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A-'Ye Cut Terra~E~ of Modern Dodavari Delta: Their Genesis

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A-'Ye Cut Terra~E~ of Modern Dodavari Delta: Their Genesis Indian Journal of Marine Sciences Vol. 29, September 2000, pp. 2 19-223 / Beachr09~ exposuJe at ~a-'ye cut terra~ e ~ of Modern Dodavari delta: Their genesis, diagenesis and indications on coastal '§ !:lbl.ll ergen~~ and .§ealevel rise l M . amk umai- ,(~JPaiiahhi R a mayya, M . .Gandhi - ~- ( De~ Studies In stitute, Andhra Uni versity, Visakhapatnam-530 00V AIi4I1Fa. -J21,aaeSh, Indi R ere~ved 1 March 1-999-;- revised 10 May 2000 G xtensive beachrock ex posures in verti cal cliFFs of wave cut terraces were observed along the coastal regions of Modern Godavari delta. Env ironmental association, geographi c location, sedimentary structures, texture and petrography of these rocks reveal that they are sediments deposited at the swash zone of beach face. In view of prevalent rapId deposlu on during earli er progradational phase of the delt a, th e sediments got buried beFore inheriting th e characteri sti cs of beach. With due progradati on of the delta, these sands were cement ed at an elevated location in supratidal regions. Their present location at or sli ghtly below the low tide water line indi cates ongoin g sealevel ri se coupled with coastal submergence. A conservati ve estimate shows 1:34 m of combined effects of th ese two process:.) 2- 2-. ' C.(. Carbonate cemented sedimentary rock composed of a tune of 3 m hi gh cliffs are found to occur that sand and carbonate skeletal fragments that occur in expose reC UlTent beds of peat and beach ridge sand s the intertidal areas of tropical and subtropical zones indicative of erosion in previously prograded lands. In are known as beachrocks. Beachrock exposures are few places, beach rocks are found exposed -- found to occur essentially along the coastline of extensively. Modern Godavari delta. Their peculiar associati on Systematic field mapping was conducted in and with wave cut terraces lying at low tide waterline, around the locations of exposures. Environmental extensive nature and the sensitivity of beach rocks to association, primary sedimentary structures and other the prevalent coastal dynamics warranted a specific associated field evidences were coll ected. Although study in terms of depositional and diagenetic hi story all along the coast except few patches, the beach of these beach rocks and the reasons for their rocks are found to occur and samples were collected occun·e nce at below low tide water line. onl y from very large exposures spanning for about Materials and Methods hundreds of meters in order to ensure sa mpling from The study area sprawls along the coastline of the Godavari delta (Fig. 1). Godavari is the largest river draining Peninsular India that forms third largest delta next to the Ganges and Mahanadi . It shows an arcuate shape. The climatic conditions of the delta are influenced by all the three seasons namely southwest monsoon (June to September), northeast monsoon (October to January) and non-monsoon (February to May). The waves approaching thi s delta are predominantly swell s with signi fica nt heights centred around 1 m. Currents range from 10-30 c m/sec with maximum speed up to 200 crn/sec. The delta is mesotidal with a tidal range of 2 m. Mean annual temperature is 27.5°C. Mean annual rainfa ll is 11 00 mm. The ri ver Godavari has protruded around 35 km into the Bay of Bengal since Neogene' . Although it has a rapid progradational hi story, during recent past, it experiences severe erosion all along its coast except 2 at river mouths . In many places, wave cut terraces to Fi g. ! - Godavari delta and sample locati ons. 220 INDlAN J. MAR. SCI., VOL 29, SEPTEMBER 2000 in situ exposures and authenci~ of samples. This has cemented along watertable (in this context, a zone restricted the number of samples to four. Pre-weighed where freshwater meets and mjxes with saline water - portion of the samples (-50 g) were treated in 1: 10 which is normally located up to a level to which He l and all owed to remain in effervescence. Acid tidally pumped seawater could rise). As the was added till there was no more e ffervescence and freshwater normally floats over seawater, this level is, then the loose and disintegrated sediments were in general, equates at or slightly above supratidal 3 washed, dried and weighed . The dried samples were region. Considering all these, presently observed sieved at half phi interval and the respective weight location of beachrock exposures at or below low tide percentages were used to calculate the size waterline could certainly indicate submergence and or 4 parameters . Data obtained from sieving were plotted sealevel rise. as cumulative frequency curves on arithmetic The mean size (Mz) vary from fine sand to medium probability paper and various stati stical parameters sand (Table 1). The average value is 2.46 indicating representing mean grain size (Mz), sorting (O'j), fine sand category. The standard deviation (O'j) varies skewness (Skj) and kurtosis (Kd were calculated. from 0.59 to 0.78 and the average is 0.65 indicating Thin sections were prepared from the rocks and moderately well-sorted nature. Skewness (Skj ) va lues 5 subj ected to staining by ali zerine red S and potassium vary from 0.207 to -0.2738 showing variation in felTi cyanide. These thin sections, so stained, were signatures of beach processes over sediments. It may studied under the petrographic microscope for also be interpreted that durin g depositi on, prevalent collecting information on rock components, avai lability of excessive sediments could ha ve caused depositional texture and di agenetic alterations. rapid burial thwarting beach processes to make Results and Discussion imprints on sediment characters typical of beach. Beach rocks, known for their rapid cementation of Kurtosis (KG) va lues show a range (0.85 to 1.12) beach sands contain di sarticulated she ll fragments and namely pl atykurtic, mesokurtic and leptokurtic objects from Modern era such as bottlenecks, etc . are signifying indirectly to diverse energy conditions in 3 occurring as exposures in wave cut terraces located at the environment of deposition . A ll the four bivariate 8 9 or below low tide waterline of Godavari deltaic plots of Friedman and Moiola & Weiser show that coastline. These are typically associated with beach the sediments of these beach rocks have ri verine face adjacent to mangrove swamps and marshy lands character predominantly than beach indicating quick which in turn, overlain by beach sands and cheni er burial of sediments and also excessive sediment influx plai ns. These exposures have the sali ent associated with rapid progradational phase of delta characteri stics of beach sand s namely possession of during depositi on. Such an interpretation of prevalent bedsets that dip seaward s, occurrence of crustacean rapid delta building, huge sediment influx and quick borings that are arranged perpendicular andlor obli que burial are ascertained through the bivari ate pl ot of lO to bedding, concentrati on of shell fragments and Stewalt . G laister & Nelson'sl l maturity trend pl ot heavy minerals and alternate bands of li ght and heavy showed that these are delta front sediments and mineral s etc. These characters of beachrocks support immature. C-M pattern of tractive current proposed l2 the inference that these are the products of by Passega indicates that these sediments were cementation of beach sand sC.. The occurrence of transported through rolling and graded suspension and beachrocks and their locati on with reference to graded suspensi on. This implies prevalent domin ant present shoreline and sealevel would serve as an role of storm surges and fl oods during the depositi on excellent indicator of previolls shoreline location and of beach rock sediments. 3 or sealevel stand ,G. From the beachrock collected at Table I - Graphic size parameters of the beach rock sediments 50 fat hom flat off Bombay, it had been adjudged to the earlier Pleistocene sealevel sand and the present Sample Mean Standard Skewness Kurtosis location under the 50 fathom water as a cause of No. (Mz) deviation (0') (SK ) (Ke) eustatic sealevel andlor tectonic subsidence? The I. 1.99 0.7788 02070 0.8538 beachrocks found off Visakhapatnam were studied 2. 1. 88 0.7280 0.09!3 0.9271 and concluded that presence of beachrocks under 3. 2.23 0.5076 0.3471 0.9290 3 4. 3.73 0.5856 -0.2738 1.1 202 water indicates sealevel variation during Holocene . Average 2.46 0.65 0.0929 0.9575 While reviewing the genesis of beach rocks, Victor Category Fille sand Moderately well Ncar Mcsokurtic 3 Rajamanickam et al. stated that beach sands get sorted s~mmetrjca' RAMKUMAR el at.; BEACHROCK EXPOSURES AT GODAVARI DELTA 22 1 Petrographic modal composition counts (Table 2) feldspars is significant indicating prolonged duration show that these rocks contain 65-85% of grains and of weathering (diagenetic) processes. Intergranular 15-35% cement with no matrix. Among grains, quartz porosity is present. It is completely filled with spars was dominant component ranging from 30 to 65 % of later origin and si lt and clay brought to those pores while feldspars range from negligible to 2%. Next to by circulated waters. Meniscus cements are also quartz, peloids dominate in proportion accounting for conspicuous. Neomorphic alteration is clearly made 13-35 %. The bioclasts range from 0-19%. Under its imprints on bioclasts and cements testifyi ng petrographic microscope, these rocks were observed varying nature of diagenetic fluids. Origin al cement to contain grain supported fabric with significant morphology must have been fibrous in nature that got grain-grain contacts. Depositional packing was altered by later stage di agenetic alteration into very preserved without much obliteration.
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