abu dhabi holocene-pleistocene carbonates and evaporites field trip

Course Description

Scope:

Field-based study of Pleistocene and Holocene carbonate and evaporite environments of central and western Abu Dhabi, UAE.

Present-day geological processes of the wind-dominated environment will be demonstrated and used by the participants to unravel the geological history of the Emirates coastline over the last 200,000 years. Participants will gather clues to solve ongoing problems designed to fully appreciate the various geological settings and sequence stratigraphic complexity.

Main Objectives:

1) To demonstrate sedimentary and diagenetic responses to relative sea level variations and climatic changes in a proximal carbonate ramp setting.

2) To foster skills in geological observation and develop carbonate depositional models based upon sequence stratigraphic observations in the field.

3) To demonstrate reservoir heterogeneity, facies correlations problems, depositional trends, aspect ratios and scale.

4) To experience modern submarine environments at first hand.

Preamble

Abu Dhabi provides one of the world’s classic localities for studying Recent and Holocene carbonates and coastal sabkha evaporites. They relate to the Flandrian transgression which started ca 18,000 Ka; climaxed ca 5Ka; and was followed by a regression due to a 1 – 2 meter sea level fall. Evidence for both the transgressive and regressive stages is captured in the sedimentary/diagenetic setting of the area. However, evidence for earlier transgressive/regressive cycles dating back to previous glacial and interglacial periods of the Quaternary over the last 100,000 – 200,000 years is also available in the Pleistocene rocks of Abu Dhabi. The area therefore provides a laboratory to study geologically ‘modern’ sequence stratigraphy with complete exposure of the sediments and rocks. It also provides an opportunity to illustrate how a range of depositional, diagenetic and erosion processes can combine to create a complex stratigraphic architecture that could be extremely difficult to fully interpret in the subsurface. An appreciation of how these processes interfere in reality should provide good lessons in lateral thinking for the geologist who is faced with limited subsurface exploration or development data and needs to construct 3D models.

Course Schedule

1	Fly to Abu Dhabi Stay at Hilton Hotel
2	Recent sediments around Abu Dhabi Island Half of the day is devoted to snorkelling on the Holocene oolite delta; in a lagoon; and on a reef (depending upon weather and water conditions). Morning: Stop 1 We will embark at the jetty of the Hilton Hotel and travel by boats to the northern beach of Sadiyat Island where each member of the party will familiarise with and practice their snorkelling techniques in the shallow water for safety reasons. Anyone who is uncomfortable should identify themselves. The beach sediments are dominated by ooliths which also comprise the low aeolian dunes along the landward side of the beach. These ooliths develop mainly on the oolith deltas that formed just beyond the tidal channels between the barrier islands. A short time will be spent describing the local offshore settings with particular emphasis on the characteristics of the oolith deltas. Stop 2 Patch reefs formed in the areas between the oolith deltas and we shall visit such a reef on the west side of the oolith delta between Sadiyat and Ras Gharab. The reef diversity is not great due to the relatively high salinities of the Arabian Gulf. Dominant corals are Porites and the ‘staghorn coral’, Acropora cervicornis. Occasional other coals are also occur. Note the large number of dead corals which is thought to be due to recent globally elevated temperatures. Stop 3 Time permitting, we shall briefly visit a Pleistocene outcrop on the northwest corner of Ras Gharab where bioturbated marine sediments overlie aeolian sediments. This will also provide an opportunity for a panoramic view of the oolith delta between Sadiyat and Ras Gharab. Stop 4 The last stop of the morning will be to snorkel on the oolith delta where it is possible to see the channel levees, large scale ripples and small dunes created by the strong tidal currents. The party will be able to appreciate the strength of these currents which are almost impossible to swim against. The party will then return to the Hilton Hotel for lunch. Afternoon: Stop 5 Having sailed westwards into the lagoon between Abu Dhabi and Bahrani Islands, we shall snorkel in the main tidal channel that extends landward from the local oolith delta which is again located at the tidally dominated gap between these two barrier islands. The channel is occupied by a thriving reef of ‘staghorn coral’. Snorkelling lagoonwards, we shall see that the channel bifurcates. Snorkelling up the shallow sea bed gradient to this confluence, we shall see how the coral community changes probably in response to water depth, temperature and salinity. Staghorn corals are subordinate here; brain corals relatively common (although not abundant) and red algal reef development is relatively mature. Abundant bivalves encrust the algal reef and echinoids are relatively common. Note also the abundant shrimp burrows guarded by small fish which share the occupation of the burrows. Stop 6 In another lagoonal channel immediately behind Bahrani, we shall see small sediment mounds formed largely due to sediment baffling by sea grasses. This is a lower energy environment than the last although currents still exist. At this locality we shall probably see the occasional Pinna living in situ within the lagoonal sediment. Wyn Hughes (GeoArabia, 1997) suggested that these are analogues of the elevator rudists that are often so common in the Cretaceous reservoirs of the region. Stop 7 This final stop of the day is on the Abu Dhabi-Bahrani oolith delta. We shall snorkel briefly off the Bahrani beach where again the high degree of sorting and highly rippled, fine grained ooliths can be seen.
3	Holocene sabkhas and the Pleistocene substrate Throughout the day, participants will be encouraged to observe the geology, gather clues and unravel the geological model of parts of the Abu Dhabi Holocene coastal sabkhas and their antecedent topography. Stop8 Much of the morning will be spent along the Mussafah Channel which traverses the mainland coastal sabkha. It is eight kilometres long and orientated almost normal to the coastline. The height of the supratidal sabkha at the landward end of the Channel is about two metres above sea level but it decreases in elevation seawards until it merges with intertidal zone. The Channel intersects the ‘Evans Line’ (Evans et al, 1969) along which the evolution of the prograding sabkha was first unravelled. Starting at the seaward end of the Channel, we shall make intermittent stops to examine the Holocene succession of the sabkha in the walls of the channel and attempt to recognise the idealised ‘sakha cycle’ which consists of: - Top (Banded) enterolithic anhydrite -supratidal Gypsum mush -upp. intertidal / supratidal Microbial mat with gypsum crystals -intertidal Lagoonal muds with large lenticular gypsum crystals -subtidal Carbonate crusts Base Lagoonal mud without gypsum -subtidal The above sedimentologial units are evident in pits dug at the surface of the sabkha as well as within the vertical sequence of the Channel walls. The party will be asked to observe, discuss and explain any additional features and differences from the idealized ‘sabkha cycle’. What sediments occur beneath the microbial mat? Is the sabkha sequence entirely regressive? What evidence would indicate a transgressive sabkha sequence? How did the lithified horizons form (ie carbonate crusts)? What is their stratigraphical significance? What is the origin of the several the red bands evident in the Channel banks? What do the shelly bands represent? Stop 9 Having driven westwards from Mussafah along the coastal highway, we shall eventually turn right towards the tombola of Al Dabb’iya. Along the Al Dabb’iya road we shall stop briefly to observe an old anhydrite pit which was dug by the AAPG to film the video, ‘Shallow Carbonates and Evaporites’. This video will be shown at the Jebel Dhanna Hotel during the evening. Whilst at this locality, the geological setting of the pit will be briefly described. This anhydrite is a different age from the anhydrite inspected in the Mussafah Channel as revealed by satellite images. This anhydrite is absent in the low area south of the pit which is occupied by deflated aeolian sand. Similar subtle depressions typically develop along the landward side of this inner anhydrite with the result that vast areas of ponded rainfall develop for many kilometers along the inner sabkhas. The ponded rainwater often remains for many weeks or months – becoming increasingly saline as evaporation proceeds and precipitating thick crusts of surficial salt (halite) that usually precipitates to form large polygons with upturned edges due to the forces of crystallization. There is evidence to suggest that, contrary to common opinion, the ponded rain water does not hydrate anhydrite to gypsum on an almost annual basis. Are there other signs of vegetation (other than microbial mats)? Stop 10 Lunch will be taken at the Pleistocene outcrop of Ras Khaf which comprises the extreme northwest point of Al Dabb’iya. We shall discuss the Pleistocene sequence in an attempt to gain insight to some of the sedimentological processes operating at that time. How many Pleistocene rock types are there? How and when were they deposited? What does the outcrop tell us about the Pleistocene sea levels? Stop 11 This is one of the Pleistocene outcrops forming a small peninsula along the northwest shoreline of Al Dabb’iya, south of Ras Khaf, where we shall discuss the significance of the Pleistocene outcrops with relation to their origins and antecedent topography of the Holocene sedimentary regime. Nearby is a well established mangrove stand which provides clues to the interpretation of parts of the Pleistocene succession. Stop 12 Returning to the main coastal highway, numerous, parallel, shelly lineaments with sparse vegetation can be observed. What is their significance? They represent linear storm beach ridges marked by superficial lags of shell fragments dominated by cerithid gastropods and clumps of halophytic vegetation (salt bushes). Prior to modern road constructions, these beach ridges provided the main vehicle tracks along the sabkhas because of their relatively firm carbonate base compared with the soft evaporitic sabkha sediments on either side. Beach rock often occurs along these beach ridges. The ridges formed about 5000 years ago when the offshore islandswere smaller and provided a much less effective barrier system; wave activity was greater; and sea level was about a meter higher than it is today. Their original relief has been reduced by deflation. Satellite images reveal that many such parallel ridges exist. They are traversed by numerous narrow tidal channels which are only rarely active. These are generally parallel in the area between Musaffah and Bu Labyad. However, satellite images reveal that they often formed ‘winged’ spits around the edges of a number of areas that are interpreted as ‘vanished jebels’. By analogy with jebels with flanking ‘winged spits’ further west, these ‘vanished jebel’ are assumed to be highly deflated Miocene inliers that have been deflated down to the level of the present day sabkha surface, ie the top of the capillary zone. Seaward of the storm beaches is the westward extension of the sabkha anhydrites, gypsum mush and microbial mats previously examined in the Musaffah Channel. The anhydrites observed in the AAPG pit are seen at Locality 9. There has evidently been two main phases of anhydrite formation. The Mussafah Channel anhydrites and their lateral equivalents seawards of the storm beaches are interpreted as being the younger of the two. Stop 13 Near Rafiq Island , we shall stop to examine very thick, extensive microbial mats (future stromatolites). Stop 14 Not far east of the road which leads to Bu Labyad, we shall briefly visit a typical Miocene outcrop comprising fluvio-lacustine sediments. Of particular interest is a well exposed bed of massive gypsum. Such Miocene outcrops (often forming classic mesas) influenced the Holocene sedimentation. Stop 15 Immediately west of stop 15 we shall walk about half a kilometre seawards across the sabkha to examine a coast-parallel zone of sparsely vegetated sand which extends across the sabkha. What is its significance? There exists a surficial scattering of marine shells representing possibly a severely deflated beach sediment. It represents the known landward limit of the Flandrian transgression. Landward of this shelly lag, there exists another depression which regularly ponds rain water for long periods. Stop 16 Turning left at the Bu Labyad cross road, we shall drive several kilometres landwards across the sabkha to examine two types of aeolian sand dunes. What are the dune types? What was the prevailing wind direction during deposition? How many aeolian phases are evident? Stop 17 About ten kilometres past Tarif we shall stop to examine the three small Pleistocene zeugen about 200 meters off the seaward side of the road.. What is their stratigraphic succession? What fossils can one find on these zeugen and what is their significance? Drive to the Al Dhafra Beach Hotel, Jebel Dhanna. Evening lecture video session.
4	Holocene sediments near Jebel Dhanna, Shuweihat and Sabkha Matti Stop 18 Drive for about 20 minutes to Shuweihat Island which exhibits excellent continental Miocene exposures of the Shuweihat and overlying Baynunah Formations. The Shuweihat Formation comprises barchanoid aeolian dunes. There is always considerable debate about the origins of the associated contorted beds. What is their origin? How can one interpret the sedimentology of these Miocene rocks? Is there any sign of evaporites? If so, how were they formed? (Time permitting, we shall drive several kilomtres into Sabkha Matti to examine modern equivalents.) The top of the Shuweihat Fm. Is taken at the level of the uppermost selenite (gypsum) horizon. The Baynunah Formation is dominantly fluvio-lacustrine and contains excellent soil horizons. It is also famous for its vertebrate fossils including elephant, crocodile, hippopotimus and turtle remains. We shall visit the site of the discovery of a famous elephant skeleton. The top of the Baynunah Fm. is a silcrete. The main reason for visiting Shuweihat is to snorkel on the windward fringing reef that extends along the western flank of the island. As we snorkel from the shore to the reef crest about 400 metres offshore, observe the type of backreef sediment. What is their origin? What is its reservoir potential? What are the reef-building organisms and why do they change across the reef profile? What sort of succession would you expect in a borehole through the backreef? The crest of a windward fringing coral reef can be seen about half a kilometer offshore. Large Porites heads (ca 12 feet high) have been observed growing in water depths down to about 30 feet on the reef front. Brain corals and staghorn corals (Acropora) are also quite common. The crest of the reef, which is often exposed at low tide, is dominated red algae (coralline algae) whose intertwining growths give rise to relatively large galleries that eventually get filled with gravel size sediment dominated by red algal detritus. Spiny echinods sometimes occupy the galleries and the surfaces of the algal framework are well populated with various marine vegetation (eg grasses) and encrusted with pelycopods. The sediment of the backreef environment is also gravelly and dominated by red algal debris. What is the likely stratification along a transect from shore to reef front? Stop 19 Driving along the beach on the west flank of Jebel Dhanna, we shall examine spit systems - both modern and cemented Holocene examples. The older cemented spits exhibit coniatolite beach rocks and embryonic teepee structures. Highly bioturbated and pelleted muddy carbonates with thin microbial mats dominate the inter-spit areas. What sort of correlation problems would you expect in this type of depositional model if found in the subsurface? Time allowing, we shall stop at the Dalma quayside to inspect a cliff of Miocene strata that exposes an interesting calcareous component within the clastic sequence. How did the carbonates form? Return to the hotel for lunch. Stop 20 Immediately east of the hotel is a large tidal channel. Several stops in the vicinity and gather clues to unravel the geological evolution of the area using the principles of Holocene stratigraphic drivers already learned from previous localities. How does one explain the occurrence of relict evaporites? How does one explain the open marine assemblage on the inner, abandoned spits? What sediments are accumulating along the present day beach? Why? What sediment type can one expect in the lagoon behind the frontal spit? What is the origin of the ooliths? What other types of oolith factories can one envisage? What type of reservoir geometries will these oolites eventually give rise to? Has this depositional setting been typical of the Abu Dhabi coastline in the past? Stop 21 About 50 kilometres east of the hotel we shall examine a well developed microbial mat with dessication polygons. This is one of the best developed and thickest microbial mats along the Abu Dhabi coastline. Stop 22 During our journey back to the hotel, we shall examine some exhumed Holocene evaporites. What is their significance? Return to the hotel. Evening debriefing and discussion session.
5	Visit Marawah Island The most complete succession of Quaternary sediments in Abu Dhabi are exposed at Marawah. The group will be asked to interpret the stratigraphic succession which comprises at least two transgressive-regressive sequences. The sediments include an extensively exposed reef dominated by red algal encrusted Porites; beach deposits; shingled oolitic bars and aeolian deposits with various. The evolving palaeotopography of the island during the Pleistocene-Holocene can be reconstructed. Return to Abu Dhabi
6	Boat trip into the lagoons north west of Abu Dhabi We shall travel by boat into the inner reaches of the lagoons behind Sadiyat and Ras Gharib islands. Much of the area has recently become accessible due to the construction of wide canals that expose in their banks excellent views of Pleistocene carbonates and evaporates. For instance, the oldest Pleistocene evaporite (subaqueous gypsum) is well exposed beneath aeolianites. Modern mangrove stands and tidal mud flats will also be accessed.

Instructor

Dr. Anthony Kirkham

Instructors may vary based on location and schedule.

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Course Provider:
 Schlumberger