Indian Journal of Marine Sciences Vol. 29, September 2000, pp. 2 19-223

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Beachr09~ exposuJe at ~a-'ye cut terra~e~ of Modern Dodavari delta: Their genesis, diagenesis and indications on coastal '§ !:lbl.llergen~~ and .§ealevel rise

l M . amkumai-,(~JPaiiahhi R a ma yya, M . .Gandhi -

~- ( De~ Studies Institute, Andhra Uni vers ity, Visakhapatnam-530 00V AIi4I.1Fa-J21,aaeSh, Indi

Rere~ved 1 March 1-999-;- revised 10 May 2000

G xtensive beachrock ex posures in vertical cliFFs of wave cut terraces were observed along the coastal regions of Modern Godavari delta. Env ironmental association , geographic 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 earlier progradational phase of the delta, the sediments got buried beFore inheriting the characteri stics of beach. With due progradation of the delta, these sands were cemented at an elevated location in supratidal regions. Their present location at or slightly below the low tide water line indi cates ongoing sealevel ri se coupled with coastal submergence. A conservati ve estimate shows 1:34 m o f combined effects of these two process:.) 2- 2-. ' C.(.

Carbonate cemented sedimentary rock composed of a tune of 3 m high cliffs are fo und to occur that sand and carbonate skeletal fragments that occur in ex pose reC UlTent beds of peat and beach ridge sands the intertidal areas of tropical and subtropical zones indicati ve 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 ex tensive ly. Modern Godavari delta. Their peculiar assoc iation with wave cut terraces lying at low tide waterline, extensive nature and the sensitivity of beach rocks to the prevalent coastal dynamics warranted a spec ific study in terms of deposi tional and diagenetic hi story of these beach rocks and the reasons for their occun·ence at below low tide water line.

Materials and Methods The study area spraw ls along the coastline of the

Godavari delta (Fig. 1). Godavari is the largest river draining Peninsular Indi a that forms third largest de lta next to the Ganges and Mahanadi . It shows an arcuate shape. The climatic conditi ons 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 swe ll 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 de lta is mesotidal with a tida l 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' . A lthough it has a rapid progradational history, during recent past, it experiences severe eros ion all a long its coast except at river mouths2

. In many places , wave cut terraces to

Systematic field mapping was conducted in and around the locations of exposures. Environmental assoc iation, primary sedimentary structures and other assoc iated f ield ev idences were collected . Although all along the coast except few patches, the beach rocks are fo und to occur and samples were collected only from very large exposures spanning for about hundreds of meters in order to ensure sa mpling from

Fi g. ! - God avar i delta and sample locations.

220 INDl AN J. MAR. SCI., VOL 29, SEPTEMBER 2000

in situ exposures and authenci~ of samples. This has restricted the number of sa mples to fo ur. Pre-we ighed portion of the samples (-50 g) were treated in 1: 10 He l and allowed to remain in effervescence. Ac id was added till there was no more e ffervescence and then the loose and dis integrated sediments were washed, dried and weighed3

. The dried samples were sieved at half phi interval and the respective weight percentages were used to ca lculate the size parameters

4. Data obtained from sieving were plotted

as cumulat ive frequency curves on arithmet ic probability paper and va rious stati st ical parameters representing mean grain size (Mz), sorting (O'j), skewness (Sk j ) and kurtos is (Kd were ca lcu lated. Thin sections were prepared from the rocks and subj ec ted to staining5 by alize rine red S and potass ium felTi cyanide. These thin sec tions, so sta ined, were studied under the pet rographic microscope for collecting information on rock components, depositional texture and di agenetic alterations .

Results and Discussion Beac h rocks, known for the ir rapid ceme ntation of

beac h sands contain di sarticul ated she ll fragments and objects from Modern era such as bottlenecks, e tc . are occurring as exposures in wave cut terraces located at or below low tide waterline of Godavari de lta ic coastline. These are typ ically assoc iated with beac h face adjacent to mangrove swamps and marshy lands which in turn , overlain by beach sands and chenier plai ns. These exposures have the sa lient characteri stics of beach sand s name ly possess ion of bedsets that dip seawards, occurrence of crustacean borings that are arranged perpendicular andlo r oblique to bedding, concentrati on of shell fragments and heavy minerals and a lternate bands of li ght and heavy mineral s etc . These characters of beachroc ks support the inference that these are the products of cementation of beach sand sC.. The occurrence of beachrocks and the ir loca tion with refere nce to present shoreline and sealevel would serve as an excellent indicator of previolls shoreline location and or sealevel stand3

,G. From the beachrock collected at 50 fat hom flat off Bombay, it had been adjudged to the earlier Pleistocene sea level sand and the present locat ion under the 50 fathom water as a cause of eustatic sealevel andlor tectonic subside nce? The beachrocks found off Visakhapatnam were studied and concluded that presence of beachrocks under water indicates sealevel va riat ion during Holocene3

.

While reviewing the genesis of beach rocks , Victor Rajamanickam et al.3 stated that beach sands get

cemented along watertable (in this context, a zone where freshwate r meets and mjxes w ith saline water -which is normally located up to a leve l to which tidally pumped seawater could rise). As the freshwater normally floats over seawater, thi s level is, in general, equates at or slightly above supratida l region. Considering all these, presently observed location of beachrock exposures at or be low low tide waterline could certainly indicate submergence and or sealeve l rise.

The mean s ize (Mz) vary from fine sand to medium sand (Table 1). The ave rage va lue is 2.46 indicating

fine sand category. The standard deviation (O'j) varies from 0.59 to 0.78 and the average is 0.65 indicating moderate ly well-sorted nature. Skewness (Sk j ) va lues

vary from 0.207 to -0.2738 showing variation in s ignatures of beach processes over sediments. It may also be interpreted that du ring depositi on, preva lent avai lability of excessive sed iments could ha ve caused rapid burial thwarting beach processes to make imprints on sediment characters typical of beach. Kurtosis (KG) va lues show a range (0.85 to 1.12) name ly pl atykurtic , mesokurtic and leptokurtic s ignifying indirec tly to diverse energy condition s in the environme nt of deposition3

. A ll the four bivari ate plots of Friedman8 and Moiol a & Weiser9 show that the sediments of these beach rocks have ri verine c harac ter predominantl y than beach indicating quick burial of sediments and also excess ive sed iment influx assoc iated with rapid progradational phase of delta during deposition. Such an interpretat ion of prevalent rapid delta building, huge sed ime nt influx and quick burial are asce rtained through the bivariate pl ot of Stewalt lO. G laister & Nelson'sl l ma turity trend pl ot showed that these are de lta front sediments and immature. C-M pattern of tract ive current proposed by Passega l2 indica tes that these sed iments were transported through ro lling and graded suspension and graded suspensi on. This implies prevalent dominant ro le of storm surges and fl oods during the deposi ti on of beac h rock sediments.

Table I - Graphic size parameters of the beach rock sediments

Sample Mean Standard Skewness Kurtosis No. (Mz) dev iation (0') (SK) (Ke)

I. 1.99 0.7788 02070 0.8538 2. 1. 88 0.7280 0.09!3 0.9271 3. 2.23 0.5076 0.3471 0.9290 4. 3.73 0.5856 -0.2738 1.1 202 Average 2.46 0.65 0.0929 0.9575 Category Fille sand Moderately well Ncar Mcsokurtic

sorted s~mmetrjca'

RAMKUMAR el at. ; BEACHROCK EXPOSURES AT GODAVARI DELTA 22 1

Petrographic modal composition counts (Table 2) show that these rocks contain 65-85% of grains and 15-35% cement with no matrix. Among grains, quartz was dominant component ranging from 30 to 65% while feldspars range from negligible to 2%. Next to quartz, peloids dominate in proportion accounting for 13-35%. The bioclasts range from 0-19%. Under petrographic microscope, these rocks were observed to contain grain supported fabric with significant grain-grain contacts. Depositional packing was preserved without much obliteration. Presence of significant grain-grain contacts and alignment of elongate' grains indicate prevalence of considerable overburden before final lithification. Grains are predominantly constituted by monocrystalline and polycrystalline, sub-angular to sub-rounded quartz followed by bioclasts of molluscan shell fragments and completely micriti sed shell fragments (peloids) . Variations in their proportions were also observed. Among monocrystalline and polycrystalline quartz grains, the latter predominates in proportion paving way for interpretation that onshore transport of sediments played major role in building beach during earlier part of the progradational history of the delta. In addition, polycrystalline quartz grains indicate gneissic suite of provenance. However, heavy mineral studies are required to ascertain this interpretation. The bioclasts range from whole tests of gastropoda to fibrous, di sarticulated molluscan shells. Peloids are irregular to rounded in shape. Minor amounts of feldspar (essentially microcline) were also found to occur in association with rounded quartz grains indicating mjxed sediment source (monocrystalline rounded quartz grains may indicate littoral drift from a long distance whereas the feldspars are indicative of lesser transport and rapid burial implying fluvi al source) and higher sediment influx during deposition. The rocks contain less or negligible amounts of argillaceous matrix indicating higher energy conditions of deposition. However, silt and clay have been added during later stage percolation of meteoric waters forming geopetal structures. Alteration of

Table 2 - Modal co mpositi on of beach rocks (in relative percentages)

Note: Quartz, feldspar, bioclast and peloid are component s of grain

Sample Grain Matrix Cement Quartz no.

Feldspar Bioclast Peloid

I. 2. 3. 4.

70 85 80 65

o o o o

30 15 20 35

55 50 65 30

2 o 13 19 15

2 o 13 negligibl e o 35

feldspars is significant indicating prolonged duration of weathering (diagenetic) processes. Intergranular porosity is present. It is completely filled with spars of later origin and si lt and clay brought to those pores by circulated waters. Meniscus cements are also conspicuous. Neomorphic alteration is clearly made its imprints on bioclasts and cements testifyi ng varying nature of diagenetic fluids. Original cement morphology must have been fibrous in nature that got altered by later stage di agenetic alteration into very fine inequigranular microspars of non-ferroan calcitic type. The shell fragments and whole tests are essentially low magnesian non-ferroan calcitic in mineralogy suggesting oxidising environments of diagenes is.

Enumeration of sedimentary structu ral, granulometric and petrographic data suggests that the sediments were deposited in beachface essentially in swash zone. As the sediment influx to the depocentre was hi gh, the sediments deposited were buried quickly before inheriting typical beach sediment characteristics. This could well be linked with the preva lent rapid progradation of delta over sea. In view of higher rate of sedimentation , enough overburden must have been laid over the sediment column deposited earli er. The pressure exerted by the overburden to underlying sediments has reali gned the elongate grains and.. during such rea lignment, the primary porosity was reduced significantly. With due progradati on, the coastal and intertidal sediments have became topographically rai sed inland region. It appears plausible from earlier study1 3 that the rapid sedimentation of sands during nOltheast monsoon (which is always associated with higher wave heights

. h f d' 14. 15 promotll1g ons ore transport 0 se 1ments rigorously than other seasons) led to undisturbed preservation of them during fair weather seasons. During fair weather seasons, the sediments got cemented to form beachrocks. These monsoonal high waves create sand deposits at higher altitude as a result of onshore sediment transport which in turn, are not accessible for waves and tides of fair weather seasons lS

. Such a seasonally non-affected conditions of beachface are said to be the favourable zones for beach rock formation 16.

The inland region was experiencing percolat ion of freshwater that moved through pores and interacted with ex isting saline water there, forming fresh-saline water mixing zone. As this zone was highl y conducive of rapid carbonate precipitation, aided with tidal pumping of saline water through interconnected

222 INDIAN J. MAR. SCI., VOL. 29 , SEPTEMBER 2000

intergranular porosity (which . supply CaC03

continuously), rapid cementation of these unconsolidated sands must have occurred with ease. Such constant pumping of seawater through the pore spaces in the originally incoherent sands due to ri se and fall of marine water leve l help significant cementation

l 7. In view of location of impermeable

mangrove swamp clay beds towards landward side, the meteoric waters collected over these sands were ought to have moved seaward side or e lse they must have simply got evaporated . The mll1lmUm

temperature of 21 °C required for calcite crystallisation was provided by the geographic location of the delta that reels under average annual

te mperature of 27.S°C. Considering the preva lent overburden that realigned elongate gra ins along with the statement that minimum of 76 cm thick overburden is required l 7 to maintain 21 0c, the role of minimum te mperature ma intained by overburden could also be infefTed. It is observed that th is increased temperature in beach sand, promoted precipitation because the apparent saturation decreased with increase in temperature l6 and there by, an increase in the paf1ial press ure of CO2. It appears pl aus ible that with all these conducive milieu, cementation occurred profusely and was influenced by evaporation of interstitial wate r and degassing of CO2. Absence of di ssolution phenomena of carbonate part icles and the presence of non-ferroan mineralogy of bioclasts and cements strongly advocate diagenesi s in mixing zone of oxidising environment. Presence of meniscus cement i.ndicates cementation in vadose zone (oxidising environment). Thi s could be translated as cementation in regions above the tidally influenced water level (supratidal reg ion) .

While analysing the evolutionary history of thi s de lta ,18 it is concluded that this de lta prograded over sea in such a way that the prodeltaic silty-clay had been first overlain by coarse sands of distributory mouth bars, barrier spit and beach. With further progradation , the lagoonal sediments deposited behind the barrier spits paved way for mangrove swamp development. Higher sedimentation rate of de lta front environments (distributory mouth bar, barrier spit and beach) built pressure on prodeltaic sediments. Since the prodeltaic sediments are highly susceptible to reduction in volume when pressure is applied, significant compression is introduced resulting in coastal submergence. As the submergence proceeds, enough accommodation space is created for further sedimentation in delta front environments. If

this process persi sts , continued delta building activity over marine regimes would take place . However, with the advent of reduction in sed iment influx to the delta front environments in view of reduced stream flow , coupled with rise in sea le ve l during recent past, previously prograded areas have started receding. The landward advancement of coast line destructs coastal land all along the coast except at ri ver mouths". By virtue of arcuate shapeliness of thi s de lta that is under the constant impact of waves approaching at an angle (i.e. oblique to the coastline and more or less paralleling genera l littoral currents) in combination with net northerl y drift of sed iments,14 it is clear that by the event of reduction in sedimen t influx to marginal marine regime and dominati on of marine agents over flu vial agents, currently thi s delta e ngages eros ional phase. The sediments so eroded, are being transported to offshore regions. Presence of clay balls and well-rounded beach rock pebbles in di stributary mouth bar confirm this asse rtion as they have striking similarity to the onland exposures. The gradient of ocean fl oor off Godavari is very less l9 possibly by the settling of offshore dri fted sediments. The offshore transport of sediments is still continuing as clearly witnessed in multi seasonal false colour composits of IRS-USS II imageries". Such type of sediment eros ion and offshore bypass ing could happen in a sett ing where ri si ng sealevel has significant role"o. Slow marine transgressions would take place where subsiding coasts are present even though a rise in sea level is not considered. The Kri shna-Godavari de lta complex in east coast f India form one among such subsiding delta coasts21 . In view of sandy nature of the coast , rising sealevel causes se vere erosion exposing previously consolidated beach rocks at or be low low tide waterline.

The observat ions that cementation of beac h sediments to form beachrocks requires minimum of 76 cm of overburden l7

, the present location of beach rocks at or be low low tide waterline, coupled with 2 m of tidal va ri ation of this de lta, when put togethe r, indicate that the beach rock cementation must have taken place above 1.34 m from present day mean sealevel. Since these beach rocks (formed 1.34 m above present day mean seale.vel) were found to occur at or below the low tide waterline, combined effects of coastal submergence and sealevel rise to a tune of 1.34 m from the time of the ir formation could be inferred. Statistical analysis of long-term tide gauge data indicates 1.2 mm/year increase in global mea n sealevel22

. 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 relative roles of sealevel rise and subsidence in this regIOn.

To sum-up these observations, the following could be concl uded. The beachrocks found at or below the low tide waterline of Godavari delta are originally consolidated beach sands deposited at the swash zone of beach face . During deposition, the sediments were quickly buried under newer sediments as there were enormous sediment influx in view of contributions from onshore/offshore sediment traJlspolt and fluvial sources. As such rapid deposition took place during major storms by onshore/offshore sediment transport of storm surges and huge fluvial sediment influxes resulting in rapid progradational phase of the delta in the past, they were kept undisturbed during fair weather seasons . The conducive environments such as mixing zone of diagenesis have greatly helped consolidating these incoherent sed iments rapidly. A ll these events took place during the earlier rapid progradational phase of the delta. During the recent past, with due reduction in sediment influx and domination of marine agents over coastal processes, the delta progradation was not able to keep pace with rising sealevel and submergence which results in widespread erosion all along the coast except at two mouths (Ni larevu and Gautami) of the river Godavari. The beachrocks, once consolidated at an e levated region , are getting exposed at or below low tide waterline currently giving c lear testimony to ongoing sealevel rise and subsidence.

Acknowledgement Authors thank Prof. A. S. R. Swamy, Co-ordi nator,

Delta Studies Institute, Andhra Uni versity, Visakhapatnam-530003 , for encouragement. Miss. V. Rajani Kumari , Delta Studies Institute, is thanked for her ass istance. This is contribution No. 13 of the Delta Studies Institute.

References I Singh I B & Swamy A S R, Modem dellas , (Andhra

University, Visakhapatnam, Indi a), 1996, p.132.

2 Ramkumar M, Recent changes in the Kakinada spit , Godavari delta. J.Ceol Soc India, 55 (2000) 183-188.

3 Victor Rajamanickam G, Mohan Rao K & Rao T C S, Inference of coastal submergence from the study of beachrock off Visakhapatnam, in Sealevel varia/ioll and i/s illlpac/ Oil coas/al environlllen/ , edited by G Victor Rajamanickam,

(Tamil Universi ty, Thanjavur, Indi a) 1990 pp. 397-409.

4 Folk R L, Petrology of sedilllel/lary rocks, (Hemphill , Texas) 1974, pp. 138- 167.

5 Adams A E, Mckenzie W S & Guilford C, A/las of sedilllentaty rocks under /h e microscope. (ELBS Publications, London) 1988 , pp. 100 .

6 Soman G R, Kale M G & Pawar N J, Sedimentological studies of Quaternary Karal (Beachrock) of coastal Maharashtra, in Qua/emary Ceology of SOlllh Asia , edited by C Mohana Doss, (Anna University, Madras, India), 1996, pp. 240-256.

7 Nair R R. Beachrock and associated carbonate sediments of the fifty fathom fl at, a submarine terrace on the outer continental shelf off Bombay, Proc Indian Acad Sci. 72 ( 1970) 148-154.

8 Friedman G M, Dynamic processes and statistical parameters compared for size frequency distribution of beach and river sands , J Sed Pe/rol, 37 ( 1967) 327-354.

9 Moiola R J & Weiser D, Textural parameters: An evaluation, J Sed Pe/rol, 38 ( 1968) 45-53.

10 Stewart Jr. H B, Sedimentary reflections on deposi tional environment in San Migne lagoon , Baja California, Mex ico, Allier Asson Pe/rol Ceol Bull, 42 ( 1958) 2567-2618 .

II G laister R P & Nelson H W, Grain size distribution : An aid in fac ies identification , Bull Can Pe/rol Ceol, 22 ( 1974) 203-240.

12 Passcga R, Texture as characteri stic o f clasti c deposi ti on. Allier Asson Pe/rol Ceol Bu/l, 41 ( 1957) 1952-1984.

13 Daly R A. The geology of Alllerican Sall1oa , (Camegie Institute Publication, Washington). 1924 pp. 93-143 .

14 Sastry J S , Vetharnony P & Swamy G N, Morphological changes at Godavari delta region due to waves, currents and the associated phys ical processes, in Qua/em ary del/as of In dia , edited by R Vaidyanadhan , (Melli Ceol Soc India. No.22) 1991 , pp.139-15 1.

15 Rao K N & Vaidyanadhan R, Evolution of the coastal landforms in the Krishna delta front , India, Trans Ill st Illdian Ceog r, I ( 1979) 25-32.

16 Ginsburg R N, Beachrock in south Florida, J Sed Pe/rol, 23 ( 1953 ) 85-92.

17 Bathurst R G C , Carbona te sedilllenlS alld /h eir diagenesis. (Elsevier, Amsterdam), 1975 pp.620.

18 Ramkumar M, Sedimentary micro-environments of Godavari delta, India. Part I Di stribution and morphodynamics,. (Andhra Univers ity, Visakhapatnam, Indi a) 1998, pp. 47.

19 Rao T C S, Pleistocene events recorded by shallow seismic profiling over the continen tal she lf off Krishna-Godavari deltas, in Qua/ernary dellaS of India, edited by R Vaidyanadhan ,. (Mem Ceol Soc Illdia, No.22) 199 1, pp.153- 163.

20 Antony E J, Beach ridge development and sediment supply, fill er J Mar Ceol , 129 ( 1995) 175- 186.

21 Bird E C F, The effect of a ri si ng sealevel on the coasts of Thailand, /lseall J Sci Tech Develop, 6 ( 1989) 1-13 .

22 Barnett T P, The examination of global sealevel changes: A problem of uniqueness, J Ceophys Res, 87 ( 1984) 7980-7988 .