Zions Bancorporation lost $1 billion of its valuation in a single day on Thursday after disclosing $60 million worth of loans it had made that were unlikely to be repaid.What led to that point was a messy, tangled web of loans that Zions said were surreptitiously subordinated by the borrowers all while the collateral was effectively eliminated.Shares of regional banks tumbled on Thursday as fears mounted around the health of their lending businesses. Zions' 13% share plunge ended up sparking worries about p ...