seq uentia l o rgani zatio n of the strata ha ve bee n in terp ret ed as slope deposits by Pickering ( I
Rcvista Bmsitcim de Gcocicncios
Giorgio Basilici & Jorge Leonardo Martins
34(:'):335 ':\46. sctcmbrc de 2(MJ..l
ARCHI TECTURAL REINT ERPR ETATION AND ACOUSTIC FACIES OF THE LOWER PORTION OF THE APIUNA UNIT (CAMBRIAN DEEP-WAT ER DEPOSITIONAL SYSTEM , SANTA CATARINA STATE , BRAZIL) GIORGIO BASILICI' & JORGE LEONARDO MART INS' Resume REiNTElI I'RUil ,tiO Dil ilRQUI1E1VR il E FACIES il CUSTfCil S Dil POR, ti O INFU I IOII Dil UN/J)il DE M 'IUNil (SI.IT EMil DEI'OSICIONil L Cil MBRlilNO DE ACUilS I'ROFUNDil S, ESTilDO DE SilN7il Cil 7il RINi\. IIRil SII.) Rochas scdim cn tarcs de rlgua pro fund a. que nflonun no lon go d n IlR 470. pcrto de Apidnu (SC) . pcnu itinuu intcrprct ar a parte inferior da unidadc de Api una CO Ill O urn sistema deposicional que evoluiu de "rumpa rica ern urcia" ate "tampa de arciuc pclito". Esta pcsquis u con tribui para 0 se rer de cxplomcno da industria de pctnilco com a produc ilo: i) de um modele con ccit ual de rc scrv utorio s em sistemas de agua profunda tipo ramp" e ii) de s ism og rumas sintericos. Neste modele sao recon hccidos do is tipos de rumpa . ( I) Uma rampa rica de ureia e carac terizada (XlI' lcn cois dc arenite , com apa rcn te casualidade de con struciio c nao ligados u c auais ulime ntudore s. Os len cdis fo rum gcrados po r tlux os bipartidos: fluxos de groins (de bris flows s.l.) na parte inferior e tlu xns turb ulentos (turbidi ty curre nts) 11 turbid~
lurbulentllow lurbidily current
T he four argil lite inter vals in unit H, interla yer cd with the sands lon e pac kages. represe nt an inte rru ption of thc coa rse sed imentat ion. Two of thesc arc thin (up to 2.6 m), an d show lenticular sha pe; they co uld mean a local interruption of sandy depos its, becau se o r the lateral shift o r the sedime ntary entry po int. The other two arc thicker (lip 10 15 Ill) and laterally co ntinuous for more than HK) m; they represent an interru ption in the sandy input. Appa rently, in the unit C coarse r input interrupt ion is not recorded. Unit D Th e unit D crops o ut X krn S W o r unit C. Accord ing to stratimetric ca lculations. unit D remains about I(Xl rn above unit C; between unit C and unit D there are no s ignifi cant outcrops, Th e un it D (Fig. 3) is 35 m thick and can be di vided in u lower IX)11ion. J9 111 thick. made up of co arser deposit s. and an uppe r portion . 16 rn th ick, made lip of filler depos its. T ile low er portio n cove rs with eros ive surface argilli te strata. and, ju st at botto m. there is a muddy co ng lo merate, whose c lasts range from peb ble to boulders. Co ng lomera tic sandsto ne an d med ium to very coarse sa ndsto ne form len ticul ar bod ies, 1.5 m to 2.5 m thick; erosive surfaces d ivide the bottom and the top of the strata and g ive a le ntic ula r s ha pe to the bed s. w hich ar c ab out 60 III w ide . perpe ndicu larly to the paleoc urre nts. Sole casts reco rd pale oc urrents to wa rd S-S\V. ln the hig her part o f the lower 1>011ion. lenticular sandsto ne strata arc rep laced from re ctan gu lar bod ies. wit h a flat top. a nd rare erosive hottom . In the upper port ion unit D c hanges radi cally the facies associatio n: len ticular rippled sands to ne strata. 2-22 e m thick. with flut bottom and undul ated top altematc with s ilty- sa ndy arg illite (Fig. S). Co rmnon ly. thc sands tone strata show current ripples directed toward W. which are at high anglc w ith respect t(>the reco rded paleoc urren ts in the lower po rtion. T he sands to ne stra ta vary laterally in th ic kness an d pinch o ut terminations arc co mmonly o bse rved at 30 III o f d istance . Th e pe rcentage distribution o f the sandstone varies from 80 c.*, to debris f/ow s.l. i"ence
/
~-
dislal
.....
-
! 180c m
-2 krn
> strata
Ihrckn. s
> gram
dllnenJn
.mJm
•
Deposits
/ proximal
-2 km
B
prox imal
Itl'l"
coarse to medium- tine sandstone
/ dislal
laminar now high -density ncn-wrbutent flo w
/
r-~~~~ =rm ' T eB--~~-::::- I
hee . very fine sandstone alternated wilh silly sandstone
Transport and depositional processes
11..11'
Fig//re 7 - Trunspon and deposiliol1al modelfor 1I10 ,W ofthe sandstone bodies ofthe units IJ and C. n i l' model consists ill a bipanitc dem"ily-slralijied jlow; constituted by a high -density IlO1 H U,.bule11l j lo\1' [debris flow s.l.) ill the lower part (Ifill by (I true turbidity cu rrent ill the IIppl'r pa n. 711c' sedimentary influence of the debris flow s.l. is greater. Th e Ilirhidit.\' current deposit is limited 10 thin layers above the debris j 10\1l s.l. or thin strata ill proxim al part of the 110": A minimum 2 km 101lg depositional model is based all outcrop data. Revista Brasileira de Geoaenciss. Volume 34, 2004
339
Architectural reinterpretation and acoustic facies of the lower portion of the Apiuna unit (Cambrian deep -water depos itional system" Santa Catarina State, Brazil)
in tile upper po rtion with the facies describ ed above. Th ese strata arc laterall y con tinuous for more thnn SO m, massive. locally c harac terized by ero sive depression s and show the thickness decreases upw..rrd. Chaotic strata form ed by loca l sliding may also occ ur.
INTERI'RETi\ T/ON The COil tact ol' thc lo wer port ion olthc unit D lie s o n ar gi llite hy mea ns or a n eros ion sur face. High co ncentration mass flo ws are respon sible for the sedimentation of the lenticular coarse deposits. Co hesive deb ris flo ws deposited muddy co ng lo me ra te and g ra in flow s .\".1. produced th e co ng lomerate sandsto ne. Th e geo metry of the bodies and the interpreted sedimentary processes a llow ded uc ing that the highconcentratio n mass Flows were dr iven and deposited in a co nlined and erosive depression. i.c, a channel. T he upward decrease in g rain size and thickness of the beds testify. durin g the progressive til ling. a loca l decrea sing in compe tence and transport capacity in the channel. The fac ies assoc iation of the upper porti o n records decrease in sandy input. dec rease in the energy of the de positional flows and paleocurrents perpendicular 1() the chan nel flo w direction. Th e de positional mechanisms o r the lenticu lar sandy strata cou ld he auriliutc d to true turbidity currents. As the facies association of the upper portion overlies in co ntinuity the c hannel deposits and as the sa ndstone stra ta sho w perpendi cul ar paleocurrents, we interp ret these as ove rbank o r levee depo sits. The upper IXJI1 ion shows similarity with the s uccessio n describ ed in the Ec ho Group (M uu i 1( 77) of the Pyrenees as deposits of channel margins. In this co ntext, the thin lenticular sandstone strata are inter preted as moderate turbidity currents, which went o ut of the channel. When mo re po werfu l flows ovc rha nkcd, they deposited thicker sandy strata with locall y eros ive bottom . Two o the r asp ect s a llow us to mak e hypoth eses o n the architectura l features of unit D: a) we note an upward dec reasing in the thick ness o r the sandstone strata. b) channel paleocurrents a re d irect ed to wa rd S ~ SW an d ov e rly ing le vee faci es ha ve paleocur rents directed to ward the \V. Thi s indica te that the channel underw ent. contempo rary to the local tillin g, a lateral migra tion to ward E. Actually. SCX) m E o f unit I) and at higher stratigraphic position. a sand y cougfomcrn tc c rops out. testifying to the lateral migration of the channel. T ilE CONCEIY I'UAL MO DEL We created a 2D mode l, where geo metrical and lithologica l data are rea l or deduced from the depositional mechanism intcrp rcuuion . The detail used in this 2D model is a co mpromise bet ween the need to c haracterize the d iffere nt dep osi tio nal un its and the po ss ibi lity of acoustic reso lution at diffe rent freque ncies: in this way the generated 1110de l is a "sim plificuuon" of the sedimentolog ical data. Below we present a sedimentary model and a lithological architecture and afterwa rds we w ill d iscu ss the s patia l a nd tempora l evolution o f the depositional units. T he unit A indicate s that the depositiona l area was gen tly inclined and devoid of sand depo sitio n d ue to insu ffic ient sandy input and/or bypassing or sandy dr iving fl ows (Fig. 9). T he high abun dance o r fine deposits (more than .' ' .' > " .. .. ,. " , • • " , . " " ' . " , , , . .. • ' . , .. ..... , .' .... ,.....' .'" " .. ' ,' , ¥'." , "~" .0' • ,~~ """ , •• ,
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Fi~IlH' / 4 - Impulsi ve responses and co rresponding synthe tic seismic sections fo r eac h concep tua l geolog ical model ill FiR. / 3. A wavelet with dominant freqnmcy of250 Hz. W(lS used ill the application offorward seismic modeling 10 the architectu ral model in Fig. 13. S('c'trstfor C011/1IWIl1.\·.
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