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 deposited at the zone of 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 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 composed of a tune of 3 m hi gh cliffs are found to occur that and carbonate skeletal fragments that occur in expose reC UlTent beds of peat and 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 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 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 (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 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 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. Presence of fine inequigranular microspars of non-ferroan calcitic significant grain-grain contacts and alignment of type. The shell fragments and whole tests are elongate' grains indicate prevalence of considerable essentially low magnesian non-ferroan calcitic in overburden before fin al lithification. Grains are min eralogy suggesting oxidising environments of predominantly constituted by monocrystalline and diagenes is. polyc rystalline, sub-angular to sub-rounded quartz Enumeration of sedimentary structural, followed by bioclasts of molluscan shell fragments granulometric and petrographic data suggests th at the and completely micritised shell fragments (peloi ds) . sediments were deposited in beachface essenti ally in Variations in their proportions were also observed. swash zone. As the sediment influx to the depocentre Among monocrystalline and pol yc rystalline quartz was hi gh, the sediments depos ited were buried grains, the latter predominates in proportion pa ving quickly before inheriting typical beach sediment way for interpretation that onshore transport of characteristics. This could well be linked with the sediments pl ayed major role in building beach during prevalent rapid progradation of delta over sea. In view earlier part of the progradational history of the delta. of higher rate of sedimentation, enough overburden In addition, polycrystalline quartz grains indicate mu st have been laid over the sediment column gneissic suite of provenance. However, heavy mineral deposited ea rli er. The pressure exerted by the studies are required to ascertain this interpretati on. overburden to underlying sediments has reali gned the The bioclasts range from whole tests of gastropoda to elon ga te grains and.. during such rea li gnment, the fibrous, di sarticulated molluscan shells. Peloids are primary poros ity was reduced significantl y. With due irreg ular to rounded in shape. Minor amounts of progradati on, the coastal and intertidal sediments have feldspar (essenti ally microcline) were also found to became topographically rai sed inland region. It occur in association with rounded quartz grains appears plausible from earlier study1 3 that the rapid indicating mjxed sediment source (monocrystalline sedimentation of sands during nOltheast mon soon rounded quartz grains may indicate littoral drift from (w hich is always as sociated with hi gher wave heights a long distance whereas the feldspars are indicative of promotll1. g ons h ore transport 0 f se d1m' ents 14. 15 lesser tran sport and rapid burial implying fluvi al rigorously than other seasons) led to undisturbed source) and higher sediment influx during depositi on. preservation of them during fair weather seasons. The rocks contain less or neg li gible amounts of During fair weather seasons, the sediments got argillaceous matrix indicating higher energy cemented to form beachrocks. These mon soonal hi gh conditions of deposition. Howeve r, and clay ha ve waves create sa nd deposits at hi gher altitude as a been added during later stage percolation of meteoric result of onshore sediment transport which in turn, are waters forming geopetal structures. Alteration of not accessible for waves and tides of fair weather lS Table 2 - Modal co mpositi on of beach rocks seasons . Such a seasonally non-affected conditions (in relative percentages) of beachface are sa id to be the favourable zones for Note: Quartz, feldspar, bioclast and peloid are component s of beach rock formation 16. grain The inland region was ex peri encing percolat ion of Sample Grain Matrix Cement Quartz Feldspar Bioclast Peloid no. freshwater that moved through pores and interacted with ex isting saline water there, forming fresh-saline I. 70 o 30 55 2 13 o water mixing zone. As this zone was hi ghl y 2. 85 o 15 50 19 15 3. 80 o 20 65 2 o 13 conducive of rapid carbonate precipitation, aided with 4. 65 o 35 30 negli gibl e o 35 tidal pumping of saline water through interconnec ted 222 INDIAN J. MAR. SCI., VOL. 29, SEPTEMBER 2000

intergranular porosity (which . supply CaC03 this process persi sts, continued delta building activity continuously), rapid cementation of these over marine regimes would take place. However, with unconsolidated sands must have occurred with ease. the advent of reduction in sediment influx to the delta Such constant pumping of seawater through the pore front environments in view of reduced stream flow, spaces in the originally incoherent sands due to ri se coupled with rise in sealevel during recent past, and fall of marine water level help significant previously prograded areas have started receding. The l 7 cementation . In view of location of impermeable landward advancement of coastline destructs coastal mangrove swamp clay beds toward s landward side, land all along the coast except at ri ver mouths". By the meteoric waters collected over these sands were virtue of arcuate shapeliness of this delta that is under ought to have moved seaward side or else they must the constant impact of waves approaching at an angle have simply got evaporated. The mll1lmUm (i.e. oblique to the coastline and more or less temperature of 21 °C required for calcite paralleling general littoral currents) in combination crystallisation was provided by the geographic with net northerl y drift of sediments,14 it is clear that location of the delta that reels under average annual by the event of reduction in sediment influx to temperature of 27.S°C. Considering the prevalent marginal marine regime and domination of marine overburden that realigned elongate grains along with agents over flu vial agents, currently this delta engages the statement that minimum of 76 cm thick erosional phase. The sediments so eroded, are being overburden is required l 7 to maintain 21 0c, the role of transported to offshore regions. Presence of clay balls minimum temperature maintained by overburden and well-rounded beach rock in di stributary could also be infefTed. It is observed that this confirm this assertion as they have striking increased temperature in beach sand, promoted similarity to the onland exposures. The gradient of l9 precipitation because the apparent saturation ocean fl oor off Godavari is very less possibly by the decreased with increase in temperature l6 and there by, settling of offshore drifted sediments. The offshore an increase in the paf1ial pressure of CO2. It appears transport of sediments is still continuing as clearly pl ausible that with all these conducive milieu, witnessed in multi seasonal false colour composits of cementation occurred profusely and was influenced IRS-USS II imageri es". Such type of sediment by evaporation of interstitial water and degassing of erosion and offshore bypassing could happen in a setting where ri si ng sealevel has significant role"o. CO2. Absence of di ssolution phenomena of carbonate particles and the presence of non-ferroan mineralogy Slow marine transgressions would take place where of bioclasts and cements strongly advocate diagenesis subsiding are present even though a rise in in mixing zone of oxidising environment. Presence of sealevel is not considered. The Krishna-Godavari meniscus cement i.ndicates cementation in vadose delta complex in east coast f India form one among 21 zone (oxidising environment). This could be such subsiding delta coasts . In view of sandy nature translated as cementation in regions above the tidally of the coast, rising sealevel causes severe erosion influenced water level (supratidal region). exposing previously consolidated beach rocks at or below low tide waterline. While analysing the evolutionary history of this delta ,18 it is concluded that this delta prograded over The observations that cementation of beach sea in such a way that the prodeltaic silty-clay had sediments to form beachrocks requires minimum of l7 been first overlain by coarse sands of distributory 76 cm of overburden , the present location of beach mouth bars, barrier and beach. With further rocks at or below low tide waterline, coupled with 2 progradation, the lagoonal sediments deposited m of tidal va ri ation of this delta, when put together, behind the barrier spits paved way for mangrove indicate that the beach rock cementation must have swamp development. Higher sedimentation rate of taken place above 1.34 m from present day mean delta front environments (distributory mouth bar, sealevel. Since these beach rocks (formed 1.34 m barrier spit and beach) built pressure on prodeltaic above present day mean seale.vel) were found to occur sediments. Since the prodeltaic sediments are highly at or below the low tide waterline, combined effects susceptible to reduction in volume when pressure is of coastal submergence and sealevel rise to a tune of applied, significant compression is introduced 1.34 m from the time of their formation could be resulting in coastal submergence. As the submergence inferred. Statistical analysis of long-term tide gauge proceeds, enough accommodation space is created for data indicates 1.2 mm/year increase in global mean 22 further sedimentation in delta front environments. If sealevel . It is hoped that dating of shells present in RAM KUMAR e/ al.: BEACHROCK EXPOSURES AT GODAVARI DELTA 223

these beachrocks would h~lp resolve the amount of (Tamil Universi ty, Thanjavur, India) 1990 pp. 397-409. relative roles of sealevel rise and subsidence in this 4 Folk R L, Petrology of sedilllel/lary rocks, (Hemphill, Texas) 1974, pp. 13 8- 167. regIOn . 5 Adams A E, Mckenzie W S & Guilford C, A/las of To sum-up these observations, the following could sedilll entaty rocks under /h e microscope. (ELBS Publications, be concluded. The beachrocks found at or below the London) 1988, pp. 100 . low tide waterline of Godavari delta are originally 6 Soman G R, Kale M G & Pawar N J, Sedimentological consolidated beach sands deposited at the swash zone studies of Quaternary Karal (Beachrock) of coastal of beach face. During deposition, the sediments were Maharashtra, in Qua/emary Ceology of SOlllh Asia, edited by C Mohana Doss, (Anna University, Madras, In dia), 1996, pp. quickly buried under newer sediments as there were 240-256. enormous sediment influx in view of contributions 7 Nair R R. Beachrock and associated carbonate sediments of from onshore/offshore sediment traJlspolt and fluvial the fifty fathom fl at, a submarine terrace on th e outer sources. As such rapid deposition took place during off Bombay, Proc Indian Acad Sci. 72 major storms by onshore/offshore sediment transport ( 1970) 148-154. 8 Friedman G M, Dynamic processes and statistical parameters of storm surges and huge fluvial sediment influxes compared for size frequency distribution of beach and river resulting in rapid progradational phase of the delta in sands, J Sed Pe/rol, 37 ( 1967) 327-354. the past, they were kept undisturbed during fair 9 Moiola R J & Weiser D, Textural parameters: An evaluation, weather seasons. The conducive environments such as J Sed Pe/rol, 38 ( 1968) 45-53. mixing zone of diagenesis have greatly helped 10 Stewart Jr. H B, Sedimentary reflections on deposi tional consolidating these incoherent sediments rapidly. A ll environment in San Migne , Baja California, Mexico, Allier Asson Pe/rol Ceol Bull, 42 ( 1958) 2567-2618. these events took place during the earlier rapid II Glai ster R P & Nelson H W, Grain size distribution: An ai d in progradational phase of the delta. During the recent facies identification, Bull Can Pe/rol Ceol, 22 ( 1974) past, with due reduction in sediment influx and 203-240. domination of marine agents over coastal processes, 12 Passcga R, Texture as characteristic of clasti c deposi ti on. the delta progradation was not able to keep pace with Allier Asson Pe/rol Ceol Bu/l, 41 ( 1957) 1952-1984. rising sealevel and submergence which results in 13 Daly R A. The geology of Alllerican Sall1oa , (Camegie Institute Publication, Washington). 1924 pp. 93-143. widespread erosion all along the coast except at two 14 Sastry J S, Vetharnony P & Swamy G N, Morphological mouths (Ni larevu and Gautami) of the river Godavari. changes at Godavari delta region due to waves, currents and The beachrocks, once consolidated at an elevated the associated physical processes, in Qua/em ary del/as of region, are getting exposed at or below low tide In dia , edited by R Vaidyanadhan, (Melli Ceol Soc India. waterline currently giving clear testimony to ongoing No.22) 1991 , pp.139-15 1. 15 Rao K N & Vaidyanadhan R, Evolution of th e coastal sealevel rise and subsidence. in the Krishna delta front, In dia, Trans Ill st Illdian Ceog r, I ( 1979) 25-32. Acknowledgement 16 Ginsburg R N, Beachrock in south Florida, J Sed Pe/rol, 23 Authors thank Prof. A. S. R. Swamy, Co-ordinator, ( 1953) 85-92. Delta Studies Institute, Andhra Uni versity, 17 Bathurst R G C , Carbonate sedilllenlS alld /h eir diagenesis. Visakhapatnam-530003, for encouragement. Miss. V. (Elsevier, Amsterdam), 1975 pp.620. Rajani Kumari, Delta Studies Institute, is thanked for 18 Ramkumar M, Sedimentary micro-environments of Godavari delta, India. Part I Di stribution and morphodynamics,. her assistance. This is contribution No. 13 of the Delta (Andhra University, Visakhapatnam, India) 1998, pp. 47. Studies Institute. 19 Rao T C S, Pleistocene events recorded by shallow seismic profiling over the continental shelf off Krishna-Godavari References delt as, in Qua/ernary dellaS of India, edited by R I Singh I B & Swamy A S R, Modem dellas, (Andhra Vaidyanadhan,. (Mem Ceol Soc Illdia, No.22) 199 1, University, Visakhapatnam, India), 1996, p.132. pp.153- 163. 2 Ramkumar M, Recent changes in the Kakinada spit , Godavari 20 Antony E J, Beach ridge development and sediment supply, delta. 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