a) is an east-west barrier island/spit complex composed of distinct ar cuate prograding beach-dune ridges, which hooks southwesterly as an active barrier spit on its western end. The barrier encloses Yalahan Lagoon, which opens to the Caribbean through Boca de Conil and through smaller inlets. Yalahan is shallow (<3 m) and mud-dominated (wackestone), 30 km long and 8 km wide. Northern and eastern shores of the lagoon are areas of active growth of mangrove marsh and mud-flat progradation. The main entrance into the lagoon is at its western end which is the site of westerly migrating submarine skeletal sandbars (Figure 9a). These feed the growing barrier-spit at the western end of Isla Holbox. The region is microtidal (tides less than 1 to 2 m). With time, the continued westward spit accretion of Isla Holbox may be more significant than the ability of tidal activity to keep it open and may lead to the complete closure of this lagoon.

The southern shoreline of Yalahan Lagoon is delineated by Pleistocene limestone outcrops and mangrove marsh which is the northern edge of the surface expression of the Holbox Fault-Fracture Zone. This fault-fracture system had a profound influence on karst development during the Pleistocene, but is today expressed by two morphologic styles along the southern shore. The eastern shoreline contains a series of finger-like coastal inlets (caletas) formed by karst solution enlargement along fracture zones. The western shoreline contains two semicircular embayments which may be Holocene flooded cenotes (collapsed karst/sinkhole systems).

This is the only segment of the Northern Yucatan Ramp which displays features that are controlled by the Holbox Fracture System. The Holbox Fault-Fracture System and its offshore extension, the Chemex-Catoche Fault Zone, distinguish the Holbox Trend from those along the rest of the northern Yucatan inner ramp.

Rio Lagartos and Chicxulub Trends

The Rio Lagartos Trend is over 80 km long and consists of narrow beaches which connect two areas of more extensive deposition which show several accretionary beach/dune ridges composed of molluscan sand (Figure9b). The narr owest portions of the barrier island have been breached by hurricanes producing spillover lobes extending southward into the restricted lagoon. These features indicate periodic storm-breaching with the subsequent inlets being rapidly healed by longshore drift. This trend extends westward to Punta Holchit where bar-spit accretion is forming a new, very narrow, lagoonal segment.

Laguna de Rio Lagartos is a long, narrow segmented lagoon (Figure 9b). Brady (1972) investigated the sedimentology of the area, focusing on the eastern lagoon. The muddy southern shore is sabkha-like with intertidal microbial mats and small salinas containing halite and gypsum. The narrow, peritidal north lagoon shore is very sandy. Water in the lagoon is constantly reddish-brown, caused by the high concentration of cyanobacteria. The lagoon floor is mud-dominated with localized evaporite crusts. Restricted conditions are prevalent in all lagoon segments with lagoon muds composed of high-Mg calcite and dolomite (Isphording, 1975). Periodic storm breaching and percolation through the barrier sands are the main lagoon recharge mechanisms. Evaporation rates are high enough and conditions restricted enough to support commercial saltworks. The narrow barrier island complex permitted major storm breaching and deposition of multiple, large spillover lobes, often as trends several kilometers long. Interlobe areas and lobe fringes in the intertidal zone have patchy microbial mat development and common mudcracks.

There is an abrupt change to the Chicxulub Trend of the Northern Yucatan Ramp (Figure 9c). This consists of very narrow barrier spit/lagoon complexes and single shore-attached beach ridges backed by mangrove marsh, which may be a terminal lagoonal fill. As with the entire North Yucatan Ramp, molluscan skeletal grains form the shoreface, beach, and dune complexes.

DISCUSSION

All Holocene and modern models have their blemishes when examined in detail (Ahr, 1998; Walkden and Williams, 1998; Wright and Burchette, 1998). Most stem from antecedent topography inherited from earlier Pleistocene highstands that also may or may not have recognized structural components. In the Kuwait wave-dominated model, structure plays a role in the modification of a pure inner ramp system. Along the channel-rich coast of parts of AbuDhabi, the near surface and outcr op distribution of Quaternary carbonates and/or siliciclastics influence the location of islands which, in turn, influences the frequency and distribution of tidal channels and sabkhas (Purser and Evans, 1973; Kirkham, 1998a). The influence on depositional style of tides and waves often change along strike of a trend. For example, the Abu Dhabi region appears to be tide-dominated, yet westward, along the Great Pearl Bank Barrier, it is wave-dominated. The changes from wave- to tide-dominated have also changed in time (Kirkham, 1998a). However, within the context of these limitations, overall sedimentologic and geomorphic conditions still allow some comparisons of the larger scale aspects of these depositional systems (Figure 10).

Comparison of Wave-Dominated and Tidal-Dominated Inner Ramp Systems

The term tide-dominated is used in the sense of Nummedal (1983) where the combination of tidal range and lagoon area/volume (tidal prism) are the most influential factors in tidal inlet frequency in contrast to just the magnitude of tidal range. Large capacity lagoons and bays receive large volumes of water during rising tides that promote strong ebb flow when tides change. Onshore winds, coincident with rising tides, enhance the effect of a strong tidal prism by increasing the water volume pushed into a lagoon but are not necessary for tidal exchange. In this fashion micro and mesotidal regimes can produce barrier systems with a high frequency of tidal inlets and ebb tidal deltas if a sufficient area behind a barrier is available to establish a prism. It must be kept in mind that some regions, like Abu Dhabi, may have been wave-dominated during earlier, higher sea-level and changed to tide-dominated upon lowering of sea-level, which enhanced the influence of antecedent topography (Kirkham, 1998a).

Lomando (1998b) compared the characteristics of the three examples discussed in this paper. These characteristics fall into three main groups: (1)formation and depositional styles of oolites; (2)comparative characteristics of associated sabkhas; and (3)principal contr ol on overall inner ramp depositional style.

The most influential factor appears to be shoreline orientation relative to prevailing wind direction (Figure10). The parallel orientation in Kuwait and the Northern Yucatan, in comparison to the perpendicular orientation in this part of the UAE, controls the long-term development of continuously prograding coastal ridges with discontinuous marine sabkhas. Early diagenetic characteristics are also strongly influenced by these contrasts. The marine sabkhas in the wave-dominated systems of Kuwait and Yucatan are sparse in early dolomites and evaporites, due to the infrequency of tide wind-driven recharge.

Fundamental differences between carbonates and siliciclastics include: (1)sour ce and sediment budget, extrinsic for siliciclastics and intrinsic for carbonates; and (2)early cementation potential, significant for carbonates but less so for siliciclastics. Hayes (1994) summarized the attributes of wave- and tide-dominated siliciclastic systems along the Atlantic seaboard of the United States. Part of these analyses includes quantifying tidal inlet spacing and frequency. On the basis of both inlet spacing and frequency, these carbonate ramp systems behave in a similar manner to their siliciclastic counterparts (Lomando, 1998b).

Structural Influences

Large-scale structural influences are readily apparent when observing the common features among headlands in the Kuwait inner ramp system. Shoal complexes, extending seaward from the tips of headlands, are focused along possible fold axes (Figure1). These shoals provide an abundance of carbonate sands for rapid, shoreline beach ridge (strand plain) progradation mainly on the down current (southern) flanks of the headlands. Fold asymmetry may also be responsible for the locations of marine sabkhas associated with headland systems. As an analogue for petroleum exploration, this model would predict a marked asymmetry in reservoir distribution and trap geometry when structurally controlled, antecedent topography is dip-oriented (perpendicular) to a coastal system. If a system retained this character through several sea-level cycles, then this could appear on high-resolution seismic as low relief offlaps on a preferred fold flank. The strand plain depositional style would produce a reservoir unit with fairly uniform porosity, but with a permeability anisotropy parallel to the beach dune ridges. Updip pinch-outs across fold noses could be controlled by the distribution of marine and continental sabkhas.

Smaller-scale structural influences are relegated to the impact of small-scale faulting between RasAl-Zour and RasBar dHalq. High r elief Holocene coastal ridges terminate on the upthrown side of faults, sometimes producing (for this flat region) significant scarps. Seaward extensions of these faults are coincident with the development of shoals oriented perpendicular to the present coastline (Figure 6). These features, along a structurally active portion of the coast, are in marked contrast to the lower relief and parallel facies distribution patterns along the structurally passive region between RasAl-Qulayah and RasAl-Zour. Structurally passive trends would have a geometry parallel to the ridge trend due to the development of wide, gypsum cemented, inter-ridge sabkhas. As a reservoir scale development model, this would predict that strong facies anisotropies and complex early diagenesis should be expected in structurally active systems. Upthrown blocks would be subjected to significant early meteoric diagenesis, such as vadose leaching or aeolian deflation (Kirkham, 1998b). Downthrown blocks would be more prone to phreatic diagenesis including meteoric or marine cementation. In a reservoir setting, the use of exposure surfaces as cycle tops, sequence, or parasequence boundaries would be complicated and misleading. Knowledge of the impact of early and synsedimentary structural influences could minimize misinterpretations.

Progradational style in the Holocene succession varies with location along the Kuwait coast. Progradation along headland strand plains produces a series of closely spaced beach-dune ridges (Figure3). Along a siliciclastic coast this style of accretion is evidence of a high sand budget in the depositional system. In the case of the Kuwait coast, carbonate productivity controls the larger portion of the sediment budget and strand plain progradation is linked to the productivity of the headland shoals and wave refraction and protective geometry of the headland itself.

The geomorphology and structural characteristics across the Northern Yucatan Peninsula are subdivided, east to west, into three structural regions (Figure7). The easternmost r egion is characterized by the Holbox Fracture Zone. The Holbox Fracture Zone is part of the north-northeast trending Eastern Block Fault District that onshore includes the Rio Hondo Fault Zone (Lesser and Weide, 1988) and the offshore Chemex-Catoche Fault Zone (Pope etal., 1993). The central region is a very gentle karst plain, spotted with thousands of cenotes (sinkholes). The western region is dominated by a large depression, rimmed by a ring of cenotes and elevation changes caused by the Chicxulub impact crater (Hildebrand etal., 1991; Pope etal., 1993, 1996). Ring faults and an ejecta blanket cr eated the topographic expression of the crater rim, which continued to be expressed by subsequent Tertiary carbonate platform deposition (Pope etal., 1996). Perry etal. (1995) coined the term Chicxulub Sedimentary Basin and postulated that reactivation of ring faults is responsible for the observed Ring of Cenotes and causes higher subsidence rates, which modified shorelines since the impact.

The Holbox Fracture Zone and Chemex-Catoche Fault Zone systems are coincident with the Holbox barrier island/spit system and the wide Yalahan Lagoon. This karst-enhanced, structural prominence which passes westward into the stable karst plain created a pronounced wide headland region. The strike-trend of the Holbox barrier island shifts coastward almost 10 km where the Rio Lagartos barrier system begins. The transition or shift in the trend occurs at the western boundary of the Holbox Fracture Zone. Rio Lagartos barrier island and restricted lagoons are a narrow but continuous trend in comparison to Holbox, and may reflect a more stable or structurally passive region associated with the stable onshore karst plain. Westward, the barrier island trend abruptly ends and changes to a single/shore-attached beach ridge coastal system backed by dense mangrove marsh. This transition occurs where the eastern edge of the ring of cenotes intersects the coastline. The ring of cenotes (Figure7) marks the rim of the Chicxulub impact crater (Pope etal., 1996). This narr ow shoreline may reflect a relatively greater subsidence rate within the Chicxulub Basin (Perry etal., 1995) and may produce a stacking or retrogradational character to this inner ramp segment.

Updip Seals for Stratigraphic Traps

Similar to potential reservoir facies discussed previously, the style, type, and location of potential updip seals for stratigraphic traps also changes along depositional strike as a result of structural influence. In the case of the Kuwait-Saudi Arabian ramp system, the coastal facies belt contains marine and continental sabkhas and dune fields. The best updip seals, analogous to ancient plays, are the discontinuous mud- and evaporite-rich marine sabkhas. These occur landward of the headland shoal and strandplain complexes. In the intervening re-entrants, dune sands are interbedded with continental sabkhas that may provide less than adequate sealing capacity. These characteristics are consistent along the entire trend and would imply that paleo-headland location would have a lower updip seal risk that intervening re-entrant portions of an exploration trend.

The Northern Yucatan trend is another example of changes along strike of facies updip of potential reservoirs. As an analogue, the potential stratigraphic seal in the Holbox barrier island complex is a low energy lagoonal mudstone and wackestone facies. On strike westward in the Rio Lagartos Trend, the lagoonal sediments change to evaporitic. Continuing westward, the updip faces complex again changes to coastal mangrove swamps. The Northern Yucatan Ramp is another example of updip sealing facies changing along strike and reflecting analogous situations of changing seal risk along strike.

Strike-Oriented versus Dip-Oriented Structural Influences

On regional to subregional exploration scales, strike-oriented continuous facies trends are often controlled by parallel structural influences (Figure1 1). Faulting has been shown to influence the location and continuity of shoal trends and lagoon margins in a strike-parallel trend (Camoin, 1983). Flexural hinge lines influence strike-parallel trends in foreland basins (Kassler, 1973; Read, 1989), but tilted fault blocks and salt domes can influence the location of isolated mounds and reefs in mid to outer ramp settings (Ahr, 1989; Purser, 1973b). When coincident with inner ramp systems, these structural influences tend to promote continuity of facies trends along depositional strike.

In contrast, dip-oriented structural features tend to impart rapid changes along depositional strike (Figure12). Facies asymmetry about folds should be expected, based on the r epetitive nature of facies distribution patterns along the Kuwait-Saudi Arabian headlands. Dip-oriented faulting can influence facies distribution patterns and the location of carbonate factories on several scales. Smaller-scale faulting can control the location of tidal channels, deltas, and their associated lagoon-tidal flat systems. On a larger-scale, the transition from one dip-oriented structural regime to another, along depositional strike, can cause significant changes in reservoir facies patterns and associated potential sealing facies. These changes include width and continuity of barrier island and shoreline trends and changes in associated lagoonal sediments. These regional changes are probably caused by changes in subsidence rates from one structural region to the next.

Another implication of understanding the role of structure on inner-ramp and inner-shelf systems is the influence on stacking patterns, stratal geometries, and sequence stratigraphic interpretations. Detailed outcrop or subsurface core-based studies are often done where outcrop exposures are best or core, log, and seismic data are densest. Care must be taken when broadly applying these results, unless consistency on strike has been demonstrated.

CONCLUSIONS

Distinguishing structural style and its influence should facilitate the application of these models to ancient sequences. The style and character of reservoir-updip seal pairs change along strike of any trend. This should be considered the rule, rather than the exception. In areas of hydrocarbon exploration, such as the Jurassic and Cretaceous carbonates of the Middle East and U.S. Gulf Coast, application of the most appropriate model will aid in reservoir modeling, and in combination with structure, will increase the likelihood (reduce risk) of accurate reservoir prediction and stratigraphic trap identification and evaluation. One key factor in applying the models to ancient sequences is the application of good paleo-climatic and paleo-geographic models.

ACKNOWLEDGEMENTS

I thank Chevron Overseas Petroleum Inc. and Hani Iskander of Chevron Overseas Petroleum Technology Co., Kuwait, for supporting this work. Salah Al-Azmi, Abdul Aziz, and other friends at Kuwait Oil Company and Tim Fairs and Henry Legarre of Chevron are thanked for their interest, help, and camaraderie in the field. Appreciation is also extended to LarryAnderson of T exaco - Saudi Arabia and the Kuwait Navy for permitting access to their bases. HattieDavis is thanked for the gr eat job of processing the satellite images, and RossPeebles (ADMA) for his guidance in the AbuDhabi sabkhas. Readers and reviewers, C.Kerans, C.Kendall, A. Kirkham, G. Evans, and J.F.Read helped improve early versions of the manuscript. Lastly, I am very grateful to a large number of Kuwaiti citizens living along the coast for their gracious hospitality and guidance in the field.

ABOUT THE AUTHOR

Anthony (Tony) J. Lomando completed his graduate studies in 1979 at the City University of NewYork where he worked on the Jurassic ramp carbonates of the U.S. Gulf Coast. The next eight years were spent in exploration and development in the Permian Basin and the Jurassic and Cretaceous systems of the Gulf Coast. For the past 12 years, Tony has been the Staff Carbonate Specialist for Chevron Overseas Petroleum supporting exploration and development and description and characterization of carbonate plays and reservoirs worldwide. Tony has conducted numerous carbonate schools and seminars for Chevron, its partners and associates in Russia, Kazakhstan, Mexico, Congo, Angola, China, Kuwait, the U.K., and U.S. He has authored over 50 papers and abstracts on many aspects of carbonate exploration and reservoir management.