Regional Tectonic Settings

Regional Tectonic Settings

1. Formation of Bengal Basin

Bengal basin, occupying a major portion of Bangladesh, lies approximately between 25^o-20^o30’N latitudes and 87 ^o 30’-90 ^o 30’E longitudes. It comprises the entire lowland of about 89000 sq km among which 57000 sq km are on land and 32000 sq km are of offshore up to 200m bathemetry (M. jain and M. Rastogi). The Bengal basin is bordered by the Chhota Nagpur plateau and Rajmahal Hills in west, Himalayan foothills in north, Meghalaya Plateau in the northeast, the Manipur-Tripura hills in the east and by Feni river at the southeast. The southern Boundary of this basin is marked by Bay of Bangal where the basin surface slopes into the Bengal deep sea fan. (Banglapedia)

The formation of Bengal Basin began about 125 million years ago, at early tertiary period as a divergent marginal basin in Indian plate drifting towards Laurasia. Before Cretaceus period the earth-landforms were divided into two plates – Laurasia and Gondwana. Laurasia consisted of mostly North American and European-Asian landmass. On the other hand Gondwanaland consisted of Indian plate including Bangladesh (Greater Rangpur and Dinajpur area), South America, Australia, Africa and Antarctica (Haroun er Rashid, 1991). In early Tertiary period the Indian plate, splited off from the Gondwanaland and moved relatively fast towards Eurasian plates. Later about 45million years ago in mid-Eocene, the drifting Indian plate collided with Eurasian plate in north and with Burmese plate in northeast resulting subduction of Indian plate in both case. The outcome was the disappearance of Tethys Sea and upliftment of the Himalaya range and the large Tibetan plateau as well as a very considerable faulting in China (Mattauer et. al. 1981). After this collision, the status of Bengal basin converted as convergent marginal basin.

The eastern part of the Bengal basin was built at Oligocene period as the northeastern part of India broke and sank into under the sea and reformed as part of Bengal basin. The older rocks of Gondwanaland surfaced and formed the Meghalaya plateau in east and Chhota Nagpur plateau in west. The area between these plateau are called the Rajmahal gap and along this gap formed the land surface of Dinajpur,Naogaon and Rangpur of Bangladesh. (Haroun er Rashid, 1991).

2. Ganges Brahmaputra Delta

Origin and Evolution of The Sundarbans

The origin of the Sundarbans ways back to the late Pleistocene period. At that time, the formation of delta in west Calcatta was in progress by the sedimentation of Ganges river. With the passage of time the flow of Ganges river migrated eastward and the formation of delta also shifted towards present Jessor area.

By the time the delta formation extended towards present Sundarban, the delta of east greater Barisal region was built by early Meghna and Brahmaputra rivers.

Later in 15^th or 16th century the course of Ganges river again changed course at eastward and led many left bank channels charging as right bank distributaries. A major left bank tributary, Teesta confluenced with Brahmaputra moved westward to meet with Ganges river in Goalando and formed the Padma river by 1830. Due to the complicated drainage pattern formed by integrated rivers, channels, creeks, estuaries, deltaic landforms of various shape, size and elevation was in built. The Kumar, Chitra, Kabadak, Nabaganga, bhairab rivers became major distributaries of Ganges river and spreaded out all over Sundarban through their numerous distributaries of each.

Going through all these chaos of river course shifting, channels and land forming, the present status of Sundarban came to existence approximately 4000 years ago. (Rob and Ashaduzzaman, 2009)

Geomorphological Units of the Sundarbans:

The Sundarbans is broadly considered as a mangrove swamp with a complex combination of tidal creaks, river channels, estuaries, tidal flats, mud flats, sand dunes, permanent, semi-permanent swamps, beaches etc. Most prominent landforms are presented in brief:

Alluvial Plains:

Alluvial plains form through long term deposition of river sediments, mostly by fluvial processes. It is extended upto the tidal plains covering the freshwater zone of river Ganges in Sundarban.

Tidal Flats or Mud flats:

Tidal flats or Mud flats are the deposition of estuarine silts, clays and animal detritus at the intertidal zone. These areas are hardly vegetated and form the middle part of tidal basin, below the vegetation supporting salt marsh.

Salt Marsh:

Salt marshes are landforms found ith e upper coastal intertidal zone between land and brackish water. These are usually salty soil with the composition of peat and mud. Vegetation on these are mostly of halophytic or salt tolerant as herbs, low shrubs or grasses.

Back Swamps or Basins:

These are drainage system features on landforms to regulate the rainwater and tidal water. It also drains out water from depression. Smaller creeks go through stages of transformation and generate river channels to drain out water from depressions. Here the water gradually losses it velocity and suspends coarser sediments by the levee and carries finer particles towards low lying swamps.

Ridges or Levees

During the flowing overflow of rivers with finer particles creates ridges at the edge of river channels through deposition of finer sediments. From the physiographic point of view the levees or ridges are the most raised lands of deltaic formation.

Tidal Creeks:

Creeks are the tidal dendritic drainage system that are of about few centimeters to several meters in depth and few hundred meters in width. These creeks form the complex network of wetlands as the smaller units to create streams and rivers with complex integration processes.

Sand Dunes:

Sand dunes are mound or ridge of sand that forms over a long period of time by trapping or windblown sands via grasses or other obstacles.

Sandy beach

Beaches are the sloping zone of loose particles such as sand, gravel, pebbles at the shoreline of ocean, sea, rivers etc. So the sandy beach is found near the shoreline for example the sandy beach at southern Dublar char.

Lithological Sequence Survey

Aims and Objectives

The Sundarbans is a tide and river dominated delta. Both the integrated rivers and channels of Ganges-Brahmaputra river and tidal activity of ocean are affecting the accretion and erosional state of landforms. From the late Pleistocene period, the gradual sedimentation of rivers, tidal activities, mangrove vegetation as well as the tropical monsoon climate have been contributing to the distinctive sedimentary layers in the Sundarbans.

The Aims and objectives of lithological survey in Sundarban:

• Study of Geomorphological landforms of the Sundarbans
• Lithological Stratification of the Sundarbans
• Identification of soil characteristics of strata
• Analysis of lithological strata to understand the formation characteristics

Study Area: Dublar Char

Dublar Char was the study area where the lithological survey was conducted. It is located at the extreme south-central part of the Sundarban of Bangladesh between 21⁰ 31´00´´N to 21⁰43´00´´N latitude and 89⁰31´40´´E to 89⁰36´20´´E longitude. It is one of the island of Sundarban Sharankhola Range, under the East Forest Division of Bangladesh in Bagerhat district. The total area of Dublar Char is about 67.93sq km with a boundary of 40.89km. The island is crisscrossed with number of river channels totaling 7328052.32sq km of area.

Methodology and Data Collection

The lithological survey was carried out in two phases, one is instant observation and analysis and another is sample collection for laboratory analysis. The total procedure can be categorized in four steps:

1. Soil profile analysis in Match-stick technique
2. Soil sample collection in Monolith
3. Soil sample collection in Boring method with gauge sampler
4. Analysis of soil sample by Troels-Smith Scheme and Color identification with Munsell color chart

Two soil profiles have been studied in two distant sites and overall five boring operation was conducted. Two of the borings are operated at each profile sites, two of them are at near intertidal zone and the last one at intertidal zone of the southeast boundary of Dublar Char.

Stratigraphic Analysis

The boring soil samples are analyzed on the basis of Troels-Smith scheme (1955). Following the stratification and characteristics of soil samples are given in tables and diagrams:

Results

From the instant observation and analysis at profile sites, three layers had been identified in site 1(fig-) and four layers in site 2(fig-).

In case of borehole samples, the stratification (fig-) shows that the soil is mainly clay dominated and the upper layers contains more or less fine sands. The clay component increases with depth. The color of the soil gets darker with depth, in general from Greyish brown to very dark grey or black. The soil layers are usually more or less iron stained, moist and organic in many layers. The soil found at 1m above and below mean sea level contained wood fragments of different sizes. Fresh herbaceous rootlets are also found in upper layers.

Except other layers, three very distinctive layers have been identified in Dublar Char soil strata. Two of them are peat layers that are found at 45 to 64cm altitude at near-intertidal zone and at -210 to -232.24 cm altitude in intertidal zone. Though the peat layers were present in those samples, the inland samples show no layer of peat. The sedimentation pattern indicates that there can be peat layers at inland in higher depth. Besides the peat layer, a sandy soil layer exists at 65 to -94cm altitude which is an exception in the sedimentation sequence in Dublar Char.